Organic evolution.pdf

Organic evolution is the theory that more recent types of plants and animals have their origins in other pre-existing forms and that the distinguishable differences between ancestors and descendants are due to modifications in successive generations.
Populations of animals and plants produce progeny at such a rate that were they all to survive, they would increase rapidly year after year.
Spectacular progressive increases in population size do not, in fact, occur. Although most populations fluctuate year by year, they remain essentially constant over the long term.
A very real struggle for existence occurs in nature. Each individual must compete for food and must cope successfully with every facet of the environment—both physical, such as climate extremes, and biological, such as diseases and predators—in order to live to produce progeny.
Each individual differs from virtually all others in its species. By Darwin’s time, striking variation in domestic animals had already been produced by selective breeding. Darwin noted that species in nature had similar potential for modification.
Here Darwin made a break with all previous suggestions on the subject. Instead of postulating that modifications are induced by the environment and are then passed on from generation to generation, he suggested that new characteristics arise from within an organism entirely by chance. (We now know that these arise as genetic mutations.) However, not all of these new characteristics will have adaptive significance or survival value, and many may even be lethal.
Some of the new characteristics enhance an individual’s success in coping with the environment and may even allow the organism to push beyond previous environmental barriers. Others will be unsuccessful, and individuals with these modifications will simply not survive to pass them along; Darwin termed this process natural selection.
With the discovery of DNA (deoxyribonucleic acid) in 1953 by Francis Crick and James Watson, the study of evolution entered yet another phase, taking it to its most fundamental level. Crick and Watson found that DNA contains the genetic instructions used in the development and functioning of all known living organisms. Chemically DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars and phosphate atoms joined by ester bonds. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information, and the main role of DNA is the long-term storage of information. Eukaryotic organisms such as animals, plants, and fungi store their DNA inside the cell nucleus, while in prokaryotes such as bacteria it is found in the cell’s cytoplasm. Within cells, DNA is organized into structures called chromosomes and the set of chromosomes within a cell make up a genome. These chromosomes are duplicated before cells divide, in a process called DNA replication.
DNA is the ultimate source.

2. th
13 Edition
Revised & Updated

3. Organic
Evolution
(Evolutionary Biology)
Veer Bala Rastogi
MEDTECH
A Division of
Scientific International

4. Evolution
(Evolutionary Biofogy)
THIRTEENTH EDITION

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Bala Rastogi

9. Evolution
(Evolutionary Biology)
THIRTEENTH EDITION
Dr. Veer Bala Rastogi
M .Sc. (Gold Medalist) Ph.D., F.A.Z. Formerly Reader, Department of Zoology
Gargi College, University of Delhi, Delhi
Ex-Member,
Textbook Evaluat ion Committee
NCERT, New Delhi
Recipient ofDistinguished Author Award 2012
by the Federation ofEducation publishers
in India, Delhi

11. MEDTECH
A Division of Scientific International
Engaging Sciences-Developing Mindsl
Evolution
(Evolutionary Biology)
MEDTECH
ADivision of
Scientific International Pvt. Ltd.
Copyright © Author
Thirteenth Edition: 2018
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Preface
Evolutionary Biology is as wide as the world of animals and plants, past and
present. Recent years have seen an immense expansion of evolutionary studies to
many areas that were scarcely touched before. The last twenty five years have
witnessed the revolutionary impact of molecular biology upon genetics and
developmental biology, the two fields of fundamental importance for
evolutionary studies. As a result evolutionists have acquired new tools and
concepts for investigating evolutionary processes. Due to rapid progress made in
the field of molecular biology, many unexpected and revealing discoveries have
already emerged. Further molecular studies will certainly modify the paradigm
of evolution as we now understand it although the extent of change remains to be
determined.
So , why did I write a book about evolution? Because, while teaching evolution,
I observed that a lot of young students are confused about exactly what evolution
is, what it does , how it works, and why it is important. This book will help all
such readers to everything out. The content of the book is meant to reflect a wide
range of evolutionary principles and to offer examples showing how
evolutionary forces work and have worked in past to give us such a colourful
and varied world. The text is intended to help the students develop a large
number of ideas or centres of interest of his own concerning evolution.

13. The text in this book is divided into five parts. Part I, begins with the Concept of
Evolution, which deals with the meaning of word 'Evolution', and the historical
development. It provides a comprehensive account of evidences to support
concept of evolution, and different theories for exploring the mechanism of
evolution. Part II deals with the mechanism of evolution. Topics on Variation
and Gene Mutations which form the necessary substrate for natural selection are
dealt in a simple and understandable form. Next two chapters deal with
chromosomal changes. Chromosomal aberrations include changes in the
arrangement of genes in a chromosome and variations in the chromosome
number. Both types of chromosomal variations advocate that no two organisms
or parts of organism are precisely alike because variations influence the
characters of individuals. Next chapter is on Isolation which
vi ~ Preface
deals with isolating mechanisms that split the species populations into separate
groups and their evolution into distinct species.
Part III includes chapters on Population , Population Genetics and Persistence of
Variability within populations leading to the formation of species. It is followed
by Genetic drift and gene flow among populations. The chapter on Natural
selection in action stresses that the evolutionary agents bring in changes in the
frequencies of alleles and genotypes in Mendelian populations that produce more
efficient adaptive relationship with the environment to ensure better survival and
comparative reproductive success . Next chapter deals with evolution of genes
and genomes. This chapter discusses how the evolutionists with the
advancement in the knowledge of molecular genetics are able to study
evolutionary changes in the genes, genomes and gene pools of populations and
how the organisms undergo mutations and get adapted to new environments.
Part IV emphasises on the basic patterns of evolution . The chapter on
Microevolution and Macroevolution throw s light on the interaction of the
elemental forces of evolution, i.e., mutations, variation, natural selection, genetic
drifts leading to microevolution. Microevolution explains the origin of new
adaptive types through a process of population fragmentation . Next chapter is
on Patterns of Evolution, i.e., sequential and divergent evolution, phyletic
gradualism and punctuated equilibrium, anagenesis and cladogenesis;
monophyletic, polyphyletic and paraphyletic evolution, divergent evolution, and
conditions responsible for adaptive radiation and the causes and significance.
This is followed by convergent evolution or adaptive convergence supplemented

14. with examples of adaptive convergence. It is followed by coevolution which is
also a pattern of evolution in which two interacting species influence each other's
adaptive changes over the time, its examples and outcome of coevolution.
Chapter on adaptations explains how the organisms enable themselves to thrive
successfully in a particular environment.
Part V provides an insight into the origin and history of life on Earth. Chapter on
'Origin of Life' covers the physical and chemical environment that existed on
ancient primitive Earth and supported the origin of first living form, the
beginning of biological evolution and gradual complexity attained there in.
Chapter on fossil records reveals the long history of life which had been partly
recorded in certain rocks of earth's crust in the form of fossils. Next chapter
summarises the whole evolutionary story as shown by fossils in geological
records. Last chapter, Evolution of Primates and Man is written from zoological
point of view, for the student needs to know first something about what actually
happened.
I have been extra cautious in presenting the text in easy and lucid language.
Diagrams are specially designed and also taken from various sources for clarity
and simplicity.
I am pleased to record thanks to universities all over the country who my
colleagues at various institutions and have provided immense valued help while
preparing this endeavour. I request all of them to send their valued suggestions
and critical review in the subsequent edition.
Preface ~ vii
I am highly thankful to Sh. Rajan Jain (Director) Scientific International Pvt.
Ltd., Sh. Vinod Chauhan (HOD-Production) and Sh. S.K. Panda (Editor-
cumCoordinator) for their tiredless support throughout in giving the shape to the
book what it is with you.
Veer Bala Rastogi

17. Contents
Prefa ce v
UNIT I. CONCEPT, EVIDENCES AND THEORIES OF EVOLUTION 1 1.
Concept of Evolution 3
1.1 Evolution and Evolutionary Biology 3
1.2 Definition of Biological Evolution 5
1.3 Basic Concept of Organic Evolution 5
1.4 Development of the Idea of Evolution 6
1.5 Evolution-A Fact or Just a Theory 14
Ke y Terms 15
Review Questions 16
Furth er Readings 16
2. Evidences for Evolution 17
2.1 Doctrine of Biological Evolution 17
2.2 Evidences for Biologica l Evolution 17
2.3 Evidences from Comparative Anatomy and Morphology (Tectology) 18 2.4
Evidences from Vestigial Organs 28
2.5 Evidences from Atavism or Reversion 30
2.6 Evidences from Comparative Embryo logy 32
2.7 Evidences from Palaeonto logy or Palaeobiology 38
2.8 Evidences from Geographical Distribution or Biogeographical
Evidences 51
2.9 Evidences from Connecting Links 55
2.10 Evidences from Taxonomy 58
2.11 Evidences from Biochemistry and Physiology 60

19. x ~ Contents
2.12 Evidences from Molecular Records 62
2.13 Evidences from Cytology 64
2.14 Evidences from Gen etics 66
Key Terms 67
Review Questions 68
Further Readings 69
3. Theories of Evolution 70
3.1 Lamarck and Lamarckism 70
3.2 Inheritance of Acquired Characters (Lamarckism) 70
3.3 Darwinism or Theo ry of Natu ral Selection 76
3.4 Mutation Theory of Evolution 92
3.5 Modem Synthetic Theory of Evolution or The Evolutionary Synthesis 95
3.6 Neutral Theory of Evoluti on 99
Key Terms 100
Review Questions 100
Further Readings 101
UNIT II. MECHANISMS OF EVOLUTION 103
4. Variation 105
4.1 Variation and Variability 105
4.2 Nature of Variation 105
4.3 Types of Variation 105
4.4 Sources of Variation 108
4.5 Variation in Number of Chromosomes (Heteroploidy) 109
4.6 Chromosomal Aberrations 11 2
4.7 Gen e Mutations or Point Mutations 11 4
4.8 Mendelian Recombination or Sexual Recombination 116
4.9 Recombination due to Exchange of Genes between Homologous
Chromosomes of a Pair 11 8
4.10 Hybridi sati on 120
Key Terms 122
Review Questions 122
Further Readings 122
5. Gene Mutations 124
5.1 Definition 124
5.2 History 124

21. 5 .3 Characteristics of Mutations 125
Contents ~ xi
5.4 Kinds of Mutations 126
5.5 Causes of Mutations or Mutagenic Agents 129
5.6 Molecu lar Basis of Single Gene Mutations or Point Mutations 131
5.7 Substitution Mutations 133
5.8 Frameshift Mutations 142
5.9 Mutation Rates 144
5.10 Effects of Mutations on Fitness 146
5. 11 Randomness of Mutations 147
5.12 Mutations and Genetic Polymorphism 148
5.13 Mutations and Evolution 148
Key Terms 149
Review Questions 149
Furth er Readings 150
6. Chromosomal Aberrations 151
6.1 Chromosomal Aberrations lS I
6.2 Origin of Chromo soma l Aberration s 151
6.3 Types of Chromosomal Aberrations lSI
6.4 Deletion or Deficiency 152
6.5 Duplication or Repeat 156
6.6 Inversion s 160
6.7 Translocation 166
Key Terms 172
Review Questions 173
Furth er Readings 174
7. Variation in Chromosome Number (Heteroploidy) 175
7.1 Changes Involving Entire Set of Chromosomes (Euploidy) 175
7.2 Changes Involving Number of Chromosomes in a Set (Aneuploid y) 176
7.3 Euploidy 176
7.4 Origin of New Species Through Polyploidy or Evolutionary Role of
Polyploidy 183
7.5 Induced Polyploidy 187
7.6 Aneup loidy 188
Key Terms 192
Review Questions 192

22. Furth er Readings 193
8. Reproductive Isolating Barriers 194
8.1 Introduction 194
8.2 History 194

24. xii ~ Contents
8.3 Role of Reproductive Isolation 195
8.4 Types of Reproductive Isolating Mechanisms 195
8.5 Premating Barriers 196
8.6 Postmating Prezygotic Barriers 203
8.7 Postzygotic Barriers 205
8.8 Multiple Isolating Barriers 207
8.9 Genetic Basis of Reproducti ve Barriers or Reproductive Isolation 208 8.10
Origin of Reproductive Isolat ion and Origin of Species 210 8.11 Evolution of
Reproductive Barriers 211
Key Terms 212
Review Questions 212
Further Readings 213
UNIT III. SPECIATION 215
9. Population Genetics, Gene Frequencies
and Hardy-Weinberg Equilibrium 217
9.1 Population Genetics 217
9.2 Population 217
9.3 Gene Pool 219
9.4 Fundamental Principles of Genetic Variation in Populations 220 9.5 Hardy-
We inberg Equilibrium 226
9.6 Hardy-Weinberg Principle and Evolution (Factors that Change Gene
Frequency ) 232
9.7 Genetic Landscape of a Population and Evolution 236
Key Terms 238
Review Questions 238
Further Readings 239
10. Persistence of Variability within Populations: Polymorphism 241 10.1
Variability within Populations 241
10.2 Polymorphism 241
10.3 Balanced Polymorphism 243
10.4 Transient Polymorphism 249
10.5 Origin of Polymorphism 250
10.6 Mechanisms to Maintain Polymorphism within Populations 250

25. K ey Terms 252
Review Questions 252
Further Readings 253

27. Contents [i] xiii 11. From Population to Species (Speciation) 254
11 .1 Species and Speciation 254
11 .2 The Species Concept 254
11.3 Species Categories 260
11.4 Origin of Species (Speciation) 262
11 .5 Modes of Speciations 262
11.6 Allopatric Speciation 265
11 .7 Peripatric Speciation of Marginal Populations 270
11.8 Parapatric Speciation 271
11 .9 Alloparapatric Speciation 272
11.10 Sympatric Speciation 272
11 .11 Consequences of Speciation 277
11.12 Rate of Speciation 277
11.13 Factors Responsible for Variation in Speciation Rates 277
11.14 Theories of Speciation 278
Key Terms 280
Review Questions 280
Further Readings 281
12. Genetic Drift and Gene Flow 283
12.1 Random Genetic Drift or Sewall Wright Effect 283
12.2 Theory of Genetic Drift 283
12.3 Salient Features of Genetic Drift 284
12.4 Genetic Basis of Random Genetic Drift 286
12.5 Genetic Drifts in Real Populations 287
12.6 Founder Effect or Founder Principle 289
12.7 Bottleneck Phenomenon 291
12.8 Genetic Drift and Evolution 293
12.9 Gene Flow 295
K ey Terms 297
Review Questions 297
Further Readings 298
13. Natural Selection in Action 299
13.1 Concept of Natural Selection 299
13.2 Salient Features of Natural Selection 302

28. 13.3 Natural Selection in Nature 302
13.4 Demonstration of Role of Natural Selection 306
13.5 Working of Natural Selection 307

30. 13.6 Components of Natural S election or Levels of Natural Selection 308
13 .7 The Results of Natural Selection 312
13.8 Models of Selection 313
13.9 Frequency Dependent Selection 322
13.10 Heterozygous Advantage or Heterosis 322
13.11 Balancing Selection and Balanced Polymorphism 323
13.12 r-Selection and k-Selection 324
13.13 Selection Pressure or Selection Intensity 325
13.14 Selection and Reproduction 326
13.15 Selection and Mutations 327
13.16 Selection and Variation 329
13.17 Selection and Adaptations (The Baldwin Effect) 330
Key Terms 330
Review Questions 331
Further Readings 331
14. Evolution of Genes and Genomes 333
14.1 Molecular Evolution 333
14.2 Molecular Phylogenie s 333
14.3 Proteins and Phylogenetic Relationship 334
14.4 Origin and Evolution of New Genes 336
14.5 Regulatory Genes and Evolution 345
14.6 Nucleic Acid Phylogenies 345
14.7 Genome and Phylogenetic Relationship 347
14.8 Convergent Molecular Evolution 347
14.9 Molecular Clocks or Evolutionary Clocks 348
14.10 Molecular Evolution in Test Tube 351
Key Terms 352
Review Questions 352
Further Readings 353
UNIT IV. BASIC PATTERNS OF EVOLUTION 355
15. Patterns of Evolution 357
15.1 Sequential and Divergent Evolution 357
15.2 Phyletic Gradualism and Punctuated Equilibrium 358
15.3 Anagenesis and Cladogenesis 359
15.4 Monophyletic, Polyphyletic and Paraphyletic Evolution 361

32. 15.5 Divergent E volution (Adaptive Radiation or Adaptive Divergence) 362
15.6 Convergent Evolution or Adapti ve Convergence or Parallel
Evolution 369
Contents ~ XV 15.7 Coevolution 372
Key Terms 372
Review Questions 372
Further Readings 373
16. Microevolution and Macroevolution 374 16.1 Microevolution 374
16.2 Macroevolution (Adaptive Radiation) 378 16.3 Megaevolution 383
16.4 Trends During Macro and Mega-Evolution 385
Key Terms 385
Review Questions 385
Further Readings 386
17. Adaptations 387
17.1 Introduction 387
17.2 Definition of Adaptations 387
17.3 Kinds of Adaptations 388
17.4 Mimicry 395
17.5 Batesian and Mullerian Mimicry 400
17.6 Co-adaptation 402
17.7 Animal Association Adaptations 404
17.8 Biotic Adaptations and Organismic Adaptations 404 17.9 Preadaptations
and Postadaptations 405
17 .10 r-Adaptations 406
17.11 k-Adaptations 407
Key Terms 407
Review Questions 408
Further Readings 408
UNIT V. FOSSILS AND HISTORY OF LIFE ON EARTH 411
18. Origin of Life on Earth 413
18.1 Origin of Life (Biopoiesis) 413

33. 18.2 Ancient and Medieval Beliefs 413

35. 18 .3 Modem Hypothesis of Origin of Life or Biochemical Origin of
Life 416
18.4 Biochemical or Chemosynthetic Origin of Life 421
18.5 The Earliest Cells 436
18.6 Where Life Originated? 437
18.7 Earliest Evidence of Existence of Life on Earth 438
18.8 Evolution o f Eukaryotic Organelles 439
18.9 Life on other Planets 439
Key Terms 443
Review Questions 444
Further Readings 445
19. History of Life on Earth 446
19.1 Geological Time Scale 446
19.2 Azoic Era 452
19.3 Archeozoic Era 453
19.4 Proterozoic Era (The Era of Former Life) 453 19.5 Palaeozoic Era 454
19.6 Mesozoic Era (Era of Intermed iate Life) 462 19.7 Coenozoic Era (Era of
Recent Life) 474
Key Terms 478
Review Questions 478
Further Readings 478
20. Fossils and Fossil Records 480
20.1 Earth's Structure 480
20.2 Classification of Rocks 480
20.3 Fossils 486
20.4 Exposing Fossils 492
20.5 Interpretation of Fossil Records 493
20.6 Law of Superposition 494
20.7 Williston's Rule 494
20.8 Cope's Rule 495
20.9 Allometry (Differential Growth Rate) 497
20.10 Determination of Age of Fossil s or Dating of Fossils 497
20.11 Significance of Study of Fossils 501

36. 20.12 Incompleteness of Fossil Records 503
20.13 Evolutionary Rate Through Fossil Records 505 Key Terms 509
Review Questions 509
21. Origin and Evolution of Horse 510
21.1 Place and Time of Origin 510
21.2 Evolutionary Trends 510
21.3 Characteristics of Modem Horse 512
Contents ~ xvii
21.4 Phylogeny 512
21.5 Side Lines 519
Key Terms 520
Review Questions 520
22. Origin and Evolution of Man 521
22.1 Introduction 521
22.2 Scienti sts Associated with Human Evolution 521
22.3 Time of Origin of Primates and Man 523
22.4 Place of Origin of Man 524
22.5 Primate Heritage 525
22.6 Special Features of Primates 525
22.7 Evolution and Adaptive Radiation in Primates 526
22.8 Compelling Causes of Evolution of Man 528
22.9 Impact of the Descent from Tree s on Primate Organisation 528 22.10
Evolutionary Trends during Human Evolution 529
22.11 Evidences from Molecular Biology in Support of
Hominid Evolution from Apes 535
22.12 Common Ance stor s of Apes and Man in Oligocene and Miocene 538
22.13 Common Ancestor s of Apes and Man in Pliocene Period ,540 22.14
Evolution of Man in Pleistocene Period 541
22.15 Cultural Evolution of Human s 551
22.16 Impa ct of Evolution on Human Brain 551
22.17 Human Races 552
22.18 Archa eological Division s of Pleistocene and Holocene Periods 553 22.19
Monophyletic or Polyphyletic Origin of Man 554
22.20 Punctuated Equilibrium in Human Evolution 556
Key Terms 557
Review Questions 557
Further Readings 558

37. Glossary 559
UNIT-I
Concept, Evidences and Theories of Evolution
Chapter 1. Concept of Evolution Chapter 2. Evidences for Evolution Chapter 3.
Theories of Evolution

38. 1
concept of Evolution
1
.1 EVOLUTION AND EVOLUTIONARY BIOLOGY
1.1.1 Evolution (L. evolvere =to roll or to unfold)
English philosopher Herbert Spencer (1820 -1903) coined the term ' evolution' to
represent the phenomenon that brings about continuous and orderly changes in
nature . The word 'evolution' was derived from Latin word evolvere where 'e'
means out and volvere means to roll or unfold.
Ql E :;:; s: OJ ::J
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Leaves on tree change colour and fall over several weeks
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39. Mountain ranges erode over millions of years
FIG. 1.1: Difference in the biological and physical evolution in time.
E volution is described as change through time. It can be used to represent any
change in physical or biological world . Lots of things in our surroundings
change over the time : the leaves on trees change colour and fall, plants grow
and die, mountain ranges rise and erode , languages and cultures change . As a
matter of fact evolution occurs at different levels and involves all the
components of universe both living or nonliving. It may be:
• at the molecular level (chemical evolution)
• at the level of physical objects like change in land topography (physical
evolution)
• at the level of stars and planets in the universe (stellar evolution or cosmic
evolution)
• at the level of living objects (biological evolution)
1.1.2 Evolutionary Biology or Biological Evolution
E volutionary Biology is defined as the process of gradual changes in organisms
to form more and more complex forms over a long period of time.
• Darwin has defined evolution as 'Descent with modification.'
• Theodor Dobzhansky has defined 'evolutionary biology' as the study of
history of evolution of newer and more complex forms of life on the Earth from
pre-existing simpler ones over a period of time.
The term 'bioevoluti on' or 'evolutionary biology' was introduced by Mayr
(1970). It is also called organic evolution or biological evolution.
Biological evolution is not just a change in time. It deals with very specific type
of changes such as changes in the frequency of different genes in the organisms
of a population or species over generations or large-scale changes leading to the
origin of new species from a common ancestor over many generations .
The central idea of biological evolution is that all life on Earth shares a common
ancestor, just as you and your cousins share a common grandmother or
grandfather.
Through the process of descent with modification, the common ancestor of life
on Earth gave rise to tremendous diversity over a veryvery long period. This

40. diversity in life is well illustrated by present day living forms and the forms that
existed in past and are documented in the fossil records.
Short-term change
Change through time •
Common ancestor
A genealogy illustrates change with inheritance over a small number of years
A B
FIG. 1.2: A. Short-term changes are seen in family genealogy. They introduce
minor
differences but do not lead to evolution ; B. Long-term changes in the organisms
of common ancestors , on accumulation for very long period produce
tremendous diversity in living forms and lead to evolution of new form by
descent with modification.
Concept of Evolution IiJ 5
Based on the concept of biological evolution, all living organisms, plants,
animals and bacteria, etc., have some common ancestor. We can say that not only
monkeys or chimpazees even birds, whales, snakes, worms and trees are all our
distant cousins .
1.2 DEFINITION OF BIOLOGICAL EVOLUTION
Biological evolution or organic evolution is defined as 'the process of continuity
of life with constant modifications.' It means living organisms modify and adapt
according to the everchanging environmental needs. These modifications keep
accumulating in the organisms generations after generations, resulting in more
complex and better adapted new species. Therefore, organic evolution is the
evolution of present complex and highly organised living beings from simpler
and less organised living beings of the past by gradual modifications
accumulated through successive generations over millions of years.
1.3 BASIC CONCEPT OF ORGANIC EVOLUTION
The basic concept of organic evolution envisages 'continuity of life with constant
modification'. It suggests that:

41. • Environmental conditions in nature are everchanging.
• Organisms have an inherent tendency to change in response to the changing
environmental conditions. This is called adaptability or adaptation.
• Such adaptive changes in organisms are inherited by the offspring and lead to
the 'Origin of new species' (Evolution).
• Since changes in the organisms are due to adaptations, new species are always
better adapted and more organised than their ancestors.
• Different members of a species, on being adapted to different environments,
diversify and evolve along several divergent lines and form new species.
• All the present day species had a common ancestor at some or other time of
their evolution (Monophyletic genealogy).
• Individuals migrate from their place of origin to varied geographical areas and
gradually adapt to different environmental conditions. This results in the
formation of several new species from one ancestral species (Divergent
evolution).
• Organisms from varied regions also migrate to a common habitat and modify
to adapt to that habitat. As a result organisms from distantly related groups
develop common features (Convergent evolution).
• Evolution is a very complex and extremely slow process. It is not possible to
see one type of animals changing to other, but presence of integrading organisms
supports the concept of evolution.
• Evolutionary changes are continuous. They occurred in past, are continuing in
present, and will continue in future.

43. 1.4 DEVELOPMENT OF THE IDEA OF EVOLUTION
Charles Darwin's name is c losel y associated with the concept of evolution and
for many people Darwinism is evolution, but the concept of evolution, for the
first time appeared in the writings of ancient Greeks. Their explanation of the
origin of living things as given by Empedocles and Anaximander supported the
notion of dynamic world and rep laced the mythological explanation.
1.4.1 Greek Theories
I. Thales (624-548 BC) propounded the theory of aquatic or marine orgin of life.
2 . Anaximander (611-547 BC) was call ed 'the earlies t evo lutionist' by Osborn
(1894). He proposed that all living beings have arisen from a primordial fluid or
slime to which they ultimately return. Th e plants and animals were formed as
this mud dried. It was presumed that man himself was first shaped like a fish and
lived in water. Later, when he became capable of terrestrial life, he cast off his
fish-like capsule like a butterfly comes out of its chry salis and assumed human
form. The theory is crude yet the implication is clear. He also proposed that
simple forms preceded more complex forms.
3 . Xenophanes (576-480 BC) contemporary to Anaximander, recognised that
fossils are the remains of organisms living in past. According to him , the
existence of fossils of marine animals on dry land indicated that the dry land was
once under the sea and that life originated in the sea .
4 . Empedocles (504-433 BC) has been hailed as 'Fathe r of Evolutionary Id ea ' ,
by Osborn. He believed in spontaneous generation and proposed that evolution
of animals was a series of attempts by nature to produce more perfect forms, The
main points of his proposition are :
• Higher forms of life evolved gradually.
• Imperfect forms (i.e., less adapted forms) were gradually replaced by perfect
forms (i.e., the better adapted).
• Plant life came first and animal life developed later.
• Perfect forms were produced by the extinction of imperfect forms.
His theory was shaped as follows:
All the matter was formed of four elements namely, air, earth, fire and water.
These were acted upon by two great force s, the love and hate , which caused
their union or separation. As a result, parts of animals were formed separately as

44. unattached organs. They joined together in haphazard manner under the
influence of love over hate. The conglomerations produced this way were mostly
monsters or disharmonious and incapable of living, but a few could function as
successful living organisms. Such successful combinations populated the Earth.
This theory provides the first glimmerings of the idea of survival of the fittest,
which formed the basis of Darwin's theory of natural selection twenty three
centuries later.

46. 5. Aristotle (384-322 BC) called 'the greatest investigator of antiquity', by
Locy (1923), was vitalist and his ideas dominated biological thoughts well over
a thousand years. He proposed that living things were animated by a vital force
or guiding intelligence, which operates constantly and improves and perfects the
living world. Aristotle suggested that the various organisms constitute a series,
the so called ladder of life in which organisms can be arranged in a sequence of
increasing complexity from non-living matter through plants to plant-like
animals (like sponges and sea anemones) or lower animals and then to higher
animals . He placed man on the top of this ladder.
Aristotle also introduced the concept of Teleology. According to this concept the
natural processes such as development or evolution are guided by their final
stage or final goal (Ie/os) or for some particular purpose. The external teleology
indicates guidance of a process towards some specified end decided by an
external mystical source. The internal teleology indicates the end point of a
process that has an understandable materialistic basis that develops from the
process itself. For example, plants are engaged in photosynthesis and animals
seek food for survival and the ultimate purpose of survival is reproductive
success .
6 . Epicurus (341-271 BC) and Soretium (99-55 BC) gave an evolutionary
explanation of origin of plants and animals. Plants appeared before animals and
humans appeared last of all.
1.4.2 Pre-Darwinian Theories
Evolutionists of medieval age were :
I . Francis Bacon (1561-1626) who reviewed Aristotelian idea and presumed that
new species could arise from the old species by degenerative process caused due
to mutability in the species. He, therefore, emphasised on variations as being the
cause for the origin of new species from the old one. He suggested that flying
fishes are intermediate between fishes and birds, and bats between birds and
quadrupeds. His work influenced the thinking of the successors.
2 . Jan Swammerdam (1637-1680), the Dutch scientist, proposed the
'Preformation Theory.' According to this theory ova contain miniature of the
adult in preformed state. The act of fertilisation (i.e., union with the sperm)
provides initiation for growth and the miniature grows into adult.AII parts of the

47. embryo lie folded together in the egg. During development these parts grow in
size, and stretch themselves.
When spermatozoa were discovered , they were called the animalcules and were
described to possess the miniature of the embryo. The eggs were presumed to
supply nourishment for the developing embryo.
The preformation theory was discarded by the valuable observations made by
Casper Friedrich Wolf (1759), who studied chick embryo and concluded that the
preformed embryo is not found either in egg or sperm . The development
includes the division of one cell and the modifications in the cells produced by
its division to form various organ systems.

49. 3. Demaillet (1656-1738) contributed mainl y on the nature and formation of
fossils. He also pointed out the sim ilarities betw een aquatic and terrestrial form
s and proposed that the terres trial forms have evo lved from the marin e form s
which were trapped in marsh es. Man y of such spec ies failed to make the
transition and had an ill-fate. He cited the examples of origin of birds from
flying fish, and men and wo me n from mermen and merm aid .
4. Maup ertius ( 1698- 1759) was the first to propose a general theo ry of evolu
tion. He propo sed that hereditary material was part iculate matter. It was tran sm
itted throu gh both maternal and patern al sides of the famil y. He thought that
hered itary part icles co uld be chan ged by env iro nme nt (ac quired charac ters)
. He a lso appreciated the role of natural se lection in evolution and of isolation
in spec iation.
5. Bonn et (1706-1793) proposed 'Embo ite me nt T heory or Encaseme nt T
heory'. It ad vocated that the initial member of a spec ies encapsulates within it
the preformed germs of all future generations. These ex iste d insid e the germ
cell s of mother. Th e theory was discredited by Prevost (1824).
6 . Wolf Theory of E p igenesis wa s proposed by Casper Friedrich Wolf to repl
ace preformation theory. According to this theory, an embryo develops by the
gradu al differentiation of undifferentiated simple tissue s into organs.
7 . Linn aeus ( 1707- 1778) is known as the 'Father of Taxonomy' . He believed
in specia l creation. He presumed that species are creat ed by God and are
immutable and fixed entities.
8 . Buffon ( 1707- 1778) be lieved in the inheritance of acquired characteristics
and the direct effect of environment on the structural modifications of organis
ms. Th ough , he never gave a con sistent theory of evolution but he did state
parts of the theory of orga nic evolu tion .
9 . Jam es Hutton (1726-1797) postulated that volcanic acti viti es bring magma
up fro m Earth's molten interior which on solidification form s new igneous
rocks. He also noted that forc es like wind, water (rain, surf), heat, cold, ice
(glaciers) and acti vities of plants and animals erode rocks and the eroded
particles are transported by water, and are deposited in layers. These layers get
compressed into sedimentary rock s. His idea of gradual geological changes
brought about by natural process is kno wn as uniformitarianism. Thi s wa s

50. greatly cham pioned by the great geologist Charl es Lyell and has greatly influe
nced Darw in.
10 . Era sm us Darwin (173 1-1 802), the grandfather of Charles Robert Darw in,
gav e the first clear statement of the inh eritance of acquired characters,
according to which the effects produced by the environment on the organi sms
are tran smitted to the offspring. The theory wa s elaborated by Lamarck in the
year 1809.
T he contributions made by Lamarck, Darwin, Cuvier, Weismann, Huxley, etc.,
are of gre at importance, since the y pro vok ed real scientific thinking of
evolutionary proc ess and their theori es are still helpfu l, but in a somewhat
modified forrn, Th e vario us modem theories have been discussed in detail
separately, henc e a brie f survey will serve the purpose here.

52. II . Lama rc k's Theory of I nhe ritance of Acquired Characters (1744-1829):
Lamarck's theory emphasises the influence of environment on the living beings.
The changes introduced by the environment are acquired by the living beings
and are inherited by the next generation. Modern supporter of Lamarckism was
Lysenko (1930), a Russian botanist.
12. Theory of Catastr op hism: The theory was formulated to explain differences
in the past and present forms of life and sharp discontinuities in the fossils
records present in the stratified rocks. It state s that there had been several
creations, each preceded by a catastrophe due to some supernatural forces and
not the geological disturbances, like volcanic eruptions, upheaveling of Earth ,
torrential rains , unprecedented increase in sea level, etc. Each catastrophe
completely destroyed the life. The new creation resulted in life quite different
from the previous one.
George Cuvier (1769 -1832) and Orbigne (1802-1832) were the chief advocates
of the Theory of Catastrophism.
Cuvier (1769-1832) is considered to be the 'Father of Pal aeontology a nd
Comparative Anatomy'. Cuvier believed in the fixity of species. The occurrence
of fossils in different rock strata was accounted on the basis of catastrophism. A
succe ssion of catastrophes have periodically destroyed all living things,
followed each time by the successive creations of new and higher forms .
13. Theory of Eternity of Life: According to this theory, life has ever been in
existence in the form as it exists today and will continue to be so for ever. It
neither had a beginning nor an end and has not changed or evolved.
However, with present knowledge, the theory cannot be accepted. The evidences
clearly indicate the gradual complexity in the organisation of living beings.
14. T heo ry of Unifor mita r ia nism: James Hutton (1785) and Charles Lyell
(1832) establi shed the concept of uniformitarianism which holds that slow ly
acting geological forces (erosion, sedimentation, disruption and uplift) resu lt in
the formation of fossil-bearing rock strata. The same forces are acting eve n
today.
Jean Baptiste P ierre Antoine de Monet Chevalier de Lamarck (1744-1829)
known for 'Theory of Inheritance of Acquired Characters'
1.4.3 Evolutionary Theories Since Darwin

53. 1. Darwin's Theory of Natural Selection (1809-1882): Darwin formulated the
theory of ' O r igin of Species by Natural Selection in 1859.'
To explain some of the phenomena, which were not suitably explained by natural
selection, Darwin propo sed some more theories. These are:
• Theory of Pangen esis: To explain how the characteristics are transmitted from
parent s to the offspring, Drawin proposed Pangen esis theory. According to

55. this theory each and every cell of the body
produces minute primordia called gemmules
or pangene. These gemm ules from all the
parts of the body are carried by the blood
to the gonads where these accumulate in the
germ cells. Each gamete represents minute
replica of parent's body.
• Th eory of Sexual Selection: Darwin pre
sumed that there is always a contest among
males for the possession of female. For this
reason they have developed various methods
to attract the female. Some are beautifully Charles Robert Darwin (1809-1882)
co
loured,
in othe
r
s
p
ecies they are provided
who propagated 'Theory of Natural Selection' with horns or they exhibit different
attractive behaviours or produce sound. This results in sexua l dimorphism,
which is very common in animals.
• Ar tificial Selection : Darwin recognised artificial selection exercised by
human beings as the commo nest method for improving the races of domestic
animals and cultivated plants and producing new varieties . He presumed that if
new races could be developed by artificial selection , the same is possible in
nature .
2. 'eismann's 10 I)' of tin ity f Ger : Considering the futility of Darwin 's theory of
pangenesis, August Weismann (1892), a staunch supporter of Darwin, proposed
that the cytop lasm of the anima l body is differentiated into soma toplasm and
germplasm. The germplasm produces gamates which transmit the characteristics
of parents into the offspring. The remain ing body of the organ- ism is formed of
somatop lasm. Weismann also emphasised that only those changes which occur
in the germplasm are heritable , changes occurring in the body (some or

56. somatoplasm) due to environmental effect are not inherited.
The essent ial features of the theory can be summarised as under:
• Ge r mplas m and Somatoplas m : Weismann proposed that the organisms com-
prise of two types of protoplasm-the ger mpla sm present in the germ cells only
and which is passed on to the offspring, and the somatoplas m, the protoplasm
forming remainder of the body that plays
no role in heredity. The germ cells of the
two parents unite during reproduction and

58. form the zygote or fertilised egg . During development zygote divides into two
daugh
ter ce lls, each of which receives an equal share of germplasm. Through germ
cells a contin uity of germp lasm is maintained generation after generatio n.
• Presenc e of Determinants: Situated in the germplasm are minute comp lex
structures .
August Weismann
(1838-1914) proposed 'Theory of Continuity of Germplasm '
These are known as determinants. The determinants can be compared with the
present day chromosomes. The characteristics of the organisms are represented
in the determinants in the form of minute physiological units, the determiners
(equivalent to genes) .
• Immortality of Germplasm: The germplasm is immortal because it perpetuates
from one generation to the next through meiotic division. The germplasm is
maintained generation after generation. The somatoplasm is mortal and dies with
the death of the organism.
• Only those variations which appear or which are introduced in the germplasm
(germinal or heritable variations) can be inherited and not those which appear in
the somatoplasm.
• The germplasm is composed of 'ids' i.e., equivalent portions of germplasm
contain all kinds of determinants present in the parent body or which are
responsible for the development of characteristics in the offspring.
• In a fertilised egg 'ids' from both the parents are contributed in equal amounts.
3. De Vries T eory of M tati n: Darwin
in his Theory of Natural Selection described
the occurrence of variations but he did not ex
plain the method of their origin. Moreover, he
emphasised on small and cumulative variations.

59. Hugo de Vries (1848-1935) suggested that ~
...
.
variations which are important for evolution are
~

60. sudden and large, which he called mutations or
t
saltations. He proposed 'Mutation Theory' in
1886 for the origin of species. "Karl Naegeli and Wanger Gulik emphasised Hugo de Vries
the presence
of some inner directive which guides Profounded 'Mu tation Theory' the course
of evolution independent of the environment.
4. Reeapit ti T eory f H ttkel: Ernst Haeckel (1811) proposed that 'Ontogeny
recapitulates phylogeny', i.e., the development of the individual repeats the
evolutionary history of the race, condensing some stages and eliminating the
others.
5. ry of Ort .. is: According to the theory of Orthogenesis the variations (or in
other words the evolutionary changes) occur along certain definite lines, guided
by some undefined or inherent mystical force. The term 'orthogenesis' was
proposed by Haeckel in 1893. There were two views regarding orthogenesis.
Karl Von Naegeli believed in the presence of some mystical principle of
progressive development in the living organisms which brings about the
particular specialisation. The theory is merely mythical and has no scientific
basis. Theodor Eimar was of the opinion that lines of evolution are determined
by laws of organic growth, aided by inheritance of acquired characters , and
proceed in specific direction.
In certain cases directional evolution has resulted to an enormous increase of
size of horns which has ultimately proved to be harmful to the organisms and has
led to their destruction.

62. 6. Isolation Theory: The role of isolation in evolution was first emphasised by
M. Wanger. He stated that any factor or mechanism which separates the
individuals of a species into groups, so that these are unab le to intermingle and
interbreed, constitutes the isolating mechanism and is helpful in the progress of
evolution. It was supported by Jordan Cellogg, Gulick and Crompron.
1.4.4 Modern Evolutionary Theory
Modern evolutionary theory has its foundation in the Evolutionary Synthesis or
Modern Synthesis that is formulated on the basis of contributions from Genetics,
Systematics and Palaeontology. It was named Neo-Darwinia n T heo ry.
The modern synthetic theory of evolution has evo lved during the last century
through accumulation of facts and theoretical conc lusions from a number of
scientists . Theodosius Dobzhans ky (1900-1975) in his book ' Ge netics and th e
O r igin of Species' emphasised the role of genetic changes in natural
populations of Drosophila in the process of evo lution. Julian Huxley (1924) and
Ernst Mayr (1942--43) have explained the mechanism of origin of variations in
higher animals and Stebbins in higher plants. Clevland, Blackeslee, Renner and
others have shown that a combination of gross chromosomal aberratio ns, rare
combinations in balanced lethal systems and obligate self fertilisation are
important factors for variation and evo lution. Rensch (1960) has suggested that
the forces operating for the origin of species also operate for the evolution of
genera, fami lies and other highe r categories.
At present the synthetic theory of evolution recognises five basic processes,
namely, gene mutations, changes in chromosome number, genetic
recombination, natural selection and reproductive isolation. The three accessory
processes also contribute to the evo lutionary phenomenon. These are migration,
hybridisation and chance in small populations.
Othniel C. Marsh ( 1831- 1989), Cope ( 1840- 1897), Mathew, Greg ry, Romer
and Simpso n in America, Woodward (1864-1944) and Wa son in England and
Broom ( 1866- 1951) in South Afr ica contrib u ed imme nsely to verte brate
palaeonto logy. J.B.S. Haldane ( 1892- 1964), Fisher (1890-1962), and Sewa ll
Wright (1889-1988)

64. Julian Huxley (1887-1975) Ernst Mayr (1904-2005) (They explained the
mechanism of orig in of variation in higher animals)
Ronald A. Fish er (1890-1962) Sewall Wright (1889-1988) J .B.S. Haldane
(1892-1964)
(They contributed to the mathematical theory for change in gene frequency in
populations under natural selection)
and S. S. Chetverikov (1920s) have provided mathematical theory for gene
frequency change under natural selection that leads to the evo lution of new
populations. Stud- ies on genus Crepis by E. B. Babcock provided support to
Neo Darwinian theory.
At the same time, when evo lutionists were busy to seek plausible explanation
for evolution, some scientists were trying to accumulate facts about evo
lutionary process. The evidences are from morphology, physiology, taxonomy
and embryology of living forms and the palaeontology (the fossi ls of previously
existing forms) . The recent techniques have been helpful in demonstrating the
evolution taking place in the laboratory within short periods of only a few years.
If organisms with very short life cycles, such as fruit fly or bacteria are reared
for several generations in laboratory, new kinds of indivi duals are observed in
the progeny. Initially, these indiv idua ls differ slightly from their parents, but as
they increase in number, differences keep on accumu lating and a stage is reac
hed when these become so markedly different from their parents that they fail to
interbreed with their parents and thus form a new species.

66. A B
FIG. 1.3: A. Change in the frequenc y of a gene for green colour in beetle that
enables it to merge with leaf colour; B. Macroevolution .
1.4.5 Evolution at Different Scales
Biological evolution encompasses changes of vastly different scales:
I . Small-scale Evolution: It includes changes in the frequency of genes in a
population from one generation to the next. This is called microevolution. It
happens on a small scale and includes changes within a single population.
Change in the frequency of gene for dark wings in beetles from one generation
to the next is an example of microevolution.
2 . Large-scale Evolution: It includes the descent of different species from a
common ancestor over many generations. It is called macroevolution. It operates
above species level and leads to the evolution of lineages. Evolution and
radiation of dinosaur lineages, evolution of horse and evolution of man are
examples of macroevolution.
1.5 EVOLUTION-A FACT OR JUST A THEORY
Concept of evolution started as a means to explain the phenomenon of a
changing living world over a period and to interpret remarkable similarities in
the living forms of diverse groups . It has now beocme a well established fact
like Newton's law of gravitation, because of supportive observations by
comparative anatomists, field naturalists, geologists , palaeontologists,
geneticists and biochemists.
Occurrence of transitional living forms like Peripatus and a variety of fossils
support the view of gradual evolution of new forms from pre-existing old
organisation. Such transitional forms are intermediate between two groups
having characters of both the groups. For example:
• Fossil Archaeopteryx with its feathers, teeth, claws and lizard-like skeleton
shows transition between reptiles and birds.
• Fossil hominids from Africa with human-like dental arch, small brain, arms
longer than present humans but shorter than modern apes, with pelvis , feet and
legs for upright walking support the view that man and apes have arisen from
some common ancestor.
1.5.1 Major Unsolved Problems of Evolution

67. Despite years of research and accumulation of a lot of knowledge in various
fields of life sciences, some challanges to evolutionary science remain
unanswered. Some of these unsolved problems are:
I. Origi f Ufe: How did living matter originate from nonliving matter? Was it a
process that happened only once or many times? Can it happen today under
natural or artificial conditions? These are some of the questions which remain
unanswered.
Inspite of well documented theory of Biochemical Origin of Life by Oparin and
Haldane, evidences are lacking for support.
2. Origj f ex : The following questions regarding sexuality and sexual
reproduction remain unanswered:
Why is sexuality so widespread in nature?
• How did maleness and femaleness arise?
• If sexuality is so important for maintaining genetic variability, why and how
many microorganisms can do without it?
• How can we account for phenomenon like parthenogenesis?
3. Origin of Phyla: Can we trace any relationship between existing phyla and
those that existed in the past. Transitional forms between phy la are almost
unknown and fossil records if any are incomplete. Hence, disagreement still
exists about the number and relation of various present day phyla.
4 . Cause of Mass Extinction: Mass extinction has occurred many times on
Earth since life originated. However, there is no agreement on the cause(s) of
repeated extinction of live forms. Asteroids are considered to be one main reason
for extinction, but are far from proven as a cause for world-wide extinction.
Similarly, punctuated equilibrium theory is considered to account for the sudden
appearance of new groups and long persistence of others, it has raised many
questions about stability and extinction of species.
1.5.2 Importance of Evolution
Evolution is not just important for Biology, it is central to it, because all life on
Earth has been shaped by evolution's key principles i.e., natural selection and
common descent with modification.

68. 1. Theory of Evolution provides a means to understand :
• the comp lexity in living world
• the mechanism of evolution of resistance to antibiotics in bacteria
• the mechanism of evolutionary adaptations
2. Theory of Evo lution tells us that we human beings are not something
different from all other living things, but we are the products of same
evolutionary processes by which other organisms have come into existence.
3. Through evo lution we become aware that:
• Living forms have descended fom other varieties of living things.
• The organisms that populate the living world are not fixed entities, but are
constantly undergoing modifica tion.
KEY TERMS
• Catastrophism •
• Germplasm Theory •
• Microevolution •
• Orthogenesis •
• Preformation Theory •
• Theory of Uniformitarianism • Emboitement Theory
Inheritance of Acquired characters Modern Synthetic Theory
Palaeontology
Somatoplasm
Theory of Eternity of Life
• Epigenesis
• Macroevolution
• Mutation Theory
• PangenesisTheory
• Teleology
REVIEW QUESTIONS
I . Summarise essential feature s of basic concept of evolution.
2. What do you mean by small-scale and large-scale evolutionary changes?
3. Define evolution. Who introduced the term ' evolution'? How does biological
evolution differs from evolution?
4. Describe the contribution of the follow ing in the field of evolution: (a)

69. Aristotle (b) Anaximander
(c) Ernpedocles (d) Aristotle
5. Name the scienti st for the following :
(a) Father of Evolutionary idea (b) Greatest investigator of Antiquity (c) Father
of Palaeontology and Comparative anatomy
6. Write short notes on:
(a) Recapitulation theory
(c) Theory of Pangenesis
7. Explain the following:
(a) Monophylectic genealogy (c) Modem evolutionary theory (e) Importance of
transitional forms (b) Theory of Continuity of Germplasm (d) Uniformitarianism
(b) Inheritance of acquired characters (d) Theory of Orthogenesis
8. Give a brief account of major unsolved problems of evolution.
FURTHER READINGS
I . Bowler, Peter J., 1984. Evolution : The History of an Idea. Berkeley
University of California Press.
2. Dawkins, Richard, 1986. The Blind Watchmaker. New York: Norton .
3. Ehrlich, Paul and Anne, 1987. Extinction: The Causes and Consequences of
Disappearance of Species. New York: Scientific American Books.
4. Futuyma, Douglas J., 1986. Evolutionary Biology (2nd Ed.), Sinauer
Associates, Inc. Publishers Sunderland, Massachusetts.
5. Gould, Stephen Jay, 1977. Ever Since Darwin, New York: Norton.
DOD

70. 2
Evidences for Evolution
2.1 DOCTRINE OF BIOLOGICAL EVOLUTION
E arth is inhabited by approximately two million species of different kinds of
living beings. They range from tiny microbes to giant-sized trees, whales and
elephants. Biologists have always tried to seek an answer to the question of 'how
did this tremendous diversity of life come to exist on this planet?' Till the middle
of nineteenth century, it was believed that animals and plants have arisen
spontaneously by specia l creation, each species being formed separately.
Th e doctrine of biological evolutio n which is now taken for granted was based
on the basic similarities seen in all living beings, both in structural organisation
and life processes. It assumes that all living organisms have evolved from some
single common ancestral form through the process of gradual modifications,
adaptations and natural selection. Darwin has described this phenomenon as '
descent with modification.' The salient features of the Doctrine of Biological
Evolution are:
• Unicellular organisms were the first to appear.
• Multicellular organisms evolved later from these simple unicellular forms.
• Early forms were simple in structure and gradually evolved into more and more
complex forms.
• Seed plants (monocot and dicot plants) in Plant Kingdom and vertebrates in
Animal Kingdom are the last to evolve from simple seedless plants and
invertebrates respectively.
2.2 EVIDENCES FOR BIOLOGICAL EVOLUTION
Doctrine of biological evolution is supported by evidences drawn from the study
of different branches of biology. These include:
I. Evidences from Comparativ e Anatomy and Morphology (Tectology) 2.
Evidences from Vestigial Organs
18 [i] Evolutionary Biology
3. Evidences from Atavism and Reversion 4. Evidences from Comparative

71. Embryology 5. Evidences from Palaeontology (Study of fossils) 6. Evidences
from Geographical Distribution 7. Evidences from Connecting Links
8. Evidences from Taxonom y
9. Evidence s from Biochemi stry and Physiology
10 . Evidence s from Molecular Records
II . Evidences from Cyto logy
12. Evidences from Genetics
2.3 EVIDENCES FROM COMPARATIVE ANATOMY AND MORPHOLOGY
(TECTO LOGY)
Comparati ve study of morphology and anatomy of various organs and organ
systems reveals that both similarities as well as differences exist in the structure
of body organs due to the similarities and differences in their functions .
Presence of basic structural and functiona l similarities in the organ systems of
organisms indicates their common ancestry. It is illustrated by:
I . Homologous organs (Homology)
2. Analogo us organs (Analogy or Homoplasy)
3. Adaptive diverge nce (Adaptive radiation)
4. Adaptive convergence
O rganisms share a unity of plan when they are closely related because of
common descent. This is substantiated by comparative anatomy and comparative
morphology.
2.3.1 Homologous Organs and Structural Homology (Same Source)
Homolo gous organs are different in appearance and perform different functions
, but are built on the same basic pattern and have a common origin (same source)
. Therefore, homology is the similarity in the basic structure of organs of
different anima l groups based on common ancestry or origin from some
common structural patterns. The concept of homo logy was introduced by
Richard Owen.
EXAMPLE
Homology in Forelimbs: The forelimbs of pterodactyl , bird and 1:
bat are modified for flying, of dolphin, seal and whale are modified for
swimming, of sheep, dog and horse for running and forelim bs of man and shrew
for grasping. The functions of forelimbs in these anima ls are entirely different

72. and so also their external "appearance. But these are constructed on the same
pentadactyl pattern , consisting of the same bones (humerus, radius-ulna, carpals,
metacarpals and phalanges), muscles , nerves and blood vessels arranged on the
same pattern.
They follow th e same pattern of development. The homology can be explained
only on the basis that all of them have evolved from the common ancestor. The
differences in appearance are due to their adaptations to carry out different
function s.
Homology can also be traced in the structure of skull, brain , nerves, muscles,
heart and blood vessels of different vertebrates. (Fig. 2.1)
Dolphin
Bird
Horse
Bat Whale Dog Shrew Flying Swimming Running Grasping
FIG. 2.1: Homology in the structure of forelimbs in bird and different mammals.
Levels of Homology in Species
Biologists have recognised homology in species at three distinct levels: 1.
Genetic homology
2. Developmental homology
3. Structural homology
1. Genetic Homology
Genetic homology is most fundamental. It exists in:
• Similarity in the DNA sequences found in different species
• Existence of universal genetic code, the same 64 codons specify the same
amino acids from bacteria to man
• Similarity in the structure of plasma membrane
• Similarity in the mechanism of transcription and translation via same RNA
polymerase
• Use of ATP as energy currency
• Similarity in the mechanism of DNA replication via DNA polymerase
2 . Developmental Homology
• Developmenta l homology is seen in overall process of development, form of
the embryos of different groups and fate of particular embryon ic tissues or

73. organs. Developmental homologies are due to homologous genes .
3. Structural Homology
• Structural homologies are similarities in adult morphology in organisms of
different groups and are the results of homologous genes .
EXAMPLE 2: Homology in th e Structu re of Heart: The heart is two-chambered
in fishes, three-chambered in amphibians and some reptiles, and four-cha
mbered in reptiles, birds and mamma ls. In fishes, heart has one auricle and one
ventricle. The auricle receives only deoxygenated blood from the body and
ventricle sends this deoxygenated blood to gills for oxygenation. In amphibians,
right auric le receives deoxygenated blood from body and oxyge nated blood
from lungs and skin. It gets mixed up in the undivided ventricle. In higher
reptiles (Croco diles), Birds and Mammals, the oxyge nated and deoxygenated
blood are completely separate d in left and right chambers of heart.
We see that the structure of heart in different groups of vertebrates presents a
gradua l modification while the fundamental structure is the same in all the
groups. (Fig. 2.2)
Aortic arch
J Carotid artery
r:...
Systemic Ventral Pulmonary artery
aorta
Veins Pulmonary vein Bulbous From body
arteriosus
Left auricle
Ventricle
Auricle Truncus (Atrium) arteriosus Sinus
venosus

75. A B
Vena cav
Right auricle
Pulmonary vein
Left auricle Left auricle
Left ventricle
Left ventricle
Right ventricle
c o
FIG. 2.2 : Heart and blood vessels arising from it in the vertebrates series
showing homology in the ir structure and distribut ion in: A . Fish ; B .
Amphibian; C. Rep tile ; and D. Mammal.
Olfactory lobe
Cerebellum
Medulla oblongata
Fish Amphibia Reptile Bird Mammal
FIG. 2.3: Homology in the parts of brain in fish , amphib ian, reptile . bird and
mamma l.
EXAMPLE 3: Homology in Brain Structure: Ranging from Fishes to Mammals, the
brain consists of similar series of parts-the olfactory lobes, cerebral hemispheres,
optic lobes and cerebellum and medulla oblongata. As we progress through the
series from Fishes to Mammals, some lobes present gradual enlargement
(cerebral hemispheres). In Fishes, the cerebral hemispheres are even smaller than
the optic lobes, but in Mammals they are so much enlarged that they cover the
olfactory lobes in front and the optic lobes behind. (Fig. 2.3)
EXAMPLE 4: Homology in Insect Mouthparts: Mouthparts in insects also show
homology. In Cockroach, these are modified for biting and chewing, in
Mosquitoes for piercing, in Butterfly for sucking, in Housefly these are spongy
and adapted for absorbing liquid food. In all these cases, mouthparts represent
modification of the basic structure and have evolved from the same prototype.
Due to different feeding habits, some mouthparts are lost and some have become
elongated and needle-shaped for piercing the host skin and sucking the blood
(Mosquitoes and Bedbugs).
EXAMPLE 5: Homology in Insect Legs: Legs in Mole cricket, Grasshopper, Honey
bee, Mantids and Water beetles are specialised for digging, jumping, collecting
pollen, catching prey and swimming respectively, but in all these cases the legs
are formed of similar five podomeres.

76. EXAMPLE
6: Homology in Plant Parts (Leaf Modifications): Thoms of
Bougainvillea (Garden Glory) and tendrils of Passiflora (Passion flower), or
Cucurbita are homologous structures. Both are modified shoots and are located
in the axil of leaves. These axillary branches in Bougainvillea are modified into
thorns for protection from browsing animals and in Passiflora and Cucurbita ,
they are modified into tendrils and help the plant in climbing. (Fig. 2.4)

78. Tendril
Boug ainvillea Passion flower
FIG. 2.4 Homology in thorns of Bougainvillea and
tendrils of Passion flower.
S imilarly, ph yllode or phylloclad e ofOpuntia and c1adode ofRuscus or
Asparagus are also homologous organs where stem is modified for carrying out
photosynthesis. These have different appearance but are modifications of stem.
(Fig. 2.5)

80. A B c
FIG. 2.5: Homologous structures: A. Phylloclade of Opuntia; B. Cladode of
Ruscus ; C. Cladode of Aspa ragus.
2.3.1 .1 Types of Homology
Homology is of following four types:
• Phylogenetic hom ology is similarity among animals or among plants of
different species. Homology in the hand of man and forelimbs of horse and bat is
an example of phylogenetic homology.
• Sexua l homology is paralleli sm in the male and female reproductive organs of
the same species.
• Serial homology has been observed among invertebrates . All arthropods have
segmented body with an exoskeleton of chitin. The exoskeleton is constructed on
the same basic pattern in all the classes of Phylum Arthropoda. In crustaceans,
all the segments of body carry paired jointed appendages. All of them are
constructed on a common structural plan, consisting of a basal two-segmented
portion, the protopodite (coxa and basis) which bears two lateral outgrowths , the
exopodite and endopodite. The appendages of various body segments perform
different functions and accordingly exhibit modifications in the basic structural
plan. The phenomenon of similarity is described as serial homology. (Fig. 2.6)
• Molecular homology is similarity in the biomolecules , such as DNA, the
genetic material, found from viruses to man.
Antennule--1
Antenna 2 o
CD
-g. Mandible---3 !!!.
0 '
Maxillula---4 Maxilla ---5
Chelate legs< 9
10
..::::::'11
Non-chelate "" 12
legs 13

81. :to Pleopods or /~ :
g: 1swimmerets\16
~ 17
S'
!!!.
18
Uropod---19

83. FIG. 2.6: Appendages of Palaemon showing serial homology, Though appear
drastically different in appearance they have same parts-protopodite, exopodite
and endopodite which are modified to carry out different functions .
Significa nce of Homology or Homologous Organs
Presence of homologous organs in different groups confirms :
• common ancestry and inter-relationship among different groups
• occurrence of divergent evolution , i.e., the ancestral individuals of the
same group when migrate to different habitats, their organs undergo adaptive
modifications in different environments and become different
2.3.2 Analogous Organs and Analogy or Homoplasy ('Same form')
A nalogous or homoplasious organs have almost similar appearance and perform
the same function but they develop independently in totally different groups
through parallel evolution and are not inherited from a common ancestor.
Therefore, analogy is the superficial similarity in appearance between organs of
different animal groups because the y carry out the same function. (Fig. 2.7)
2.3.2.1 Analogy in Animals
EXAMPLE 1: Ana logy in Win gs: Wings of an insect (Dragonfly), bird (Eagle),
mammal (Bat) and reptile (Pterodactyle) perform the same function of uplifting
the body in the air, but their basic structure is totally different. The wings of an
insect are mere expansio ns of bodywall without any skeletal support . They are
mere flaps of chitin, stiffened by a series of ' veins' . In Pterodactyle, each wing
is an enormous fold of skin supported by enlarged fourth finger of the forelimb.
In bird, the flight surface is formed by feathers attached to the bones of forelimb.
In bat, the wing is formed of a fold of skin and is called pat agium. It is
supported with elongated and outspread phalanges of last four digits (2nd, 3rd,
4th and 5th digits). (Fig. 2.7)
EXAMPLE
2: Analogy in Body Shape: A fish, an Ichthyosaur and a whale have
stream-lined body and are adapted for aquatic existence . But these belong to
three different classes of vertebrates with no traces of common ancestry. (Fig.
2.8)
Phalanges

84. Metacarpals Carpals Radius and ulna
A B FIG. 2.7: Analogous org ans : A. Wings of insect; B. Wing of bird

86. Metacarpals
Carpals1!:~~"'$;i.............J
Radius Skinand ulna
(Patagium)
(fused)
Humerus C
FIG. 2.7: C. Wing of mamma l (bat); D. Wing of an extinct reptile (pterodactyl).
A.lchthyosaur B.Eel

88. C.Fish
FIG. 2.8: Analogy and adaptive convergence in:
A. Ichthyosaur; B. Eel; C. Fish; D. Whale.
EXAMPLE 3: Analogy in Fins of Fish and Flippe rs of W hale: Fins of fish and
flippers of whale are completely unrelated structures, but have similar
appearance and perform the same function to help in swimming. Their structural
details are totally different. (Fig. 2.9)
2.3.2.2 Analogous Structures in Plants
EXAMPLE 1: Potato and Sweet potato have similar tuberous appearance due to
storage of food, but Potato is stem and Sweet potato is root.
EXAMPLE 2: Cladode of Ruscus or Asparagus are analogous to leaves of other
plants. Cladode looks like leaves and carries out photosynthesis like leaves but is
modified stem.
E. AMPI. . 3: Tendrils help in climbing, but they have different origins. They are
modified stipules in Smilax, petiole in Clematis, leaflets in Pea, whole leaf in
Wild pea and axillary buds in Passiflora.
Analogous organs are developed in the evolutionary process through adaptations
of distantly related organisms to the same mode of life.
Pterygiophores
Phalanges
Fin rays
A. Fin of a shark B. Flipper of a whale
FIG. 2.9: Analogy in the fin of A. Shark; B. Flipper of whale.

90. B c D EA
F IG. 2.10: Analogous tendrils developing from different parts in plants: A. Stem
tendril (from axillary bud) in Passiflora , B. Leaf tendril (from leaf) in Lathyrus;
C. Petiole tendril (from petiole of leaf) in Clematis ; D. Stipular tendril (stipule
developed into tendril); E. Leaflet tendril (Apical leaflets modified into tendril in
Pisum).
A 1PI. 4. Spines have different origin, but carry out the same function of
protection against browsers and dessication. Spines in Opuntia and Berberis are
modified leaves but in Flacourtia the stem and its branches are modified into
spines. (Fig. 2.11)
Stem thorns
Leaf Leaf
spines
spine
\ --...-ItI1"t
A B c
FIG. 2.11 : Analogous organs in plants: A. Spines of Opuntia (modified leaves);
B. Spines

91. of Berberis (modified leaves); C. Branched thorns of
Flacourtia cataphracta (modified stem).
..-it.'i-i1• .I.
Differences between Homologous and Analogous
Organs
Homologous Organs
1 . They have the same basic structural plan.
2. They are found in closely related organ isms which arise from some common
ancestor.
3. They differ in appearance.
4. They are modified to carry out different functions.
5. They have different inte rnal structure .
6. They lead to adaptive divergence or divergent evolution.
Analogous Organs
1 . They have totally different structural plan.
2. They are found in totally unrelated organisms.
3 . They have s imilar appearance.
4. They develop to carry out the same function.
5. They have similar inte rnal structure .
6. They lead to convergent evolution or adaptive convergence.
2.3.3 Adaptive Divergence and Adaptive Convergence
Stud y of evolution of different plant and animal groups rev ea ls the phenom
enon of adaptive divergence and adaptive convergence influenc ed by the env
ironme ntal or habitat differences or similarities .
• Org ani sm s of the same or close ly relat ed groups when occupy different hab
itats, they assum e different appearanc es and their homologou s structures
exhibit grea t divergenc e in the form and function . Thi s is called adaptive
divergence. Adapti ve divergence leads to adaptive radiation. It is evo lution in
several spec ialised directions from a co mmon genera lised ancestral form .

92. • Organi sms of distantly related or totally unr elated groups are found to develop
similar adaptations, while living in the same or sim ilar hab itat. Their ana logous
struc tures, though apparently similar are constructed on different basic plan s.
Thi s is ca lled adaptive convergence or convergent evolution. Both ada ptive
divergence and adapti ve conve rge nce provide strong evide nce in support of
evo lution and presence of great biologic al diversity.
2.4 EVIDENCES FROM VESTIGIAL ORGANS
Th e vestigial or rudimentary organs are the useless remnants of structures or
organs which were prominent and functional in ancestors. The se are often
undersized, degenerated and nonfunctional.
2.4.1 Vestigial Organs in Man
Man alone possesses nearly 100 vestigial structures.
I . Vermiform appendix in man is the remnant of caec um which is large and
functional in herbivorou s mammals. It contai ns bacteria that produ ce enzyme
cellulase for the diges tion of cellulo se.
T he presence of nonfunctional appendix in man indicates that ancestors of man
(the early primates) had a much coarser diet, feedin g on resistant vegetable
matter. But the descendants changed food hab its, the caecum and appendix
being no longer useful and gradually reduced.
2 . Auricular muscles of external ear are used in many mamm als for collecting
sound waves from the surroundings. Comp lete set of muscles for their
movements is present in the external ear of man but these muscles are
nonfunctional.
3 . Nictitating membrane or Plica semilunaris is the third eyelid in the inner
angle of each eye in man and many mammals. It corresponds to the nictitating
membrane but it is compl etely unstretchable and nonfunctional.
4 . Vestigial Tail Vertebrae: Early embryo of man possesses an external tail but it
is shed off much before the adulthood is attained. Rarely, a child may be born
with a short visible tail. In adults the tail is represented by a string of caudal
vertebrae, which constitute the coccyx (tail bone). (F ig. 2.12)
5 . Lobe of the external ear is of no practical benefit to man , although serve d

93. the purpose of sound gathering in the ancestors of man.
6. Wisdom teeth are the third pair of molars. They are vestigial. These are last to
erupt or even fail to erupt.
7. Canines in man are reduce d due to taking soft food and noncarnivorous habit.
8. Mammary glands in males are rudimentary.
9. Bod y hair in human s are of no use and are vestigial remain s.
2.4.2 Vestigial Organs in Other Animals
Not only man but almost all the plants and animals possess vestigial organs. A
few of them are cited below:
I. Vestiges of Hindlimbs and Pel vic: Both Whale and Python have vestiges of
bones of hindlim bs and pelvic girdle embedded in the flesh of abdomen. This
shows that both of them have evolved from ance stors which had functional
hindlimb s. In Snakes these have disappeared becaus e of burrowing habit and in
Whales they are lost because of aquatic habit.
Nipples of mammary glands in
man
B
Auricular or ear
muscles~iiili.i:ii\
Nictitating
membrane
or plica
semilunaris
C
Segmental muscles of abdomen
Pyramidal
--l-:HI. Ileum muscles
Rud imentary tail E F G FIG. 2.12: Some vestigial organs in human body : A.
Mammary glands , chest hairs and coccyx ; B. Nictitating membrane or plica
semilunaris; C. Auricular or ear muscles; D. Wisdom tooth

94. and pointed canine ; E. Rudimentary tail ; F. Abdominal muscles; G. Caecum
and append ix.
2. Vestigial Wings: Flightless birds (Kiwi of New Zealand and Ostrich of Africa)
possess vestiges of wings supported by tiny replicas of usual bones of a bird 's
wing.
3 . Splint Bones: In Horse leg, the splint bones represent the metacarpals of 2nd
and 4th digits.
4. In animals living permanently in deep caves, the eyes are rudim entary or
vestigial.
Ribs Flightless or vestigial wings
Pelvic
Vestigial pelvic
Splint girdle
girdle and
hindlimb Parts of girdle bone
Femur
A B C o E
FIG. 2.13: Vestigial organs in different animals: A. Vestigial pelvic girdle and
bones of hindlimb in Python ; Band C. Vestiges of pelv ic girdle in Whale ; D.
Vestigial wings in flightless bird, Ostrich ; E. Splint bones in the leg of Horse.
2.4.3 Vestigial Organs in Plants
There are vestigia l structures in plants also. For exa mple:
1 . Cutin-covered stomata are present on the stem s of cacti plants.
2. In plants related to Prickly pear, the leaves are functional, but in Ruscus and
Prickly pear and in underground stems , the leaves are scale-like and vestigial.
3. Rudimentary stam ens on some Aspa ragus plants and nonfunctional pistils on
others, actually represent the vestiges of ancestral monoecious Asparagus plants.
4. Cycad sperm that are passively transported to the egg cells have nonfunctional
flagella .
Significance of Vestigial Organs
The occurrence of vestigial structures in present day forms indicates that these
structures were fully developed and functional in the ancestors from which these
present day forms have evolved. Due to change in habit, these structures were
not needed by the ancestors and have gradually reduced to vestiges . The wide-
spread occurrence of vestigial organs provides evidence for the occurrence of
organic evolution.

95. 2.5 EVIDENCES FROM ATAVISM OR REVERSION
Ata vism or reversion is the reappearance of those ancestral characteristics in an
organi sm or in the organisms of a group , which do not occur normally or which
represent the reminiscent of normal structures possessed by the individuals of
othe r groups.
Such abnormal structu res are known as atavistic cha r acte rs . There are several
examples of reversion or atavism in man and other animals. In such cases
abnormal characters or structures appear in the embryo or in the adu lt, which
were not present either in the parents or grandparents but were found in some
remote ancestors.
2.5.1 Examples of Atavism
Atavism, of course not very common, is well illustrated by numerous examples:
1. Cervical Fistula in Man: In fishes there are five pharyngeal pouches which
open to the exterior. In man, in the norma l course , only one pharyngeal pouch
perforates to form an opening from the pharynx to the exterior in the form of
external ear canal and eustachian tube . But rarely, the neck may possess an
additional opening through which throat or nasal cavity communicates with the
exterior. This represents the opening of an additional pharyngeal pouch to the
exterior and is known as cervical fistula.
2. Tail: Tail is absent in man, but occasionally a human baby is born with a short
fleshy tail. It is devoid of vertebrae and is removed by surgeon with no trouble.
3. M amma r y Glands : Humans posses s j ust one pair of mammary glands in
the pectoral region. The same condition is noted in all the primates but in pigs ,
these occur in two rows one along either side of the chest and abdomen.
Sometimes, extra-mammary glands or their nipples appea r in man .
Human tail
Cervical fistula
resulting from./.-persistent
~
pharyngeal slit
A B

96. 4 . Hair on the Body and Face in Irish Dogman: Man is characterised by scanty
hair on the body and no hair on face but in the relatives of man (apes) hair are
present profusely. A man was born in Russia with profuse development of hair
on the face and body (Irish dogman).
Pointed canine tooth

97. Axillary nippleJiJE
Thoracic nipple
Abdominal nipple
Rudimentary Thick hairs
gill slits on body
C o F
FIG. 2.14: Atavism in the human body: A. Tail in a human baby; B. Cervical
fistula; C. Rudimentary gill slits; D. Mammary glands in different regions of the
body; E. Pointed canine; F. Hair on the body and face in Irish dogman.
5 . Disappearance of Phalanges
of 2nd and 4th Digits in Horse:
Carpals
In modem horse the third digit is
Tarsals-~~..,
most prominent and its metacarpal
bears a hoof, whereas the 2nd
and 4th digit s are absent and
their metacarpals are represented
as splint bone s. Occasionall y
a hor se possesses one of the Metacarpal-3

98. Splint bones---I
t wo splint bones (2nd and 4th)
Metatarsal-3
with phalanges and rarely with
a reduced hoof. This represents
the ancestral character since the
prehistoric horses possessed three
......--- Hoof
--E. .
toes in each foot (Fig. 2.15).
6. Homodont Dentition in
Hindlimb Forelimb
Piscivorous Cetaceans: Cetaceans FIG.
2.15: Splint bones, phalanges and hoof in the are mamm als, charactersied by
forelimb and hindlimb of horse. heterodont dentition. But piscivorous cetaceans
possess simple, conical homodont teeth somewhat similar to predaceous reptiles
or some fishes. Their ancestors are known to possess heterodont teeth of
mammal s. This could be considered to be a true reversal of evolution . (Fig.
2.16).
2.5.2 Types of Atavism
Atavism can be categorised into Famil y atavism, Race atav ism and Atavism
teratology.
I. Family Atavism: It includes sudden rea ppearance of a charact er or characters
in the offspring afte r remain ing lat ent in the fam ily for seve ral generations.
Th is phenom enon is controlled at gene level and can be explained by simple
Mendelian laws of inheritance. For example, app earance of red hair in a child,
whose pa rents and grand- parent s all possess bla ck hair, but the red hair were
seen in some members of the family several generations ago. It mean s the gene
or genes contro lling red hair character could not gain expression for several
generations due to some specific reason and were transmitted unnoticed or in the
latent condition but appeared sud denly.
2 . Race Atavism: It includes those cases of reversal where one or more charac
ters of one race appear in the indi viduals of another race. For example, the
profuse growth of hair on the bod y and face of Irish dogman and the presence of
additional mammary glands or their rudiments in man are race atavistic charac
ters.
3. Atavism of Teratology: Th is includes the appea rance in a race of such
abnorma l characters which were norma l in other supposedly ances tral races.

99. The appea rance of cervical fistula in man, which actuall y corresponds to the
gill-slit, or the appearance of externa l hindlimbs in a humpback whale or the
homodont dentition in piscivorous cetaceans are examples of teratolo gy.
T hese changes co uld not be account ed by the changes in the ge ne pool s of the
populations but can be explained by the change in developmental field s. There
are developmental fields for each and every structure in the developing embryo.
For example, in mammals there are distin ct deve lopmental field s for incisors ,
can ines, molars and premolars. Some physical factors such as temperature, pH ,
or chemica l factors may suppress or change the development and differenti
ation introducing some change in the teeth struc tures . It means the environment
can directly affect the development of a tra it.
T he principle of reversibility was advocated by L. 0 0110 in 1893 and is now
known as DoUo's law. It states that living organisms do exhibit evolutionary
irreversibility (the reappearance of ancestral characteristics). The law has no
exceptions and is rather a generalisation.
A B
F IG. 2.16: The evolution of homodont dentition in piscivorous cetaceans from
heterodont ancestors is an example of atavism: A. Skull of a primitive
creodont carnivore; B. Skull of Zeuglodon. an Eocene whale; C. Skull of a
modern porpoise.
2.6 EVIDENCES FROM COMPARATIVE EMBRYOLOGY
Early in the nineteenth century, Von Baer had not iced remark able similarity
among vertebrate embryos, whose adults are markedly different. Darwin and
others concluded that early developmental stages are more conservative or
evolutionari ly stable than
late stages or the adults . Ernst Haeckel (1834-1919) was impressed by the
generalised pattern of development and the general resemblances between the
embryos of different groups of animals . Haecke l formu lated the ' Rec
apitulation Th eory' or ' Bioge netic Law'. It says "Ontogeny recapitulates
phylogeny". Ontogen y is the life history of an individual starting from ovum and
phylogen y is the evolutionary history of the group. It includes sequence of adult
ancestors which must have incurred during the evolution of the group of this
individual. It means an individual during its development repeats the most

100. important changes which its ancestors have undergone during the long course of
their evolution.
Haeckel meant that early stages of deve lopment recapitulate the adult ancestra l
forms . The homologies were traced at the following levels:
I. Similarities in early embryonic deve lopment of animals
2. Resemb lance in the vertebrate embryos
3. Resemb lance in invertebrate larvae
4. Temporary embryo nic structures
5. Development of vertebrate organs
6. Retrogressive metamorphosis
7. Recapitulation Theory and Biogenetic Law
2.6.1 Similarities in the Early Development of Animals
The earl y developmental stage s of all the multicellular animals are similar. All
start their life from a fertilised egg called zygote . It undergoes repeated
cleavages and develop s into morula, blastula and gas t r ula. In gastrula, three
germinal layers, i.e., ectoderm, mesoderm and endode r m, are formed . These
germinal layers give rise to the same types of parts in all the animals. Later,
development in different groups diverges. The nearer the relationship in the
adults, the greater is the similarity in their development. This support s the
common ancestry of all animal s. (Fig . 2.17)
Blastocoel

102. Blastopore Gastrula
Zygote
Eight-cell stage Blastocoel
Endod
erm +-Ectoderm
x ·
Blastula Cross section (hollow ball) of blastula
FIG. 2.17: Early embryonic stages (up to gastrula stage) in the development of a
multi cellul ar organism.
A B c D E F G H

104. FIG. 2.18: Similarity in the early embryos of some verteb rates: A. Fish; B.
Salamander ; C. Tortoise; D. Pigeon ; E. Pig; F. Cat; G. Rabbit; and H. Man
2.6.2 Resemblance among Vertebrate Embryos
T he embryos of Fish, Salamander, Tortoise, Pigeon, pig, Cat, Rabbit and Man
during early stages of development resemble each other so closely that it is
difficult to distinguish them from each other. They all possess:
• Similar head with rudiments of eyes and ears
• Pharyngea l clefts or gill clefts, notochord and embryonic tail
• Limbs which develop as limb buds
• The notochord which is replaced by vertebral column in all the vertebrate
embryos
Th e similarity in embryos of divergent forms of vertebrates indicates their
common ancestry, and the degree of similarity in embryos indicates the degree of
evolutionary relationship of adults.
2.6.3 Resemblance in Invertebrate Larv ae
Pr esence of trochophore larva in annelids and molluscs indicates their ongin
from the same ancestor. Similarity in the bipinnaria larva of Echinodermata and
tornaria larv a of Hemichordata suggests that both echinoderms and
hemichordates have evolved from the same common ancestor.
2.6.4 Temporary Embryonic Structures
The embryos of certain animals develop some temporary nonfunctional
structures which disappear before hatching or birth. For example:
I . Visceral pouches or gill clefts
develop in the embryos of all
the land vertebrates, but are not
present in the adult . Gill slits are Visceral useful in fishes because they
live
pouches
in water but are of no use for land
Dorsalvertebrates, still they develop in hollow
the embryo. nerve cord 2. The embryos of all vertebrates
Notochorddevelop notochord
which is
replaced by vertebral column in Postanal---\"
tail

105. adults.
3 . Tooth buds develop in the embryos of toothless whale and birds, though
adults do not have teeth.
FIG. 2.19: Chordate characters that appear in all the chordate embryos.
Development of nonfunctional structures in the life history of a bird and whale
suggests that:
• Birds and Whale have evolved from toothed ancestors, and
• Their embryos repeat for a short period the ancestral character, i.e., the
presence of teeth.
2.6.5 Development of Vertebrate Organs
The de velopment of various organs like kidneys, gonoducts, gonads, heart ,
aortic arches, brain, ear, etc. , in the embryos of all the vertebrates follows the
same basic plan and indicates a common ancestry. For example, the highly
developed four-chambered heart of birds and mammals develops as a two-
chambered tube similar to heart of fishes. It becomes three-chambered like
amphibian heart and later on four-chambered heart . Such a basic plan of
development of different organs in all the groups of vertebrates supports their
common ancestry.
2.6.6 Retrogressive Metamorphosis
Adults of certain animals have degenerated features and do not show any
resemblance with other animals of their group or any other group. But, their
larvae have helped in establishing their phylogenetic relationship. For example:
• Sacculina is a parasitic crustacean that lives as ectoparasite on crab 's abdomen.
The adult has a sac-like body but its larva resembles nauplius larva. During
metamorphosis, larva undergoes degenerative metamorphosis and loses
appendages, gills, mouth , alimentary canal , sense organ s, etc. The taxonomic
position of Sacculina was established on the basis of its larva. (F ig. 2.20)

107. 36
Iil Evo lutionary Biology
Compound eye Antennule

109. •
•
•
Antenna
Root like processes of SaccuJina
Bo dy
of
Crab
FIG. 2.20: Saccu lina . The paras itic crustacean show ing dege nerated structure.
Herdmani a is an ascidian. Its chordate nature has been established from its
larva, which is free swimming and possesses all the three chordate characters.
During metamorphosis larva loses all the chordate characters and changes into
the adu lt ascidian. The adult has a purse-like body and no chordate features.
(Figs 2.21A and B)
Neoteny: In some animals (e.g., axo lotl larva of Ambystoma) the larva fails to
undergo metamorphosis. It develops gonads, attains sexual maturity and starts
reproduc tion. This is called neoten y or paedogenesis.
Retention of primit ive or larval features by adults provides evidence in favour
of evolu tion. Under specifically favoura ble circumstances natural selection
favours retention of primiti ve or larval characters.
I ncurrent
siphon
to mouth
Dorsal, hollow nerve cord
Excurrent Atrium siphon
P harynx /' with
numerous
slits
Anus Tunic
-
+-
\...::----:::

110. -.::
Intestine Oesophagus Stomach
FIG. 2.21 : A . An ascidia n adult without basic chordate features; B. Larv a of
Herdmania showing chordate features.
2.6.7 Recapitulation Theory and Biogenetic Law
The recapitulation theory
renamed Biogenetic law
was proposed by Von Baer (1828). It was revised and by Ernst Haeckel (1868).
According to recapitulation
theory, every organism during its development repeats or recapitulates in an
abbreviated form the evolutionary history of its race . In Haeckel 's words,
Ontogeny recapitulates phylogeny. Ontogeny is the developmental history of an
individual, while phylogeny is its ancestral history or the history of the race. An
organism repeats its ancestral history during its development which can be
illustrated by the human development as follows :
The fertilised egg may be compared to the single-celled ancestor of all the
animals and the blastula to a colonial protozoan, which might have been the
ancestor of all the Metazoa. Gastrula (two-layered cup-shaped mass of cells)
represents the Coelenterate ancestor and the embryo with the development of
mesoderm represents triploblastic stage as in flatworms.
The early human embryo with a dorsal hollow nerve cord, a well developed
notochord and a series of pharyngal clefts represents the fundamental chordate
characters. With the development of a piscine heart, paired arotic arches,
primitive pronephros and a tail, it resembles a fish embryo. Later on, it
resembles reptilian embryo, and finally develops mammalian characteristics.
During the seventh month of intrauterine development human embryo resembles
a baby ape, being completely covered with hair and having proportionately
longer forelimbs. It means embryonic development (ontogeny) in man
recapitulates the history of the race (phylogeny). This provides support to
recapitulation theory. Other examples that support recapitulation theory are:
• Larva of Herdmania (i.e., Ascidian tadpole) has chordate characters, but adult
Herdmania is without notochord, nerve cord and tail.
• Development of Frog includes tadpole larva which is aquatic and has fish-like

111. characters like gills , gill slits, tail with a tail fin and lateral line sense organs.
This suggests that Frog (i.e., amphibians) has evolved from some fish-like
ancestor.
• Gymnosperms are not dependent on water for fertilisation, but flagellated
sperm
and water dependency for fertilisation is found in primitive gymnosperms like
Cycas and Gingko. This shows phylogenetic relationship between Gymnosperms
and Pteridophytes.
• Presence of filamentous protonema during development of Moss and Fern (that
resembles the filamentous green algae) suggests algal ancestry of Bryophytes
and Pteridophytes.
• Oak trees from Southern United States retain their leaves throughout the year,
whereas oaks from Northern United States are deciduous and shed their leaves
during winter.
Von Baer's principle of embryonic differentiation constitutes a better guide to
embryological evidence for evolution. According to this principle:
• General characteristics appear in the development early and specialised
characters later on.

113. 38 ~ Evolutionary Biology
1 . Recapitulation is seen in invertebrate animals also. For example: Adult
insects have three pairs of walking legs but in embryo. each body segment
carries one pair of primordia of legs. This shows that the insects have evolved
from an ancestor having segmentally arranged appendages.
2 . Biochemical recapitulation is also found in organisms. For example : (a)
Fishes excrete ammonia.
(b) Adult frogs and other amphibians excrete urea while tadpoles excrete
ammonia as in fishes .
(c) Birds excrete uric acid but embryos excrete ammonia first and urea later
on.
• From the more general, the less general and finally the specialised characters
appear.
• An animal during development departs progressively from the form of other
animals.
• Young stages of an animal do not resemble with the adults of different groups
but they resemble with their embryos.
2.7 EVIDENCES FROM PALAEONTOLOGY OR PALAEOBIOLOGY
Palaeontology is the study of fossil remains of p lants and animals that lived in
the past. Fossils (Latin; Jossilum = something dug out) are actual remains ,
traces or impressions left by the organisms that lived in the past and got
preserved in the sedimentary rocks. These include bones, teeth, shells and other
hard parts of animals or impressions of plants pressed into shale or insects
trapped in tree resin. Over the last two centuries, palaeonto logists have studied
fossils in Earth's different strata all over the world and have pieced together the
story of past life. The chronological sequence of fossils in the rock strata
illustrates the sequence of evolutionary events and has helped in building the
broad historical sequence of biological evolution.
Study of plant fossils is called palaeobotan y and of animal fossils
palaeozoology. Leonardo de Vinci (1452-1519) of Italy is called the 'Fathe r of
Palaeontology' and Cuvier (1800) the 'Founder of Modern Palaeontolo gy'.
Fossil records provide the most direct evidence of evolution, whereas all other

114. evidences are indirect.
2.7.1 Formation of Fossils
Fossils are formed in different ways based on the environmental conditions . The
fossils may include origina l remains of the hard parts (bones, teeth, shell, etc.)
in the sedimentary rocks, petrifaction of hard and soft parts, carbonised films,
molds (impressions of organisms in rocks), casts (molds filled with foreign
material) and as actual remains in peat, amber, asphalt and ice.

116. The land anima ls may also get fossilise d in amber (hardened resin) , asphalt
(hardened tar), volcanic ash, peat bogs and sand deposits or in ice.
2.7.2 Types of Fossils
Fossils are of the following types:
I. Unaltered Remains of Entire Organisms: Under exceptionally favourable
conditions, the entire anima l body gets preserved in ice, petroleum spring,
asphalt , resin, amber and oil-soaked ground . Woolly mammoths from Siberia in
Arctic Tundra remain ed preserved in ice for thousands of years. Actually, this
area is described as ' nature's cold storage or warehouse'.
2 . Petrified Fossils (Altered Fossils): Petrified fossils are formed by the
replacement of organic parts of dead and decaying organisms molecule by
molecu le by mineral s. The process is called petrification. Petrified fossils are
formed in the sedimentary rocks on the bottom of lakes, rivers or sea when
animals or plants or their parts get buried in the sediment. The process of
petrification successfully preserv es the hard parts. Under very favourab le
conditions, even the finest details of soft tissues, like muscles or other organs are
also preserved by the replacement of their organic material with mineral s.
• Amber is hardened resin. A number of insects and arthropods are found
preserved in the amber deposits of Oligocene Epoch from middle Tertiary Period
along Baltic Coast.
• Asphalt found in the tar pits of Rancho La Bera in Los Angeles (California) has
preserved a number of birds and mammals.
• Oil soaked ground in Poland has remains of complete Woolly Rhinoceros.

117. _.

119. ..... .._...........~~~..
"C,.o,
A. Surface erosion B. Sedimentation and C. Fossils in and sedimentation
sedimentary rock sedimentary rock
FIG. 2.22 : Formation and exposure of fossils in sedimentary rocks .
Pseudomorphs and Pseudofossils
Pseudomorphs are the casts of bodies of individuals that lived in the past. These
are formed when remains of living beings embedded in sedimentary rocks are
completely dissolved by infiltering water and the space so created is secondarily
filled with crystals forming their casts.
Pseudofossils are not fossils . Sometimes igneous rocks formed of minerals
develop crevices . Their mineral substances crystallise and develop into patterns
that resemble outlines of plants , their leaves, etc. Such rocks which are actually
without plant remains but appear so, are called pseudofossils.
3 . Molds and Casts: Natural molds are formed by the hardening of material that
surrounds the buried organisms. Their bodies disinteg rate leaving hollow
cavities, called molds. These molds retain the exact body shape of the organisms
preserved. They get filled with natural deposits of minerals which harden to
form exact cast of the original organism. Both molds and casts provide external
details of the body's shape, size and form of the organisms. Fossils of men and
their domestic animals of Pompeii city which got buried by volcanic eruption of
Mount Vesuvius in AD 79 were preserved as molds and casts .
Microfossils as Fuel Indicators
Fossils of spores and pollens (polynofossils) and of other vegetal remains of the
past are used as indicators of environmental conditions whether these were
favourable for the accumulation of organic matter and its conversion to fossil
fuels by the transformation and subsequent thermal alteration. By quantitative
analysis of microfossils , the approximate location and configuration of near
shore marine deposits is determined. This provides information about the sites of
formation and accumulation of hydrocarbons and coal.
Fossil fuels like coal, gas and petroleum are formed from remains of
phytoplankton, marine and terrestrial algae and lipid-rich plants.
4 . Prints or Impressions: The foot prints of anima ls or impressions of leaves,
stems, skin and wings, etc., left in soft mud are preserved when it changes into a
rock.
5. Tracks and Frails: The foot prints or tracks and trails of moving animals left in

120. the soft mud are preserved when the soft mud hardens into rocks, preserving the
prints. The foot prints and trail fossils are called fossils.
6. Coprolites: These are fossils of faecal matter or droppings. These are found in
association with the anima l fossils. Their study may provide information
pertaining to their food habits .
2.7.3 Determination of Age of Fossils or-Dating of Fossils Paleontologists use
following methods to determine the age of fossils or rocks that contain fossils:
I. Stratigraphy: Stratigraphy provides sequential arrangement of fossils in the
rocks from which relative age of fossils can be determined. It shows that the
lower

122. strata of rocks contain the oldest fossils and uppermost strata contain the recently
formed fossils.
2. Radiometric Dating: It relies on half-life decay of radioactive elements to
allow scientists to date the rocks and the fossils contained there in directly. This
is also called radioactive clock method or radioactive dating method.
The method of radioactive dating was introduced by Boltwood in 1907. It is
based on degradation of radioactive nuclei into stable nonradioactive element by
loosing electrons. Each radioactive element has its half life which means one
gram of a radioactive element changes into half a gram in a specific time. The
half life of each radioactive element is fixed. For example, 238uranium changes
to 206lead (2
06
Pb)
in 4.5
billion years and radioactive carbon J4C changes to radio-isotope of carbon
J2 C in 5,579 or 56 x 10
3
years .
• Uranium-Lead Method: In this method, the amount of 238uranium and
2061ead in a given rock is estimated accurately and the age of the rock is
calculated on the basis of half life. This method can be used only to determine
the age of very old rocks and fossils because of very long half life of uranium
(4.5 billion years).
• Radioactive Carbon Method: Radioactive carbon CC) method of determining
age of fossils was suggested by W.F. Libby (1950). J4C is a radio-isotope of
carbon J2c. Its half life is about 5,600 years. When bones are formed, small
amount of 14C is incorporated and its amount remains constant throughout the
life of an organism. Upon death the radioactivity is gradually lost. By
determining the amount of radioactivity in the bones, it is possible to
approximate
the time of death or fossilisation. Since the half life of J4C is small, radioactive
carbon dating method can give the age of fairly recent fossils (about 11,000
years to recent) .
• Potassium-Argon Method: This method is recently used to determine the age of
the earliest known hominoid fossils from East Africa. The half life of radioactive
potassium is 11 .6 x 10
9
years .

123. 2.7.4 Fossil Parks in India
Fossil park is a large area of fossil bearing rocks dug out and preserved to show
fossils in rocks . In India, the fossil parks are:
• Birbal Sahani Institute of Palaeobotany, Lucknow
• About 50 million years old Fossil Forest at Mandla District, Madhya Pradesh
• About 260 million years old Coal-forming Forest in Odisha
• About 100 million years old fossil forest in Raj Mahal Hills in Bihar
• National Fossil Park, Tiruvakkarai in South Arcot District of Tamil Nadu
2.7.5 Geological Time Scale
By studying the types of fossils in different rock strata and determining their age
by radioactive dating method, geologists have constructed a geological time
scale or stratographical scale . This scale is the calendar of the Earth's past
history indicating

125. the evolution of life through time recorded in the sequence of rocks. The geolo
gical time was developed by Giovanni Arduiana (1760). It is estimated that the
Earth was formed about 4.6 or 5 billion years ago and life on the Earth
originated about 3.6 billion years ago. This period of history of life on the Earth
has been divided into six major periods called eras which are divid ed into
periods and periods into epochs.

127. Fossils become
abundant
FIG. 2.23 : Geological time scale.
B y determining the age of rocks and fossils contained therein, a geological time
scale has been reconstructed. It depicts characteristic climatic conditions,
occurrence of specific living plants and anim als and adaptive modifications in
them.
2.7.6 Mass Extinction
Ther e have been mass extinctions throu ghout the history of the Earth at the end
of Ordo vician , Devonian, Permian , Triassic and Creta ceous periods. The
recent mass extinction occurred in Cretaceous Period at the end of Mesozoic
Era, about sixty million years ago when dinosaurs sudden ly became extinct.
Geologists found that high concentration of metal iridium occurs in a thin layer
below the surface soil in all parts of the Earth. Metal iridium is rare on the Earth
but occurs in large quantities in meteorites. The rocks which are found to be rich
in iridium are estimated to be 60 million years old. From these observations, it is
concluded that a comet or meteorite hit the Earth around that time with an
impact of several thousands of hydrogen bombs and resulted in mass extinction.
Walter and Luis Alvarez (1977) concluded that striking of meteorite with the
Earth could have formed a cloud of dust, blocking out the Sun and causing
animals and plants to freeze and die.
2.7.7 Molecular Clocks
Molecular clocks a llow scientists to use the amou nt of genetic divergence
between organisms to extrapo late and estimate dates of past life. When
sequence of nucleotides in DNA of the cytochrome c gene in vario us organisms
is com pared, it becomes evident that the greater is the number of differences in
the nucleotide sequences in the gene for cytochrome c, the longer the time they
have diverge d. The changes in nuc leotide sequences in cytoc hrome c gene are
pres umed to have accumulated at a constant rate. This phenomenon is known as
'molecular clock'.
2.7.8 Evidences From Fossils
The study of fossils revea ls existence of life in the past and provides direct

128. evidence for the course of evolution of plants and animals. The sedimentary
rocks with fossils can be compared with the leaves of a book with evolutionary
history written in the language of fossils . The fossil records establish the
following facts:
2.7.8.1 Distribution of Fossils in the Successive Strata
The distribution pattern of fossils shows that ancient fossils present in the bottom
rocks are simple, while the most recent fossi ls found in the uppe r strata are
more highly evolved. This shows that fossi l forms became more and more
complex as we proceed towards recent rocks. The fossi ls of man, the most
highly evolved anima l, are found only in the recent rocks.
2.7.8.2 Missing Links (Transitional Forms)
The transitiona l fossil organisms which possess characters of two different
groups of present day living forms are called missing links. These show
evolutionary relationship between two different groups. With the help of missing
links, the evolutionary sequence of major vertebrate groups have been drawn .
Birds
Fishes ----+ Amphibia ----+ Reptiles

130. { Mammals
Coracoid Furcula
Claws
Digits
Ulna
Numerus
Tail feathers Hind limb
Digits
FIG. 2.24: Restoration of fossil bird, Archaeopteryx lithographica (the ancient
lizard-bird from upper Jurassic limestone rocks from Germany). A few examples
of missing links are:
I. Archaeopteryx lithographica is a connecting link between reptiles and birds.
Its fossil was obtained by Andreas Wagner in 1861 from limestone rocks of
Solenhofen in Bavaria, Germany. These rocks were from Upper Jurassic Period
about 180 million years old. The fossil bird was of the size of crow and had both
reptilian and avian characters .
Reptilian characters of Archaeopteryx are:
• Presence of teeth in jaws
• Fingers having claws
• Long tail with free caudal vertebrae
• Presence of scales
• Sternum without a keel
• Bones solid and nonpneumatic
• Ribs single-headed and without uncinate process
• Metacarpals are separate and carpometacarpus absent
Avian characters of Archaeopteryx are:
• Presence of feathers on the body
• Forelimbs modified into wings
• Jaws drawn into a beak
• Presence of v-shaped furcula or wish-bone
• Limb bones and girdles bird-like
• Foot has four clawed digits and a long tarsometatarsus

132. Evidences for Evolution
~
45
IM:'-."}
Ge ological Time Scale of Earth
(To be read from belowupward, duration of each Epoch and period is in million
years)
Eras Periods Epoches
1 . Coenozoic (Era of
modern life)
(Age of
Mammals and Angiosperms)
(65 million years)
1. Quaternary
2. Tertiary Recent
(1 million yrs)
Pleistocene
(2-2.5 million yrs) Pliocene
(6-7 million yrs)
Dominant Animal and
Plant Groups
Age of mammals: modern
man, apes, monkeys and other mammals, birds and insects.
Appearance of primitive man; extinction of large mammals.
Emergence of man ; evolution of modern mammals (horse, camel, elephant).
2 . Mesozoic (Era of intermediate or medieval life)
1 . Cretaceous (135 million yrs)
(Age of
Reptiles

133. and Gymnosperms) 2. Jurassic
(180 million yrs)
3. Triassic
(225 million yrs)
3. Plaeozoic (Era of
ancient life)
1. Permian (275 million yrs)
Miocene
(25 million yrs)
Oligocene
(38 million yrs) Eocene
(54 million yrs)
Palaeocene (65 million yrs) Formation of first man-like ape; mammals
dominating and at the height of evolution.
Extinction of archaic mammals; rise of first modern mammals
D iversification of placental mammals (Eutherians), carnivores and hoofed forms
appeared.
Rise of placental mammals and evolution of modern birds.
Appearance of archaic eutherian mammals; rise of modern birds and teleost
fishes ; extinction of giant reptiles and toothed birds. Dwindling of
gymnosperms and increase of angiosperms.
Age of gymnosperms and reptiles: dominance of dinosaurs; rise of toothed birds;
spread of reptiles. First angiosperm and first bird appeared.
Rise of first dinosaur and egg laying mammals; extinction of primitive
amphibians. Abundance of cycadophytes, gymnosperms Abundance of primitive
reptiles, decline of amphi bians; extinction of many marine invertebrates;

135. rise of modern insects.
Contd ...
(Age of Amphibians)
(Age of Fishes)
(Age of Invertebrates)
4. Proterozoic (Era of Primi tive life)
5. Archaeozoic (Era of dawn of life)
2. Carboniferous (345 million yrs (Pensyl
vanian +
Mississipian)
3. Devonian (395 million yrs)
4. Silurian
(430 million yrs)
5. Ord ovician (500 million yrs)
6. Cambrian (600 million yrs)
2000 million yrs
3800 million yrs Precambrian
Period
Age of amphibians: Spread of ancient sharks; first reptile appeared; rise of
insects.
Age of fishes : Origin of amphibians; abundance and diversifica- tion of fishes.
Origin of jawed fishes and wingless insects; wide expansion of invertebrate
phyla.
Origin of vertebrates and jawless fishes; corals and trilobites abun- dant;
diversification of molluscs.
All invertebrate phyla estab- Iished; trilobites dominant.

136. Sedimentary rocks ; origin of first simple marine in vertebrates; shelled
protozoans, coelenterates. (fossils scanty)
Origin of life , fossils of primitive bacteria-like and algae-like forms found
(fossils rare).
The fossil of Archaeopteryx shows that b irds evolved from reptiles. Huxley
called birds as g lori fi ed reptiles. The presence of a wish-bone (furcula) in the
skeleton of a fossil cursorial and carnivorous dinosaur, Velociraptor, shows that
birds are closest to dinosaurs and have evolved from them .
2 . Seymouria is a gr oup of
fossi l ancestr al r eptil es f rom
Carbonife ro us period and i s a
missing l ink betw een amphibians
and reptiles. Dr. T.E. White made
the detailed study of Seymouria . A
3 . Ichthyostega from Devon ian
Period and Carboniferous Period
is a missing link between fi sh and
amphibia (Fig. 2.25).
4. Cynognathus w as a wolf
size d and mammal-like repti le

138. th at lived in Triassic Period. It FIG. 2.25: Ichthyostega, a missing link between
fish
r esembl ed r ep ti les as well as
and amphibian: A. Skeleton and B. Restoration.
mammals. It is regarded as ancient repti le ancestor of mammals.
Present
10 million years ago
20 million years ago
30 million years ago
Rodhocetus kasrani :
Its reduced hind limbs did not help in
walking or swimming. Rodhocetus swam with an up-and-down motion like
modern whales
40 million years ago
50 million years ago
60 million years ago Hypothetical
mesonychid skeleton
Amb ulocetus natans
that walked on land like modern sea lions and
swam byflexing its backbone and paddling with its hind limbs like modern otters
FIG. 2.26 : The fossils of Ambulocetus and Rodhocetus marine mammals have
filled in the gaps between the mesonychids, the hypothetical ancestral form and
whales.
5. Ba silosaurus is the connecting link between aquatic mammals and their
terrestrial ance stors. It resembl ed whale but had hindlimbs.
6. A series of fossil s of extinct marine mammals like Ambulocetus and
Rodhocetus help in tracing the evolution of present day toothed whales from
hypoth etical four-legged terrestrial mesonychid mammal (Fig. 2.26) .
7. The fossil Pteridosperms (the seed ferns) that existed in Carboniferous and
Pennian period s, form a connecting link betw een ferns and gymnosperms.
Their leaves resembl ed those of ferns, but the stem showed seco ndary growt h
and seeds like gymnosperms .
2.7.8.3 Evolutionary History of Groups

139. Foss il records of different classes of vertebrates appear in chronological
sequence.
Fossil fishes are the first vertebrates, amphibians next, followed by reptiles and
then birds and mammals.
2.7.8.4 Evolutionary History of Individual Forms or Pedigree Studies The
palaeont ologists, by the study of fossils, have traced out the com plete
evolutionary history of some animals such as Horse, Elephant, Camel and Man.
E volution of Horse (Fig. 2.27): Fossil history of horse was described by Othniel
C. Marsh in 1879. It started about 60 milli on years ago in the plains of North
America. The ancestors of horse were small-size d, fox-like form s present in
Eocene Period of Coenozoic Era. They had four toes in forefoot and three toes in
the hindfoot.
They increased in size and acquired modifications to suit grass land life for fast
runnin g. The stages in the pedigree of horse have been summarised in Table 3.4.
The
continuous change of a c haracter in a specific direction within an evolving
lineage is called evolutionary trend. A lineage may show several trends both
progressive (showing increase or improvement of character) or retrogressive
(showing degeneration or loss of character). In the evolution of horse, both types
of evolutionary trends are shown in Table 3.3.
Teethbecame larg er and
harder with broad cutting
surface reflecting a change
in diet from soft leaves to
Body size increased, perhaps in response to
more abrasive grass.

140. I
selection by predators .
Cl
c: .~
Selection for fast running on the open plains favoured the evolution of long,
stout and strong legs with large, hard hooves and reduction in the number of
digits.
The ancestor horse with small teeth; browsing on soft vegetation on the ground
2. Mesohlppus Forefoot::-:--l .J'"
Tooth A t n
Paleotheres
FIG . 2.27: The evolution of the horse during last fifty million years from small
woodland browsers to large plain-dwelling grazers . Three features which
changed during evolution include size, leg anato my and tooth anatomy.
ii·N_.JCI Progressive and Retrogressive Trends in Evolution of Horse Prog res
sive Evolutionary Trends
1. Progressive increase in size and weight.
Retrog ressive Evolutionary Trend s 1. Gradual loss of toes in both forelimbs
and hindlimbs.
2. Lengthening of limbs . 2. Gradual loss of canines in both upper and lower
jaws.
3. Loss of hair from the body. 3. Lengthening of middle toe in both fore-
li mbs and hindlimbs .
4. Increase in height.
5. Increase in length of neck.
6. Enlargement and complexity of brain , specially cerebral hemispheres.
7. Lengthening of facial region of head .
8. Increase in the size and complexity of molar teeth .
.'(,,-jl:l'''1 Stages in the Evolution of Horses

141. Name of Stage Scientific Com Epoch
Name mon
Name
Time of Height Evolu(in cm)
tion (in million years)
1. Eohippus Dawn Eocene
(Hyracothhorse
erium)
60 40 cm (Terrier dog size)
40 60 cm 2. MesohipInterOligopus mediate cene horse
3
3. MerychipRumi- Miocene pus nant
horse
25 100 cm (size of ass)
Only 3rd (2nd and 4th digits as splint)
4. Pliohippus PlioPlio- cene cene horse
10 108 cm Only 3rd
(2nd and 4th digits as splint)
5. Equus Modern Pleisto- horse cene 0.5
160 cm Only 3rd
(2nd and 4th digits as splint)
No. of Fingers
No . Molars of
Toes
3 Short
crowned, for grinding.

142. 3 Short
crowned, for grind- ing.
On ly Long
3rd crowned, toe for grazing. pres-
ent
Only Long
3rd crowned, toe for grazing. pres-
ent
Only Long
3rd crowned, toe for grazing. pres
ent
2.7.8.5 Land Pattern
4
F rom the study of distribution of fossils, palaeontologi sts had explained the
gradual changes in the Earth pattern. These changes are described as continental
drifts or plate tectonics by Alfred L. Wegener in 1929.
A. About 375 million years ago, around Carboniferous Period, Palaezoic Era, the
present day continents formed a single big landmass called Pangea. The
ancestral Pacific Ocean was Panthalassia Ocean and the ancestral Mediterranean
was Tethys Ocean.
B . About 180 million years ago, i.e., near Triassic Period of Mesozoic Era, this
landmass divided into two land masses:
• Laurasia, the Northern half, consisting of North America, Europe and Asia.
• Gondwana, the Southern half, formed of South America, Africa, Australia,
India and Antarctica.
50 ~ Evolutionary Biology. _

143. .
1<:'
~
"'j
Laurasia '
o~ ;...CJ....../
~ ~
"?"~ .f.~.~ ngea
_...... ............... .
..........
"1! IS6 \ Africa J•
Gondwana':?~ ~~ i .····f India-. ~20\':>.. -"
~
( {
! ....
\
~~~\
"'
\ ..
· · · · · · · · · · ··A~ l;~~ii~ p..\}'"
L
A B
c D
FIG. 2 .28: Gradual change in the position of different continents by cont inental
drift : A. Landmass in Palaeozoic era (Carboniferous Period -Pangea ; B. In
early Mesozoic Era about 185 Mya) , Earth divided into two land masses:
Laurasia and Gondwana; C. In late Mesozoic Era (about 135 Mya) continents
started separating; and D. In the beginning of
Coenozoic Era (about 65 Mya) modern land pattern established.
C. About 135 mill ion years ago, near the end of the Jurassic Period, South
America drift ed away from Africa and India brok e loose, moved north ward
and rammed into Asia .
D . About 65 million years ago, near the end of the Mesozoic Era and beginning
of the Coenozoic Era, North America separa ted from Europe. The Westward
drift of North and South America resulted in the widening of North and South
Atlantic oceans.
The above conclusions are based on the following observations: I. Early fossils
from various island s are not much different indicating that these were
interconnected .
2. Fossils became more and more different and latest fossils and present fauna
and flora of different islands is much different.
2.8 EVIDENCES FROM GEOGRAPHICAL DISTRIBUTION OR
BIOGEOGRAPHICAL EVIDENCES

144. Biogeography is the study of distribution of animals and p lants on Earth in
space and time . The study of geographical distribution of animals is called
zoogeography . Some striking differences between the distribution of certain
flora and fauna are explained on the basis of evo lutionary inter-relationship as
follows :
2.8.1 Biogeographical Division of the Earth
P.L. Sclater (1858) divided the Earth surface into six biogeographical reg ions or
life zones called realms based on the distribution of birds. Later in 1876, Wallace
reorganised these biogeographical realms on the basis of distribution of
terrestrial and fresh water vertebrates. These realms are:
I. Nearctic: North America up to Mexican Plateau.
2. Palaearctic: Asia (north of the Himalayas), Europe and Africa (north of the
Sahara Desert) .
3. Neotropical: Central and South America, Mexican lowlands and West Indies.
4. Oriental: Asia (south of the Hima layas including India, Sri Lanka, Malaya
peninsula), Sumatra. Borneo, Java, Celebes and Philippines.
5. Ethiopian: South Africa and the Sahara Desert, Madagascar and adjacent
islands .
6. Australian: Australia, Tasmania, New Guinea, New Zealand and Oceanic
islands of Pecific Ocean .
These realms are separated from one another by major barriers like seas,
mountains and deserts . For example, Oriental Realm which includes India, Sri
Lanka and Malaya Peninsula is isolated from Palaearctic Realm by Himalayan
Mountains and from Ethiopian and Australian Realms by sea (F ig. 2.29).
2.8.2 Diversity in Fauna and Flora
In many cases , countries which are very near to each other and have simi lar
climatic conditions differ in the fauna and flora. For examp le:
I. Madagascar is only 260 miles from East Coast of Africa but its inhabitants are
markedly different.
2. The climate of Australia, South Africa and Western South America is very
much the same , but the fauna and flora in each region are strikingly different.
3. The fauna of North Africa and South Europe, which are wide ly separated by
Mediterranean Sea, is much more ident ical than in the above cases .

145. Different fauna is present in Madagascar and mainland of Africa because
Madagascar separated from Africa about 65 million years ago and their fauna
and flora evolved separately and independently. They became markedly
different.
Similarities in the fauna (flightless birds, various invertebrates and fresh water
fishes) of Australia, South Africa and Western South America can also be
explained on the same basis .
52 ~ Evolutionary Biology
The similarity in the fauna of North Africa and So uth Europe is because these
were connected together for long time and living forms from one island could
migrate to other. Because life evolved together on these continents, their fauna
and flora show similarity.
FIG. 2.29: Six biogeograph ical realms of the world.
2.8.3 Discontinuous Distribution of Closely Related Species In some cases
closely related species exist in widely separated places with no representatives in
the intermediate territory. For example:
I. Lungfishes: The surv iving three genera of lung fishes are found in three
different continents:
I. Protopterus in Africa 2. Neoceratodus in Australia
3. Lepidosiren in South America

147. D South American Lungfish(Lepidosiren para doxa)
• African Lung Fish (Protop terus)
• Australian Lung Fish(Neoc eratodus)
FIG. 2.30: Diagram showing discontinuous distribution of lungfishes.
2. Tapirs are found in tropical America and Malay Islands .
allies I1mas are found in South 3. Camels occur in Asia, while their nearest
America.
4. Elephants are found in Africa and India
climate in Brazil.
but not in places with identical
5 . Alligators occur in South-eastern United States and Eastern China .
6. Magnolias, Tulips and Sassafras grow naturally in eastern United States and
eastern China only.
The discontinuous distribution of these forms is explained as follows : I. These
forms enjoyed wide distribution. Their extinction in the intermediate region
resulted in widely separated populations. For example, North American
continent was connected with Asia till early Coenozoic Era. The climate of the
whole region was much wanner during Mesozoic Era. Alligators were widely
distributed over this region . In late Coenozoic Era, the climate changed due to
Ice Age. Because of low temperature alligators became extinct in most of the
region except South-eastern United States and eastern China, which remained
unaffected by glaciation. The natural occurrence of Magnolias can also be
explained the same way.
2. The evolution of Horse and Camel occurred in North America, from where
they migrated to Eurasia and South America through land connections existing
till Pleistocene Period , but became extinct in all other areas including North
America except in few areas where they are present now.
2.8.4 Restricted Distribution
Mon otremes or egg laying mammals occur only in Australian Island.
Marsupials, the pouched mammals are exclusively found in Australia, New
Zealand and South America.
This restricted distribution of Monotremes (egg-laying mammals) and
Marsupials (pouched mammals) only in New Zealand and Australia is explained

148. on the basis that Australia, New Zealand and South America were once
continuous with the mainland of Asia, but got separated in late Cretaceous
Period much before the appearance of carnivorous eutherian mammals. Placental
mammals, being better adapted, eliminated monotremes and most marsupials on
Asian mainland. But the primitive marsupial mammal s in Australia survived,
flourished and diversified because placental mammals could not reach there .
Placental mammal s evolved in Asia along different lines.
2.8.5 Life on Oceanic Islands
An oceanic island is one which never had any connection with the mainland . It
is formed from sumbmerged volcanic mountains which are pushed up above the
surface of sea. On being exposed, the mountain peaks cool down providing a
suitable surface for animals and plants that reached there.
The animals and plants of oceanic islands in general , resemble those of the
nearest mainland, yet include distinct species. For example, Darwin during his
historic

150. 54 [i] Evolutionary Biology
voyage in 1825 found that the animals of Galapagos Islands resembled those of
South American mainland.
Galapagos Islands
Study of life on different Galapagos Island s nicely illustrates the role of
isolation in the subsequent evolution of animals and plants. The prominent
inhabitants of these islands are :
I. Iguana: It is the giant lizard about 1.25 meters long and is represented by two
species, one terrestrial and other marine (Fig. 2.31).
FIG. 2.31: Iguana . the giant lizard of Galapagos Islands.
FIG. 2.32: Giant tortoise on Galapagos islands.
2. Giant Land Tortoises: On Galapagos Islands these tortoises have attained
length of about I meter and weight up to 225 kg. Such giant tortoises are missing
from mainland because on mainland these were exposed to mammalian
competitors. Absence of mammals on these islands presented congenial
atmosphere with no competitors permitting unhindered increase 10 size.
3 . Darwin's Finches: Darwin observed that the finches on mainland of South
America were all of one type, possessing short, straight beaks for seed crushing.
These birds from vari ous Islands of Galapagos differed in size and shape of the
bill due to adaptation to different food types avail able. Darwin differentiated
thirteen species of finches and grouped them into . six main types:
• Large ground finches
• Cactus ground finches feeding on cacti
• Warbler finche s
• Insectivorous tree finches
• Vegetarian tree finches
• Woodpecker finches

152. Darwin's finches offer an excellent example of adaptive radiation . The ancestral
finches on reaching different islands occupied all empty ecological niches in the
absence of competition and evolved into different species.
Evolution favours appearance of similar forms under similar circumstances or
under similar environmental conditions. Convergence or convergent evolution is
the evolution of similar forms in different lineages when exposed to the same
selective
pressures. Divergence or divergent evolution is the evolution of different forms
in the same lineage when exposed to different selective pressures.
Tortoises and Finches on Ga lapagos Islands present divergent evolution.
Typic al mainland type (ancestral) Finch

153. +
IGalapagosLarge ground
finches finch
+

154. Warbler I
finch
Cactus ground
1
finch
Food,o",m ~
Insectivorous
1
tree finch
~
+ 1
Vegetarian
tree finch
Woodpecker
1
(tool using) finch
~
Large Cactus seeds seeds and nectar Flying Large insects insects
Buds and Insect fruit larvae
FIG. 2.33: Darwin 's finches on the Galapagos Islands show ing adaptive radiati
on. Common ancestor evolved into many different species for exploiting
different food sources. such as seeds, fruits, buds , insect larvae and insects.
2.9 EVIDENCES FROM CONNECTING LINKS
W hile classifying animals one comes across certain anima ls or small animal
groups which exhibit characteristics of more than one group. Such animals or
animal groups are called connecting lin ks between these two groups. For exa

155. mple:
I. Viruses: Viruses are connect ing link between nonli ving and living because of
the following characteristics:
• Nonliving Characters
(a) Viruses can be crystall ised like inorganic and organic compounds.
(b) These are devoi d of enzyme system, so cannot carry out any of the
metabolic activities.
(c) These are unable to respond to external stim uli.
(d) Do not grow in size.
• Livin g Characters

157. (a) Viruses multip ly (reprod uce), though inside the living system. (b) Viruses
undergo mutations.
(c) Viruses possess property of recomb ination and heritability.
2. Euglena: Euglena is a protozoan. Like plants, it has chlorophyll and
chloroplasts. It is holophytic or autotrophic and synthesises food by photosynth
esis.
Like animals, Euglenasbody is covered with pellicle. It reproduces like animals.
Therefore, Euglena is said to be a connecting link between plants and animals. 3.
Proterospongia: It is a colonial protozoan. Its cells are similar to choanocytes or
collared cells of phylum Porifera.
4. Peripatus: Peripatus is a connecting link between Annelida and Arthrop oda
because it exhibits characteri stics of both the phyla Annelida and Arthropoda.
• Annelidian Characters
(a) Presence of nephridia as excretory organs.
(b) Body pseudosegmented and worm-like.
(c) Presence of simple eyes.
(d) Thick cuticle.
• Arthropod Characters
(a) Clawed and pseudosegmented walking legs.
(b) Presence of trachea as respiratory organs.
(c) Presence of antennae.
Perip atus shows that arthropods have evolved from annelid ancestors. 5.
Neopilina: Neopilina belongs to class Monoplacophora of Phylum Mollusca.
It s first living specimens were dredged from a depth of 3500 meters off the
Pacific Coast of Costa Rica (Mexico). Until the discovery of living Neop ilina,
molluscs were regarded as unsegmented animals and were thought to have
evolved from marine flatworms because of following similarities:
(a) Soft, flattened and unsegmented body.
(b) Foot ventral, muscular and creeping.
(c) Embryology of two groups is similar.
D iscovery of Neop ilina has provided support to annelid-molluscan relation

158. ship.
• Ann elidan Characters of Neopilina
(a) Metameric structural plan.
(b) Segmentall y arranged 5-6 pairs of ctenidia, 6 pairs of nephridia , 8 pairs of
r etractor muscles and 2 pairs of auricles .
(c) Eggs divide by spiral cleavage as in polychaetes.
(d) Presence of trochophore larva as found in polychaete s.
• Molluscan Characters of Neopilina
(a) Possesses shell and mantle like other molluscs.
(b) Possesses a flat muscular foot as found in Chiton.
(c) Body is soft and dorso-ventrally flattened.
6. Balanoglossus: Balan oglossus belongs to Subphylum Hemichordata. It is
intermedia te between nonchordates and chordates.
7. Chimaera: It is a connecting link between cartilaginous fishes and bony fishes
and exhibits characteristics of both:

160. Evidences for Evolution Ii] 57
• Characteristics of Cartilaginous Fishes
(a) Cartilaginous endoskeleton.
(b) Mouth ventral.
(c) Dorsal fins two in number.
(d) Placoid scales present in young ones.
(e) Claspers present in males.
• Characteristics of Bony Fishes
(a) Mouth small and with fleshy lips
(b) Gills four pairs
(c) Cloaca absent; anus and urinogenital apertures are separate. (d) Teeth fused
to form tooth plates that join the jaws.
I .
"(Flagellum \
Chloroplasts
.J L ......-=-- Nucleus
Virus Euglena Proterospongia
Clawed appendages Spines
Poison spur
Peripatus Neopilina Echidna FIG. 2.34 : Some connect ing links: Virus
(bacteriophage); Euglena, Proterospongia, Peripatus, Neopilina and Echidna.
Front
First dorsal fin
clasper

162. Gill slit Pectoral fin
FIG. 2.35: Chimaera , a connecting link between cartilaginous and bony fishes .
8 . Lungfishes : Lungfishes, Neoceratodu s, Lepidosiren and Protopterus, are
connecting link between fishes and amphibians. Like other bony fishes , these
are fish-like in appearance, have paired fins, dermal scales, gills, lateral-line
sense organs and ears represented by internal ear only. Their amphibian features
are presence of internal nostr.ils or chonae, lung (modified air bladder) and heart
with imperfectly divided auricles. They have fleshy fins supported by bon es and
capable of walking on the bottom of ponds and wate ry mudflats for short
distances.
9 . Prototheria: The egg laying mammals, Echidna and Ornith orhynchus, the
only living prothotherians of class Mammalia, are connecting link between
reptiles and mammals. Like reptiles, prototherians have cloaca, lay shell ed
Pectoral fin A. Neoceratodus
B. Lepidosiren
/
Pectoral fin C. Protopterus
FIG. 2.36: Lungfishes: A. Neoceretodus, B. Lepidosiren. C. Protopterus.
yolky egg s and their embryonic development is similar to reptiles. Like
mammals, these have mammary glands and nourish their bab ies.
2.10 EVIDENCES FROM TAXONOMY
Th e sci ence of naming, describing and classifying organisms is known as
taxonomy. Classification started as an 'artificial system' of cat alogu ing of
innumerable living organi sm s as a librarian classifies and catalogues books. It
developed into a 'natural system ' based upon natural affinities and actual kinship
(based on genetic relationship) found in the organisms. It was devised by
Linnaeus. It is, therefore, concluded
FIG . 2.37: The egg laying prototherian mammals: A. Echidna, the spiny anteater
; B. Ornithorhynchus, the duck-billed Platypus.
th at resemblances in animals are because these have arisen from a common
stock, and differences in them are chiefly due to adaptations to different types of
environments. All the animal phyla when viewed together seem to have
relationships with one another. It is po ssible to arrange them in a series of
increasing complexity on

165. Sponges
FIG. 2.38 : The phylogenetic tree showing the evolutionary relationship among
animals.
a ladder-like diagram. It is easy to concede that protozoans come at the bottom
and chordates at the top of ladder.
Taxonomists have summarised their studies in the form of tree-like diagram in
which phyla represent major branches of the tree of life (Fig 2.38). These are
divided into several small branches, classes, which are divided into orders. This
taxonomic tree with its branching system like a real tree represents an inter-
relationship among groups of organisms descended from a common ancestor and
modified along different lines. The living animals constitute the terminal twigs
of the phylogenetic tree and
do not exhibit any d irect relation ship. The fundamental relationship lies in the
main branches and the trunk and the connecting links either exist as remote
ancestors persisting today or died in past and may be represented by fossils.
T he mere fact that animals could be graded in order of increasing complexity is
an evidence of evolution. The natural system of classification is based upon
similarities and such similarities in structure could be only due to their origin
from common ancestors .
2.11 EVIDENCES FROM BIOCHEMISTRY AND PHYSIOLOGY
The most con vincing evidence of common ancestry comes from the basic
similarities seen at the molecular level in the chemical composition, enzymes,
hormones, blood proteins and various physiological activities in all living
beings.
2.11 .1 Molecular Homology
It is the similarity in the molecular structure of important biomolecules. The
remarkable similarity in important macromolecules in the organisms of different
groups indicates degree of closeness between them. For example:
1. Homology in the Structure of DNA and cytochrome c: 98.2 per cent
homology is found in the base sequences of DNA of Man and Chimpanzee and
100 per cent molecular homology occurs in the amino acid sequence of their
cytochrome c.

166. 2 . Similarity in Metabolic Processes: The process of protein synthesis,
biosynthesis of various organic molecules in the body and catabolism of organic
substances during respiration involve the same biochemical reactions and same
enzymes from bacteria to human beings. In all living beings, oxygen is used in
the oxidation of glucose , and carbon dioxide and energy are released . The
energy released is stored in ATP molecules.
3. ATP is the energy currency in all livi ng cells.
4. Chlorophyll is the photosynthetic pigment in all oxygenic photosynthetic
organisms both prokaryotes and eukaryotes.
5. Cytochrome c present in all the eukaryotic cells acts as an electron acceptor.
6. Nitrogenous waste in all living organ isms is produced in the form of
ammonia. It is excreted in the same form in aquatic invertebrates but in
terrestrial invertebrates and vertebrates, ammonia is changed to urea. In birds,
snakes and lizards , urea is changed to uric acid.
7. Blood groups in man are A, B, AB and O. Apes have blood groups A, B, AB
while Monkeys do not have A, Band 0 blood groups. This shows that Man is
more closely related to Apes than to Monkeys.
2.11.2 Blood Proteins and Precipitation Method to Test Relationship
Blood proteins also help in tracing the phy logenetic relationship. Serological
tests by precipitation method are used for finding out relationship among
different animal groups. This method was introduced by Dr. George H.F. NuttaI.
These tests have shown closeness between man, gorilla and chimpanzee; tiger,
lion and leopard; dog,
cat and bear ; birds and crocodiles. Man is closer to old world monkey s than to
new world monke ys. The closeness is determined by the amount of precipitate
formed in the blood of apes and monke ys with antihuman serum. The lesser the
precipitation less is the closeness.
P recipitation Method for Testing Relationsh ip : Freshly drawn human blood is
allowed to clot in a dish and the serum is collected. Small quantitie s of serum
are then injected at definite intervals of one or two days into the vein of a rabbit.
The rabbit' s blood develops antibodies again st human blood. After a definite
number of doses the rabbit is allowed to live for some days and then bled. The
blood is collected and allowed to clot. The serum is then preserved. It is
"antihuma n serum".

167. Th e antihuman serum is added to the blood of other animal s and the degree of
clumping or clotting is noted . It has been noted that the clotting is more in
closely associated animals and reduces as the relation ship become s more
distant.
Wh en antihuman serum is mixed with the blood of dog or cat, precipitation does
not occur but with that of primate s (Ape s and Monkeys) precipitation is very
fast. Moreover, the old world monkey s show slight reaction and the new world
monkeys show a faint reaction , indicating that man is more closely related to
old world monke ys than the new world monkey s.
Diff erent types of serum s can be prepared in this manner and relation ship can
be tested. Guttal and Graham Smith came to the following conclu sions after
conduct ing thousands of blood tests:
I . Anticarnivora sera give maximum and more clear reaction s among the blood
of Carnivora than the blood of other animal groups. The reaction s are more
quick and prominent with more closely related forms.

169. Antihuman serum precipitates
Rabbit
Human Gorilla Monkey Horse
(Immunisation) blood blood blood blood
FIG. 2.39 : Representation of precipitation test by using rabbit contain ing
antibodies against sheep serum .
2 . Anti pig ser um reacts promptl y with the blood of species belonging to pig
family, moderate reacti on with rumin ant s and cam els. Antillma serum gives a
moderate reaction with the blood of camel.
3. Antiwhale serum produces maximum precipitation only with the blood of
other whales and slight reaction with that of pig and ruminants.
4. Strong antiturtle serum gives reaction only with the blood of turtles and
crocodiles. It gives negative results with that of snake and lizard's serum.
5. Blood protein tests have given following information:
• Man is nearest to great apes, chimpanzee and gorilla and next nearest are old
world monkeys, new world monkeys and tarsiers .
• Dogs, cats (including lions , tiger s, leopards) and bears indicate a close
relationship.
• Sheep, goats , deer and cows are closely related.
• There is affinity between lizards and snakes and between turtles and crocodiles.
• The relationships have been observed in invertebrates specially Crustacea,
Insecta and Mollusca.
The species of a single genus exhibit very close serological similar ity. Genera of
the same family show moderate reaction and families of same order show slight
but detectable serological similarity.
2.12 EVIDENCES FROM MOLECULAR RECORDS
According to evolutionary theory, every evolutionary change involves the
substitution of a new gene for the old one and the new allele arises from the old
one by mutation. Continuous accumulation of changes in the DNA coding for
proteins leads to evolutionary differences. The organisms that are more distantly
related accumulate a greater number of evolut ionary differences, while two
species that are more closely related share a greater portion of their DNA. This
pattern of divergence is clearly seen in human haemoglobin polypeptide.
2.12.1 Molecular Clocks

170. Highly conserved proteins like haemoglobin and cytochrome c provide the best
molecular clocks for determining rate of evolution and trace evolutionary
relationship between different groups. Though, all proteins appear to accumulate
changes in the sequence of amino acids over time, the rate of accumulation of
changes/mutations in different proteins is different. Therefore, proteins evolve at
very different rates .
Molecular Clock
Accumulation of molecular changes in the proteins at a constant rate due to
changes in the nucleotide sequences in the genes for particular proteins is re-
ferred as molecular clock.
The longer is the time since the organism s diverged, the greater is the number of
differences in the structure of specific proteins and also the number of difference
s in the nucleotide sequence of the genes for the specific proteins. When such
molecular
change s accumulate at a constant rate, the phenomenon was called molecular
clock by Zuckerkandl and Pauling (1965 ).
2.12.2 Molecular Homology as Seen in Molecular Structure of Haemoglobin
H aemoglobin is formed of two identical alpha polypeptide chains and two
identical beta chains. Each a-chain is formed of 141 amino acids and each 13-
chain has 146 amino acids. Chimpanzees, Gorillas, Orangutans and Macaques
that are closely related to human s, have fewer differences from human s in
amino acids of 13-chain of haemo globin than from distantly related mammal s,
like Dogs. The haemoglobin of nonmammalian vertebrates differs even more
and the nonvertebrate haemoglobin has much more different compo sition .
2.12.3 Molecular Evolution of Cytochrome c
Cytochrome c is the respiratory pigment present in all eukaryotic cells. It forms a
part of electron transport system and accept s electron s from H+ions in all
eukaryotes. It is formed of 104 amino acids.
Cy tochrome c is a protein that has evolved at a constant rate. When a graph is
plotted between the time since each pair of organisms diverg ed and the number
of nucleotide differences, the result is straight line as shown in Fig. 2.41.
The diagram shows that longer is the time of separation of two group s, the

171. larger is the number of nucleotide substitutions in their genes for cytochrome c.
For example:
Human Macaque Dog Bird Frog Lamprey

173. Q)
E
F
45
67
10 20 30 40 50
Number of am ino acid differences between the
haemoglobin polypeptide of other
animals and a human one
FIG. 2.40: Haemoglobin molecule reflects evolutionary divergence, i.e., the
greater the evolutionary distance from humans, the greater is the number of
amino acid differences in polypeptide cha in of haemoglobin of vertebrates.
64 [j] Evolutionary Biology
Human/Kangaroo 100
V>
c
.Q
~
iii 75
.0
::J
V>
Q)
:2
(5
50 Horse/
Q)
"0 Donkey
::J
Z
Sheep/Goat 25
Human/ Dog/ Cow Cow
Rabbi
U '
Roden
~ Human/Rodent

174. Pig/ Cow
Millions of years ago
FIG . 2.41: Diagram showing the evolution of cy1ochrome c gene in different
mammals from horse/donkey to humans . The graph plotted between time of
divergence of each pair of organisms (like sheep/goat or goaUcow or
human/cow) and the number of nucleotide substitutions results in a straight line
suggesting that the gene for cy1ochrome c evolved at a constant rate.
• In chimpanzee and human , cytochrome c genes are identical,
• In human and kangaroo , there is substitution of nucleotides In a period of 125
million years.
• In human and cow, the substitution is about 75 nucleotid es during the period of
120 million years.
2.12.4 Phylogenetic Trees
Th e pattern of substitutions in the nucleotide sequences in globin genes during
the entire course of evolution can be laid down in the form of a phylogenetic
tree. It represents the evolutionary history of the gene and it closely resembles to
the evolutionary relation ships predicted by anatomical studies. For example,
whales, dolphin s and porpoises are grouped together. Similarly, human, monkey
and ape have very similar haemoglobin and are grouped together.
The pattern of accumulation of changes seen in the genes encoding proteins
provide strong direct eviden ce for evolution.
2.13 EV IDENCES FROM CYTOLOGY
2.13.1 Cell Structure and Cell Organelles
The basic structure of a cell is the same in almost all the organisms. All animal
cells have similar organelles such as cell membrane, endopla smic reticulum ,
Golgi
~ -Hemoglobin
Macaque Porpoise Cow
Pig
Insect and annelid
globins

175. Annelid
wo;;;r~m;---=::;:a_
..
~ Original
Insect globin gene
Fig. 2.42: Phylogenetic tree based on the evolution of globin gene. The length of
various lines corresponds to the number of nucleotide substitutions in globin
gene.
co mplex, ribosomes, mitochondria and nucleus. All the plant cells have cell
wall, chloroplasts, ER, mitochond ria, ribosomes and nucleus. All cell organelles
have same structure and carry out same functions in all nonspecia lised and
specialised cells.
2.13.2 Protoplasm
T he chemica l composition of all living beings is reduced to four main chemical
elements: Carbon, Hydrogen, Oxygen and Nitrogen. The se combine to form
four classes of organic compounds: Carbohydrates, Fats, Proteins and Nucl eic
acids. They form a colloidal solution which is called protoplasm . It forms
physical basis of life in the cells of all living beings.
2.13.3 Chromatin
Th e essential component of nucleus in every living cell is the chromatin
material or hereditary material that organises into chromosomes during cell
division. The chromosomes have fairly constant chemical composition in the
living animals being
Per cent Man 100
Chimpanzee 100
Gibbon 94
Rhesus monkey 88
Capuchin monkey 83
Tarsier 65
Snow loris 58
Galago 58
Lemur 47
Tree shrew 28
Mouse 21
Hedgehog

176. Chicken
19
10

178. FIG. 2.43: Diagram showing relat ive similarities between human DNA and
DNA of other vertebrates.
composed of DNA and proteins. The basic unit of DNA is a nucleotide
consisting of a molecule of phosphoric acid, one molecule of pentose sugar
'deoxy ribose' and a purine or a pyrim idine as nitrogenou s base.
Th e chemical composition of DNA is basically the same in all living beings
except for differences in the sequence of nitrogenous bases. It means they all
have graduall y evolved from some common ancestor.
2.13.4 Genetic Code
It is surprising to note that the same genetic code having triplet codons is found
from viruses to man. Moreover, in all living beings, for each amino acid, the
same codon exists.
2.14 EVIDENCES FROM GENETICS
The final line of evidence for evolution is drawn from Genetics, 'The Science of
Heredity '. It has been established now that genes ' the hereditary determiners'
are
The degree of similarity between the DNAs of two species can be estimated by
the pairing property of DNA strands. The DNAs are melted into single strands.
The single-stranded DNA of species A is broken in small fragments . These
fragments are brought in contact with the single-stranded DNA of species B. The
fragments of DNA of species A will pair with those fragments of DNA of species
B which have complementary base sequence.
I I I I I II
TCACCTG
Unpa ired segment Paired segment DNA of speciesA
I I I I I I
GAATAG
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
I I I I I I I

179. TCACCTG
Unpaired segment
A T C C A A
c1 ~ ; ; ; -,.- ~
T
I I I I I I I tI_L~_~_~_' ~ I
Paired segment DNA of species B T T T A C C
I I I I I I
FIG. 2.44: Similarities in the base sequence in DNA of species A and species B.
Any fragments of DNA strand of species A that are not complementary to any
region of species B remain unpaired. The greater the degree of pairing in the
DNA strands of species A and species B, the closer are the similarities in the
genetic messages contained in the DNA of two species.
quite constant and are inherited unchanged generation after generation. But
genes undergo changes producing mutations and va r iation.
These changed genes determine the c haracter in a different direct ion than the
original. The natura l forces of isolation and natural selection operate on these
mutations.
The selection and breeding of domesticated animals and cultivated plants for the
past several thousand years provide models as to how some evolutionary forces
operate in nature. Organisms change with the ever changing environment and
many of these variations are heritab le. Mutations always affect their genetic
milieu leading to the development of new characteristics. Some of these
modifications are beneficial to the organisms in quest ion. By selecting and
interbreeding these selected varieties, new races have been estab lished by man.
The same selection and interbreeding might have occurred and are occurring on
large scale in nature, and would establish new species.
• Adapt ive Divergence •
• Atavism •
• Biogenetic Law •
• Discontinuous Distribution •
• Homology •
• Missing links •
• Recapitulation Theory •
• Vestigial Organs
KEY TERMS

180. Adaptive Radiation
Antisera
Connecting Links
Divergent Evolution
Homologous Organs
Palaeontology
Retrogress ive Metamorphosis
• Analogous Organs
• Biogeography
• Convergent Evolution
• Fossils
• Homoplasy
• Phylogeny
• Reversion
REVIEW QUESTIONS
I. ( a) Briefly summarise the doctrine of biological evolution.
(b) List the variou s evidences that support this doctrine.
(c) How evidences from vestigial organs in human beings support evolution?
2. (a) What is evolution?
( b) Do you suggest that the present forms of living beings have evolved and not
made?
(c) How far does the science of palaeontology and comparative anatom y favour
organic evolut ion?
3. Discuss any two evidences for the support of organic evolution .
4. Write a short essay on the evidence s in support of the theory of evolution . 5.
Discuss fully any three of the evidenc es in favour of the idea of organic
evolution. 6. Give an account of the geographical and palaeon otological
evidences of evolution. 7. Write an essay on the evidences in support of the
theory of evolution as illustrated
by embryology and comparati ve anatomy.
8. Describe the palaeonto logical and embry ological evidences of organic
evolution . 9. Write an account of the morpholog ical and embryological
evidences in support of
organic evolution .
10. Explain the significance of connect ing links and embryos in the study of

181. evolution. II . Describe the missing links.
12. What is biogeograph y? How biogeographical studies provide evidence in
favour of
evolution ?
13. Explain the following terms:
(a) Palaeontology (b) Phylogeny
(c) Fossils (d) Geological time scale
14. Enlist vestigial organs found in man. How will you account for the
following:
(a) Discont inuous distribution of anima ls
(b) Restricted distribution of Monotremes
(c) Occurrence of vestigial organs in organism
(d) Parallel or convergent evolution in animals living in the same area. 15.
Explain which of the following are homologous and which are analogou s and
why?
(a) Fish scales and cuticular exoskeleton in Arthropods
(b) Wings of a bird and a bat
(c) Nai ls and claws in mamm als
(d) Nails in mammals and claws of scorpion
(e) Ginger and sweet potato
(f) Trunk of an elephant and hand of a chimpanzee.
16. Explain the following:
(a) Homology and homologou s organs
(b) Analogy and analogous structure s
(c) Divergent evolut ion or evolutionary divergence or adaptive radiation
(d) Convergent evo lution or adaptive convergence or parallel evolution (e)
Vestigial organs
(f) Atavism or reversion
17. Why is Archaeopteryx is called a connecting link between repti les and birds.
18. Name the fish/lizard which is called living fossil.
19. Explain the following :
(a) Fossils
(b) Mass extinction
(c) Pseudomorphs
(d) Connecting links
FURTHER READINGS

182. I. Appleman , P. (ed.), 1970. Darwin: A Norton Critical Edition. Norton ,
Newyork .
2. Badash, L., 1989. The age of the Earth debate Sci. Amer., 261 (2),90-96.
3. Barnett, S.A. (ed.), 1958. A century of Darwin. Heinemann, London .
4. Conn . H.w., 1900. The Method of Evolution. Putnam's, Newyork .
5. Darwin, c., 1859. On the Origin of Species by Means of Natural Selection or
the Preservation of Favoured Races in the struggle for Life. Marry, London .
6. De Beer, G., 1966. Vertebrate Zoology, Sedgwick and Jackson , London.
7. Forey, P.L., 1988. Golden Jubilee for the Coela Ganth Latimeria Chalummae.
Nature, 336, 727-732.
8. Ghiselin , M.T., 1969. The Triumph ofthe Darwinian Method, University of
California Press, Berkley.
9. Gould , S. J., 1977. Ontogeny and Phylogeny , Harward University Press,
Cambridge, M.A.
10. Haber, F.c. s, 1959. The Age ofthe World Johns Hopkins University Press,
Baltimore. II . Lack, David Lambert, 1947. DarwinsFinches Cambridge
University Press, Cambridge, England.
12. Leconte, A., 1888. Evolution , Its Nature. Its Evidence. and Its Relation to
Religious Thought. Appleton, Newyork .
13. MacFadden , B.1., 1992. Fossil Horses: Systematics, Palaeobiology, and
Evolution of the Family Equidas. Cambridge University Press, Cambridge,
England. 14. Rudwick , M.1.S. 1973. The Meaning of Fossils, Macdonald ,
London . 15. Simpson, G. G., and W. Beck, 1965. Life, 2nd ed. Harcourt Brace,
and World, Newyork .
16. Woeken, IT., 1986. Light and Life Processes. Vass Nostrand Reinhold,
Newyork .

184. DOD

185. 3
Theories of Evolution
Various theories regarding evolution have been po stulated by different
evolutionists. Out of them, the most important ones are:
I. Lamarck's theory of inheritance of acquired characters (Lamarckism) 2.
Darwin's theory of natural selection (Darwinism)
3. De Vries theory of mutations (Mutations)
4. Modern concept of evolution (Synthetic theory)
3.1 LAMARCK AND LAMARCKISM
Jean Baptiste Pierre Antoine de Monet Lam arck (1744 1829) was a French
naturalist, well known for his doc trine of evolution, 'Inheritance of Acquired
Characters'. It was published in 'Phi/osophie Zoologique ' in the year 1809, the
year Darwin was born. Though, Lamarck's name is inseparably linked with the
theory of evolution ' , he had attributed to biology in a number of other ways. He
introduced the word 'Biology' to emphasise the kinship betwe en living beings
(i.e., plants and animals). He also provided detailed investigations of fossils and
living forms , introduced the term s invertebrates and vertebrates and emph
asised distinction between them . Moreover, Lamarck arranged all living
creatures of past (fossil animals) and present (living ones) in one broad line or
'staircase' leading uphill to the primates with man being at the top of stairs .
3.2 INHERITANCE OF ACQUIRED CHARACTERS (LAMARCKISM)
T heory of inheritance of acquired characters or Lamarckism can be defined as
'The changes in structure or function of any organ acquired during the life-time
of an individual in response to changes in the surrounding environment are
inherited by its offspring and keep on adding up over a period of time'.
These changes accumulate for generations and lead to the origin of new species.

187. 3.2.1 Postulates of Lamarckism or Inheritance of
Acquired Characters
Lamarckism is based on following three factors or postulates:
3.2.1.1 New Needs
Changes in the environment create new needs in living organisms so that these
are better suited to the changed environment. The organisms have to put in
special efforts for the fulfillment of the new needs. These efforts lead to changes
in the habits or behaviour of the organisms. The new need results in the
formation of a new organ or a part in the body. If the need continues that organ
or part also continues to grow.
3.2.1.2 Acquisition of New Characters
New characters are acquired by the living beings in two ways :
1. By Use and Disuse: The new habits involve greater use of certain organs
to meet the new need, and disuse or lesser use of certain other organs in changed
conditions. The continuous use of an organ or organs keeps them functional and
makes them stronger, larger and more efficient. Continuous disuse of an organ or
organs leads to gradual reduction in their size and to their final disappearance.
Vestigial organs are example of such non-functional organs in the modem forms.
These were functional in the ancestors. By the differential use and disuse of
various
body parts , an organism would change by acquiring some characters and
modifying
some others.
The changes acquired or accumulated in an organism during its life time by the
use and disuse of organs or by the influence of the environment are called
acquired
characters.
2. Effect of Environment: Changes in temperature, light, medium, food, etc.,
influence the functioning and behaviour of living organisms and introduce
changes
in their structure. It means organisms acquire new characters due to direct or
indirect
influence of environment.
3.2.1.3 Inheritance of Acquired Characters

188. The characters acquired by an organism during its life time are passed on to the
next generation. In every generation some new characters are acquired or the
older ones keep on increasing or improving. As a result, after a number of
generations, the species becomes different from the parental one and evolves
into a new one.
3.2.2 Evidences in Favour of Lamarckism
3.2.2.1 Evolution of Long Neck in Giraffe

190. According to Lamarck, giraffes have evolved from some deer-like ancestors who
had short neck and forelimbs and grazed on the grass . As the climate of this area
gradually became arid, its rich vegetation was replaced by a few high trees. The
lea ves of these high trees were the only food available to the ancestors of
giraffe. For obtaining food from the tall trees, they had to continuously stretch
their neck and forelegs. The continuous stretching of neck to reach the tree
leaves resulted in gradual elongation of neck and forelimbs . The increase was
transmitted to the members of next generation, in which further elongation
occurred due to similar efforts. This, in due course of time, resulted in the
present day long neck of giraffe.
3.2.2.2 Evolution of Snakes
According to Lamarck , the ancestors of snakes were limbed and lizard-like.
They lived in thick forest. Out of fear of mammals, who were more powerful ,
the snake's ancestors started creeping on forest floor and living in narrow
crevices or burrows. For creeping among the vegetation or burrowing in the
narrow crevices , they stretched their body, which gradually became elongated.
The legs which were of no use in creeping and burrowing but were a hindra nce
gradually disappeared.
3.2.2.3 Evolution of Aquatic Birds
Aqu atic birds like ducks, swans and geese, have arisen from terrestrial ancestors
by developing web between the toes for wading in water (adaptation to aquatic
mode of life). Web developed because the ancestral forms had to spread their
toes and stretch the skin between them to rest on water. Reduction in the size of
wings occurred due to their continuous disuse.
3.2.2.4 Evolution of Feet in Horse and Deer
Ancestors of modern horse lived on soft ground in jungles. When jungles were
re- and deer had to graze on hard grass and to Modern giraffe with long neck
placed by dry grassy plains, the horses

192. A B c o
E FIG. 3.1: Evolution of long neck in giraffe according to Lamarck : A. Ancestor
with short neck able to eat grass; B. Neck lengthens a little by stretching in an
effort to reach leaves; C. Offspring with slightly longer neck; D. Offspring
lengthens neck further by stretching as leaves from lower branches are
consumed ; E. Very long neck after a number of generations.
walk on dry land. These changes in habit and habitat were accompanied by
changes in the molars and premolars, reduction in the number of digits and
lengthening of the legs. The foot posture gradually changed to unguligrade
which was suited for swift running over hard ground.
Some Other Examples
• Clasping birds have developed sharp and curved digits due to constant perching
on the twigs or branches of the trees.
• Eyes are reduced in moles because they live underground in burrows.
• Muscles of pinna are reduced in man but are well developed and functional in
rabbit, dog, elephant, etc., because they live in jungles and use their pinna to
collect sound waves from the surroundings.
• Biceps muscles in the right arm of blacksmith are more developed because of
the continuous use of
• his arm.
• Flightless birds (Kiwi of New Zealand, Ostrich from Africa and Emu from
Australia) are believed to have descended from flying birds. When they first
reached New Zealand, they were able to fly. On settling there, they did not find
any need to fly as there were no enemies on land and there was plenty of food. In
due course of time, they lost the ability to fly and wings got degenerated due to
disuse .
• The evolutionary theory of Lamarck was warmly supported by Etienna
Geoffroy (1772-1844), Professor of Geology at the Faculty of Science at Paris.
3.2.3 Criticism of Lamarckism
Lamarckian theory was exposed to severe criticism and Lamarck had to defend it
throughout his life. Cuvier and Weismann were the great critics of Lamarckism.
Some objections which even Lamarck could not answer were :
1. Tendency to increase in size has been noted in many forms, but many times

193. evolution causes reduction in size . For example, in angiosperms, the trees are
primitive and shrubs , herbs and grasses have evolved later and are more
advanced. Moreover, persons constantly busy in reading and writing and using
their eyes more than others often develop impaired sight.
2 . New organs develop when the organisms feel their need is also not true. If the
development of new organ or structure depends upon the desire, why the man
who has long desired to fly like birds has not developed the
wings.
3. Reaction to the environment may have some weightage. Organisms do react to
the environment but environment causes temporary changes in their organisation
and these changes can not be inherited to the offspring. Similarly, it could not be
understood that how use or disuse of an organ will produce a change in its size
and how this change will be inherited to the offspring .

195. 4 . Experiments ha ve discarded the concept of inh eritanc e of ac quired char
acteristics. For example, if one of the parents become s blind or deaf or lame
before producing the offspring, they do not produce blind, deaf or lame
offspring. Mutilations and wounds of parents do not appear in the offspring.
Small feet in Chinese women, piercing of nose and ears in Indian women and
circumcision of penis in Jews and Muslims are age old customs, yet these have
no hereditary effect. The strong muscles of a wrest ler are not inherited by his
children.
5. August Weism ann (1904), in his famous experiment , cut off the tails of rats
for about 80 generations, but tailless offspring were never born.
6. Pavlov, a Russian physiologist, trained mice to come to food on hearing the
bell. But training was not inherited.
7. 'Theory of continuity of ger mpIasm' proposed by Weismann (1892) and Men
de l's laws of inh eritance were hard blow to Lamarck 's theory of inheritance of
acquired characters. According to germplasm theory, each organism has two
types of cells:
• Germ cells, which are found in gonads and pass on their hereditary material to
next genera tion.
• Soma tic cells, which form the body and do not pass their hereditary material to
the offspring.
The environment, and use and disuse of organs affect somatic cells only. This
means acquired characters are restricted to somatic cells alone and do not
influence germ cells. As such these can not be inherited. Lamarck's theory of
evolution is now considered as an erroneous assumption.
3.2.4 Neo-Lamarckism
Acc ording to Neo-Lamarckians , mainly T. H. Morgan and Cope, the acquired
characters which become incorporated in the germplasm are heritab le and
accumulate generation after generation resulting in the origin of new forms or
new species.
3.2.5 Experiments in Support of Inheritance of
Acquired Characters
The fo llowing experim ents were carried out in support of Neo-Lamarcki sm: I.
Bonner carried out numerous transplantation experiments within native and

196. unnatural environments and found that variation produced were inherited to their
future generat ions.
2. White mice which were reared at a high temperature (20°-30°C), were found
to develop a longer body, a long tail and longer hindlimbs. This character was
found
to be transm itted to the offspring generation after generation.
3. Tower exposed some potato beetles to abnormal conditions of temperature and
humidity at a stage when their reproductive organs were developing. The
offspring
of these beetles showed colour variation and these were passed on to offspring.

198. 4. Brown Scquared described that certain diseases like exophthalmia,
haematoma and dry gangrene are caused by injuries in the restiform body of the
brain and are inherited to the offspring.
5 . McDougall trained rats to follow certain 'escape routes' from a tank of water
and the training was given for about 45-50 generations. It was found that there
was a decrease in the number of errors made in learning the prob lem generation
after generation.
6. Cells exposed to X-rays or treated with certain chemica ls (like colchic ine,
mustard gas, etc.,) develop changes in chromosome structure (chromosomal
aberrations).
3.2.6 Neo-Lamarckian Explanation
Neo-Lamarckians explain the above observations as follows:
3.2.6.1 Formation of Germ Cells from Somatic Cells
In case of a sexual reproduction and vegetative propagation, germ cells are
derived from the somatic cells. These have chromosomes and genes, simi lar to
the parent. For example, plants raised from tubers, stem cuttings or from leaf
buds or underground stem inherit genes and chromosomes of the parent plant.
Driesch raised comp lete embryo from a part of egg or from isolated early
blastomeres.
3.2.6.2 Effect of Environment on Germ Cells through Somatic Cells
Heslop Harrison demonstrated that a pale variety of moth , Selonia bilunaria,
when fed on manganese-eoated food , a melanic variety of moth is produced.
This breeds true to its colour showing permanent change in the genotype and
phenotype of offspring. He also observed a melanic variety of this moth in areas
where food plants were infected with manganese salts from the industrial smoke.
The influence of manganese is through somatic cells on to the germ cells .
3.2.6.3 Direct Effect of Environment on Germ Cells
Tower exposed young ones of potato beetle to abnormal conditions of
temperature and humidity. The heat treatment did not produce any somatic
change in the beetles themselves, but their offspring showed marked colour
changes in the next generation. These changes were passed on to the succeeding
generations.

199. Muller demon strated the role of X-rays in producing heritable variation in
Drosophila by changes in the chromosome structure. C. Auerback in
collaboration with Robinson and Carr produced heritable changes in Drosophila
by using certa in chemical mutagen, like mustard gas. On the basis of these
observations, Neo-Lamarckism proves that:
I. Germ cells are influenced by the environmental changes.
2. Germ cells may carry acquired or somatic variation to the offspring
(Harrison's experiment).
3. Germ cells may be affected directly by the environmental factors (Tower's
experiment).

201. 4. Somatic characters are the result of interaction between genes and
environment. It means environment does affect the gene expression.
3.3 DARWINISM OR THEORY OF NATURAL SELECTION
Charles Darwin , the author of ' Origin of Species' was a born naturalist. He is
known for the most plausible and testable explanation about the mechanism of
evolution of life on Earth, known as 'The Theory of Natural Selection'.
Darwins's contribution was two fold:
• He developed the concept of descent with modification and provided a large
number of evidences to support it.
• He perceived the concept of Natural Selection as the primary force behind
descent with modification.
Darwin's concept of descent with modification and Natural Selection was the
outcome of information gathered from following sources:
3.3.1 The Voyage on Beagle
Charlas Darwin (1809-1882) was born in Shrewsburg, in Western England. He
was a born naturalist. Even as a boy he loved to read nature books or fishing,
hunting

203. BindloelMarchena Towe r/ Genovesa
Santiago/James
FIG. 3.2: A. The route followed by HMS Beagle during its voyage of the world
for exploration and collection; B. Galapagos
Islands of oceanographic and biological information around the world.
and collecting insects . Darwin's father, who was a physician could see no future
for his son and sent him to medical school. But Darwin found medicine boring
and quit it to join Cambridge University to become a clergyman.
At Cambridge, Darwin became favourite of botany professor , John Henslow,
who paved his way for voyage on HMS Beagle as a naturalist.
In December 1831, Darwin got an opportunity to travel by HMS Beagle for a
voyage of world exploration, planned by British Admiralty. The voyage lasted
for five years. During this period (1831-36), the ship visited some of the islands
of Atlantic Ocean, many points on the coast of South America, Southern tip of
Africa, South East Asia, and some islands of South Pacific. Darwin carefully
noted the flora, fauna and geology of these islands and collected numerous living
and fossil specimens.
He described his observations in his book 'Naturalist Voyage around the World'.
3.3.2 Darwin's Observations
Darwin made following important observations:
3.3.2.1 Geographical Variation in Animals and Plants
• Different islands which are widely separated but have similar climate and
topography have different flora and fauna. For example, plants and animals
living in temperate regions of South America are much different from plants and
animals from temperate regions of Europe. They closely resemble to animals and
plants of tropical forests of Brazil and the grasslands of Argentina.
• The plants and animals living on nearby islands are related but differ amongst
themselves and from those found on the mainland.
• When Beagle sailed to Galapagos Islands, about 600 miles (i.e., 900 km) away
from the west coast of South America, Darwin found a living laboratory of
evolution. He came across giant tortoises, recognisably different on each island.
He saw strange iguanas, mocking birds, penguins and cormorants, etc.
• On Galapagos Islands, Darwin identified 13 species of finches, each species
occupying a

204. Albemarle
FIG. 3.3: Three species of tortoises of Galapagos Islands.
different island. They differed

206. from each other as well as from the finches present on the mainland of South
America, from which they all are supposed to have evolved. These birds are
popularly known as Darwin's finches (Fig . 3.4).
Darw in never imagined that islands, about 80 or 90 km apart and most of them
in the sight of each other, formed of the same rocks, having the similar climate,
rising nearly to the same height, would be inhabited by different animals. It was
found to be true for animals as well as for plants. The plant genus, Scalesia, is
restricted to the Galapagos, Out of its six species, each was restricted to a single
island.
Darwin presumed that Galapagos Islands were volcanic islands. They got
colonised by animals and plants that migrated from mainland of South America.
On these islands, the ancestral forms diversified into separate species in response
to different environments on different islands.
Tree Finches
M edium
insectivorous tree finch
Vegetarian tree finch
S mall
insectivorous tree finch
Large
insectivorous
tree finch
Woodpecker-type finch Mangrove finch
Ground Finches

208. S mall ground finch
Large cactus
ground finch
Medium ground finch
Cactus ground finch
Sharp-beaked ground finch
FIG. 3.4: Different types of finches on Galapagos Islands. The beak of these
finches is adapted for gathe ring and eating different types of food. The tree
finches have beaks adapted for eating insects and ground finches have beaks
adapted for eating seeds of cacti.
Theories of Evolut ion Iil 79 3.3.2.2 Malthus Essay on Human Population
In 1838, Darwin went through the essay on 'The Principles of Populations' by
Thomas Malthus. He documented that rate of reproduction in both animals and
plants is very high and the population tends to increase many times more rapidly
than the available food supply. The food supply increases in arithmetic ratio, but
the population increases in geometric ratio. This leads to competition. The
concept of competition among the living beings for their survival offered the
basis of Theory of Natural Selection.
3.3.2.3 Artificial Selection by Animal and Plant Breeders
Darwin was familiar with the selection practice carried out by plant and animal
breeders for certain desirable traits in domesticated plants and animals. Darwin
himself was a pigeon breeder and he knew that the diversity in colour, size, form
and behaviour in pigeons could be achieved by breeders by artificial selection.
He saw close parallelism between artificial selection by breeders and selection in
nature.
3.3.3 Perceiving Natural Selection
Arriving back to England in October 1836 , Darwin started analysing his notes
and by early 1840 worked out the major features of his hypothesis. In 1844, he
wrote a long essay on descent with modification and underlying mechanism of
natural selection. But he was reluctant to publish it and continued to compile
more and more evidences to support his hypothesis. When Darwin was busy in
giving shape to his hypothesis, he received manuscript for evaluation from

209. Alfred Russel Wallace, another naturalist from Malaya Archipelago in June
1858. Wallace had developed a hypothesis of natural selection nearly identical to
Darwin's theory of natural selection. The paper named 'The Tendency of
Variation to depart from Original Type' contained complete detail s of the theory
of 'Origin of Species by means of Natural Selection ' .
On June 30, 1858 , Charles Darwin and Wallace had presented a paper jointly to
the Linnaean Society of London in which they used the term 'evolution' to
describe the progressive changes in successive generations of living organisms.
The theory of evolution helped to explain the following two things :
• The similarities between related organisms are due to their descent from a
common ancestor.
• The differences between them are the result of variation inherited from one
generation to the next.
The theory of evolution by natural selection announced jointly by Darwin and
Wallace in 1858, made little impact on the world of science. Finally, in 1859,
Darwin published his observations and conclusions under the name 'Origin of
Species'. Darwin's ideas were well received by many scientists and by a large
group of public . There were debates and discussions, but due to ill health,
Darwin could not

211. participate in them . The defence and support of the theory was taken up by
Joseph Hooker and Thompson Huxley.
3.3.4 Basic Postulates of Darwinism
Darwin's theory of natural selection is based upon the following five
fundamental propositions:
3.3.4.1 Overproduction or Rapid Multiplication
All living beings have an inherent tendency to produce offspring of their own
kind in large number for the perpetuation of their race. This is called geometric
increase. The number of their offspring produced is much more than can be
supported by a particular environment and can possibly survive.
EXAMPLES:
• Bacteria multiply after every 20 minutes. Within 24 hours , a single bacterium
will produce 4.7 x 10
27
bacteria weighing about 2,000 tonnes.
• A Paramecium divides three times in 48 hours. If all its offspring survive and
multiply, in five years the mass of paramecia will be equal to ten thousand times
the mass of earth.
• A codfish produces over a million eggs in a year. If all the eggs develop into
fishes, the whole Atlantic Ocean will be full of codfish in five years.
• Oyster lays about 1,14,000,000 eggs in a single spawn. If all the eggs develop
and young ones survive, after five generations , there would be more oysters
than the estimated number of electrons in the visible universe.
• Elephant which is the slowest breeder, starts breeding at the age of 30 years
and during its life time of 90 years produces only six offspring. If all the young
ones survive, a single pair of elephants would produce 1,90,000 elephants in 750
years.
• A rabbit produces about six young ones in a litter at one time, four litters per
year and the young ones start reproducing at the age of six months.
• A single evening primrose plant produces average 1,18,000 seeds while a fern
produces few million spores.
Regardl ess of the rate of reproduction of a species, its number remains roughly
constant under a fairly stable environment. For example, in the life history
ofFasciola (liver fluke), the adult fluke produces thousands of eggs per year
which escape out of the host faeces. Further development of eggs is possible

212. only if these get suitable environment. The miracidium larvae hatching out of the
eggs can survive in water only for a short period. Their further development is
possible only if they get the secondary host, snail (Limnea or Planorb is), and
are able to reach the pulmonary sac. During their development inside snail body,
they have all the chances of being exterminated along with snail which forms
food of large number of carnivorous animals. Finally, the cercaria larvae coming
out of the snail body are exposed to their enemies, like-insects, crustaceans and
others. Their metamorphosis into the adult is possible only when they are eaten
up by the sheep. As a result, only few of the thousands of eggs reach up to the
adult stage.
Limited Resources
Spac e in the Universe is constant. The ultimate source of food for plants and
animals also remain s constant. Therefore, the carrying capacity of the
environment does not allow the population to grow indefinitely. Inspite of
enormous reproductive potential of living beings , under natural conditions, the
number of individuals of each species remains nearly constant over a long period
of time.
3.3.4.2 Struggle for Existence
According to Darwin, individuals multiply in ' geometric ratio ' whereas space
and food remain almost constant. Organisms face a three-fold struggle to ensure
living, to obtain maximum amount of food and suitable shelter. The three-fold
struggle operates at following levels:
I. Intraspecific Struggle: The intraspecific struggle is the competition among
the individuals of the same species or in closely related forms to gain the upper
hand. This type of struggle is very intense, because the need and approach of all
the competing organisms is precisely the same . Example of intraspecific
struggle is offered by the young trees in a forest. Their seedlings spring up in
cluster s. Some of them die soon due to lack of sufficient soil, moisture or food.
But many of them are able to grow and their tall branches begin to mingle
blocking light and air of weakling s.
2. Interspecific Struggle: This is struggle between the organisms of different
species living together. Individual s of one species compete with those of other
species for similar requirements, i.e., food, shelter and breeding places . Arnold

213. observed an everlasting struggle for food in the form of predator-prey interaction
and described it as 'Each slew a slayer and in turn was slain'. A lizard feeds on
ants and is preyed upon by snakes , and kites feed on both.
3 . Struggle with the Environment: This is the struggle of living forms against
extreme heat or cold, against excess of moisture or drought, against lightning ,
storms , earthquakes and volcanic eruptions or shortage of food, water and
oxygen , lack of light, presence of predators, absence or shortage of shelter and
space, etc. For example, in North and Central States of America, a severe winter
with late snow cuts off food supplies and causes extermination of quails .
Likewise dinosaurs at the end of Mesozoic Era became extinct due to
environmental catastrophe.
3.3.4.3 Variation and Heredity
The everla sting competition among the organisms has compelled them to
change according to the conditions so that they can utilise the natural resources
and can survive successfully. Therefore, it is difficult to find out any of the two
individuals alike. Even the progeny of the same parents are not exactly alike in
all respects. These differences are known as variation. Without variation
individuals of a population will be identical and there will be no possibility of
any selective advantage and possibility of evolution to occur. All the variation
are not significant from
Reproductive
fecundity
(Tendency for geometric
increase in number )
Struggle for+ existence
Environmental
(Competition)stress +
(Limi ted resources )
Heritable
variation
(In s tructure and behaviour) Natural selection
More offspring by fittest
organisms (Persistence
of adaptiv e traits )
Evolution
+
Environmental
Change in the genetic cha
nges
makeup of population by
n atural selection (Adaptation) FIG. 3.5 Graphic representation of main conceptual

214. explanation for evolution by natural selection as given by Darwin .
evolutionary point of view becau se some of them are changes caused
temporarily in some organisms and are not inherited to the offspring. The
variation which once appeared in the parent generation, continue to appear in the
progeny generation after generation forming raw material for evolution. These
variation are now known as her itable var iation. The dual or biparental heritage,
mechani sm of cell division at the time of gamete formation (i.e., meio sis),
effect of X-rays or other radiation s and other atmospheric chan ges introduce
changes either in the gene s or the chromosomes of the germplasm, and are
respon sible for producing variation.
Va riation may be harmful , neutral or useful. Harmful variation will make its
possessor unfit in the struggle for existence and will lead to its extermin ation.
For example, a change leading to chloroph yll deficiency in a plant will result in
lower rate of photosynthesis and weak ening of plant. Such weak plants will
naturally be weeded out. On the other hand, individuals possessing useful
variation will be more success ful in the struggle for existence. For example,
developm ent of thick exoskeleton in terre strial organi sms protects them against
water loss. These animals are better suited to terrestrial life than those with soft
skin because of the danger of dessication.
3.3.4.4 Natural Selection and Survival of the Fittest
During th e struggle for existence
onl y tho se individuals could
s urv iv e w h ic h exhibit such
va ria tion that are proved to
be mo re beneficial
th e hard ships and
in facing
ri gours of
env iro nme nt i.e. , which are
better adapted to the changing
cond ition s of the environment.
Those individuals or races which
cannot tolerate these hardships
stand no chance in the struggle FIG. 3.6: Variation in population of giraffe and

215. for existence . ln Darwin 's word s natural selection favouring the long neck .

217. the nature appears saying, " Thou art weighed in the balance ofnature and art fo
und wanting".
Darwin b elieved that as man selects domesticated plants and animal s with
desired characters, the nature selects or favours only those individuals which
have more favourable variation and are better adapted to the existing and chan
ging environment. The less fit or unfit organisms are left out in the struggle for
existence. For example:
• In a herd of herbivores, only the weak are unable to flee fast or which are less
alert are attacked by the predators.
• In deserts, plants which are able to avoid loss of water by transpiration or
which have long roots to absorb water from deeper layers of soil are able to
survive the drought period.
• Only those animals are able to survive and live in very cold region s of Tundra
which have thick fur to conserve body heat.
• Darw in illustrated the phenom enon of natural selection and survival of the
fittest by taking example of Lamarck 's giraffe. According to Darwin, ancestors
of giraffe exhibited great variation in the length of their neck and legs. Since
grass was scarce on land they had to eat leaves from tall trees.
-iM:jl¥il Differences between Lamarckism and Darwinism
Properties Effect of environment
Lamarckism
Environmental effect is the only factor for evolution.
Use and disuse of orga ns
Arrival/survival of the fittest
Inheritance of acquired
Overpr oduction and struggle for existence Organs which are used more often,
become more developed and organs which are not used, get vanished.
It considers the origin and arrival of the fittest.
It considers the inheritance of characters acquired characters.
Lamarckism is silent on this topic.
Variation Lamarckism considers

219. only the adaptat ions for changing environment. It is silent on other sources of
variation.
Darwinism
Besides environment,
other factors, such as gene recombination, mutation, genetic drift also play
important role in evolution. It does not say anything about the use and disuse of
organs.
It considers only the surv ival of the fittest.
It considers the inheritance of only useful variation.
Darwinism says that all organisms produce more offspring than can possibly
survive. Due to limited food and space these offspring struggle for their
existence. Darwinism considers all sources of variation and that variation are the
ultimate cause of evolution.
Naturally, giraffes with long neck and longer legs had advantage over those with
shorter legs and neck, because they could get food more easily and had better
chances of survival. These forms could fed and reproduce more successfully and
became abundant. On the contrary, giraffes with short neck starved and died, and
gradually became extinct.
3.3.4.5 Origin of Species
From above observations Darwin made certain conclusions and summarised
them under the heading "Origin of Species by Natural Selection" as follows:
As a result of struggle for existence , variability and inheritance, the successive
generations tend to become better adapted to their environment. These
adaptations are preserved and accumulated in the individuals of the species and
ultimately lead to the origin of new species from the old ones. The environment
is ever changing and it leads to further changes and the appearance of new
adaptations in the oganisms. As natural selection continues, the latter
descendants after several generations become markedly distinct from their
ancestors. Furthermore, certain members of a population with one group of
variation may become adapted to environmental changes in one way, while
others with a different set of variation may become adapted in a different way.
As a result two or more species may arise from a single ancestral species.

220. Origin of new species by natural selection as envisaged by Darwin can be
summed in the form of the following Wallace's chart.
_
• ,.:4:II••J Wallace's Chart Facts
A. 1. Rapid multiplication
2. Limited space for shelter
3. Stability of populations
B. 1. Struggle for existence
2. Variation and heredity
C. 1. Survival of the fittest
2. Continued changes (adaptations) Inference
Struggle for existence
S urvival of the fittest and natural selection
Origin of new species
3.3.5 Evidences in Support of Darwinism
Darwin's theory of natural selection is today recognised as the main factor in the
evolution of organisms. Following evidences support Darwin's theory of natural
selection.
1 . Artificial Selection: There is close parallelism between natural selection and
artificial selection. By controlled breeding and natural selection for several
generations, many new varieties of plants and new species, races or breeds of
domestic animals have developed. New races of dogs, horses, pigeons, poultry,
sheep, pigs, goats, etc., have been developed by man by artificial selection. If
man can produce new varieties in a short time, nature with more time and vast
resources can easily produce new species by selection.

222. Darwin's Theory
Ancestors of giraffes had short neck that they stretched to reach food.
Ancestors of giraffes had necks of various lengths.
The ir offspring had longer neck that they again stretched to reach food. Natural selection due to
competition supported the survival of longer-necked giraffes and their offspring.
The continued stretching of neck resulted in long neck in toady's
giraffe
A
Only long-necked giraffes survived
B FIG. 3.7: Evolution of long neck and long legs in giraffe as suggested by A.
Lamarck 's theory of inheritance of acquired characters and B. Darwin's theory
of natural selection.
2. Mimicry and Protective Colouration: Mimicry and protective colouration as
seen in certain animals can be achieved only by gradual changes occurring side
by side both in the model and mimic occupying the same area.
3 . Coevolution: The joint evolution of two interacting species, in responce to
solution imposed by one over the other is called coevolution. This coevolution is
very common in flowering plants where position of nectaries in flowers and
length of proboscis of pollinating insects show corresponding increase. Darwin
suggested that a race between flower and insect leads to corresponding
modification between the length of floral tube and length of insect's proboscis to
ensure pollination for flower and availability of nectar for insect.

224. 3.3.6 Criticism of Darwinism or Objections to Darwin's Theory of Natural
Selection
Darwin 's theory was so reasonable and well documented that most biologists
soon accepted it. But it was attacked by certain clergymen and certain scientists
such as Sir Ric ha r d Owen and Adam Sedgwick.
The following objections were made against Darwini sm:
1. Darwin's theory stresses upon sma ll fluctuating variation, which are to a large
extent nonheritable and can play no part in the process of evolution.
2. It does not explain the effect of use and disuse and the presence of vestigial
organs.
3. Darwin did not differentiate betwe en somatic and germinal variation and
considered all variation as heritable.
4. Many of the differences between the specie s are not of adaptive value but are
simp ly incidental effects of genes. How and why such incidental features are
preserved, accum ulated and inherited?
5. Why are the inherited characteristics not diluted when the organisms posses
sing them breed with others without them ?
6. Darwinism exp lains the survival of the fittest but not the arrival of fittest.
7. Natural selection does not account for the beginning of organ or how an organ
can be useful in initial stages.
8. It does not exp lain overspecialisation of a particular structure, which in some
cases has led to the extinction of its possessors like antlers of extinct Irish deer
and large specia l tusks of Jefferson mammoth.
9. It could not tell whether instinct s are acquired and modified through natural
selection or not.
10. Natural selection does not explain the origin and development of coordinated
sets of structures and coadaptations, whose effectiveness depends upon
perfection. Adapt ations such as elec tric organ s of fishes, mimicry in Kallima
butterfly, etc., cannot be explained by Natural Selection.
II. Darwin proposed 'ar tificial selection' for improving races of dome stic plants
and animals but these could never lead to permanent specific variation. 12.
Natural selection does not explain the evolution of terrestrial animals from
aquatic forms .
13. Specific objection was raised to the theory of sex ual selection proposed by
Darw in. The theory involves passivity on the part of male and an active choice
on the part of fema le for a more beautiful, attractive and more powerful male.

225. 3.3.7 Supplementary Theories of Darwin
Darwin proposed three additional theories to exp lain certain objections against
the theory of Natural Selection. Thes e are:

227. 3.3.7.1 Theory of Sexual Selection
In some animal species, ma les differ from females of the same species in their
morphology. Their males possess certain specialised characters for attracting the
mate for reproduction . These are called secondary sexual cha racters. Some of
the examples are:
(a) brilliantly coloured feathers in male peacock
(b) antlers in male deer
(c) beautiful and brightly coloured bands on the body of male tiger
Lion

229. Wild boar
Dear
Elephant
FIG. 3.8: Secondary sexual characters in males of some of the mammalian
groups.
(d) mane in male lion
(e) moustache and beard in man
if) nuptia l pads of male frogs
(g) vocal sacs in male frogs
(h) brood pouch in male sea horse
(i) scent glands in male mammals
(j) musical notes produced by male birds and certain insects.
Darwin proposed sexual selection theory to explain the occurrence of these
characters . Because females select the male partner for mating, so males have
developed various characters to attract females . It suggested that males which
are provided with attractive characters are selected by females for mating. There
is competition among males to get the females. This competition leads to
struggle or fight among the males of certain species .
Salient Features of Theory of Sexual Selection
1 . In almost all species, males are more numerous than females . This gives an
opportunity to female for selecting the male.
2. Males are more attractive in most mamma ls and birds.
3. Males are provided with brilliant colours, conspicuo us ornamentation,
pleasing behaviour, mating dances, melodious songs and sounds. Females lack
such features.
4. There is competition, quarrel and fight among males for the possession of
females.
5. Sexual selection operates only on one sex (i.e., males) of the species , but
natural selection operates equally on both the sexes.
6. Sexual selection is associated with reproduction while natural selection is
concerned with the survival of the individual.
3.3.7.2 Darwin's Theory of Pangenesis
To explain the in heritance of characteristics from parents to the offspring ,
Darwin proposed the "Theory of Pan genesis" , which is now totally discarded.

230. The theory can be briefed as follows :
• All the somatic cells of the body produce minute particles, the pan gen es or
gemm ules which are carried away by the blood stream and are accumulated in
the germ cells.
• Each gamete or germ cell represents a minute replica of the parent's body and
is capable of developing into the adult with minutest details.

232. • In the developing individual each pangene or gemmule regulates the
development of the organ from which it originated from the parent.
• Gemmules are produced continuously at all stages of development.
• Sometimes certain gemmules might lie donnant for several generations and
then develop. This results in the appearance of characteristics in the offspring
which were present in their ancestors (atavism) and are not seen in the parents.
Therefore , according to this theory every somatic cell produces gemmules and
the actual germ cells are the sites of collection of gemmules coming from
different somatic cells. The theory is of historical importance only, since the
detailed structure of sperm and ovum has been studied and the entire process of
their formation is clearly evident.
3.3.8 The Experimental Support to Theory of Natural Selection
Darwin's theory of natural selection was accepted because of its direct approach
and practical nature. The staunch supporters of Darwinism were Wallace,
Thomas Henry Huxley, Ernst Haeckel, August Weismann, J.B.S . Haldane,
Sewall Wright, H.J. Muller, T. Dobzhanskey, Kettlewell , Herbert Spencer and
Mendel.
Many biologists carried out experiments to provide support to Darwinism. These
supporters of Darwinism are known as Neo-Darwinians. They introduced a
number of new facts to make the idea of natural selection more conceivable.
Some of the experiments conducted in this series are as follows :
1. Weldon's Experiments with the Shore-Crabs of Plymouth Sound: Weldon in
his experiments with shore-crabs placed a large break near the mouth of
Plymouth Sound River. This slowed the rate of flow of river water and China-
clay deposition increased. This caused the death of numerous crabs. The
survivors had a slightly narrow frontum and there was a progressive narrowing
of the frontum in succeeding generations. This shows that natural selection
operates upon minute fluctuating variation, and under changed environmental
conditions, it favoured narrow frontum in crabs helping them in their survival
and perpetuation.
2. Cesnola's Experiments with Mantis: The role of natural selection was
illustrated by Cesnola in Mantis religiosa by fixing them on plants . Those

233. having colour markings harmonious with the plants survived, whereas all others
were eaten up by the birds.
3. Poluton's and Sander's Experiments with Butterfly Pupae: The survival value
of protective colouration was also exhibited by Poluton's experiment. The
numerous pupae of butterflies with different colours were placed under
conditions which favoured protective colouration. Some of them were also kept
in non-harmonious background. The protective colouration was found to have a
survival value.
4. Davenport's Experiment with Chicken: Chickens with black-white, barred and
checkered colour pattern were left in the field. It was found that the chickens
with plain colours were killed by hawks while those with barred and checkered
colour pattern were spared because they were inconspicuous from the
surroundings.
5 . Industrial Melanism in Peppered Moth: Industrial melanism was studied by
R.A. Fisher, E.B. Ford and H.B.a. Kettlewell. Their study was based on
peppered moth, Biston betularia . The original population of this moth had
mottled grey wings

235. that blended perfectly with lichen-covered tree-trunks in Great Britain . Until
1848, these non-melanic forms nicely blended with surrounding and were
protected from the predatory birds. With dramatic rise of industrialisation in
Europe (specially Manchester), the atmosphere became sooty and tree trunks
became dark in colour due to the absence of lichens. One black moth appeared in
Manchester in 1845. This is called carbonaria or melanic form. In 1848, this
black variety of moth formed 10% of the moth population, but by 1898 it formed
99% of the total moth population. Genetic studies show that melanism is
produced by a single dominant gene and it follows typical Mendelian
inheritance. The change in the gene, genotype and phenotype frequencies of
carbonaria appears to be correlated with the spread of industry and change in the
surroundings. This can be explained as follows :
The grey or light coloured moths in soot-covered area were easily located by
predators and preyed upon. The black coloured moths matched and camouflaged
with the soot-covered surroundings and were able to escape from their predators.
Thus natural selection operated and favoured the black moths.
To pro ve that natural selection was the cause of changed gene frequency,
Kettlewell bred large stocks of the non-melanic and melanic varieties of
peppered moth. These were marked and released in two areas:
• The nonpolluted rural area of Dorset
• The highly polluted area of Birmingham
He filmed insectivorous birds feeding in the two localities and also recaptured
the
surviving marked moths. The results of one such experiment are given in Table
3.3.
Industrial melanism , therefore, presents an excellent example of natural
selection. This has been observed in a wide variety of moths (about 70 species in
Great Britain and about 100 species in USA) in addition to peppered moth.
Natural Selection of Pe ppe red Moth in Two Loca lities (From Marshal/1978)
Nonmelanic Melanic
6. Resistance to DDT: In agriculture and also in houses DDT or some other
insecticide is sprayed to control insect pests. But none of the insecticides brings

236. about 100% death of any insect species. Some insects are resistant to insecticides
and are able to survive. The resistance to DDT is a character controlled by genes.
This gene is present in the population of the species but in the absence of DDT
has
P lace
1. Dorset 1955
(Unpolluted)
1. Released
2. Recaptured
3 . % of recaptured 2. Birmingham 1953
(Polluted)
1. Released
2. Recaptured
3. % of recaptured 496 473 62 30 12.5 6.5
137 447
18 123
13.1 27.5
no selective value. On DDT spraying, the insects with DDT resistant gene are
able to survive while others die. The insect population with DDT resistant gene
evolved due to selective advantage against insectic ide.

238. 3.3.9 Neo-Darwinism
In th e last 25 years, a number of new facts have been added to the knowledge of
evolution, and theory of Natural Selection has been reanalysed in the light of
genetics, Mendelism, population genetics and biological species concept. This is
called Neo-Darwinism.
N eo-Da rwinism is the merger of Darwinian selection and genetic theory. This
concept evolved after a book published by Julian Huxley ( 1942) entitled '
Evolution: The Modern Synthesis'. Theodosius Dobzhansky reviewed Darwin
ian concept of evolution by Natural Selection in Mendelian populations. In his
book ' Genetics and Origin of Species' (1937), he presented chromo somal
studies of Drosophila populations and interrelation among its different species.
E.B. Bobcock provided botanical support to the ' Neo-Darwinian theory' by
studying plant genus, Crepis. Stebbins (1950) provided an account of 'Variation
and Evolution in Plants'. Rensch (1947, 1960) showed that phenomena of
microevolution were adequate to explain macroevolution . According to Neo-
Darwinism, origin of species can be explained on the basis of:
• genetic variability in population or variation in the gene pool of populations,
introduced by mutations, variation and heredity
'''-·HI¥MI Differences between Darwinism and Neo-Darwinism
Darwinism
1. Theory of origin of species was proposed by Darwin.
2. Mechanism of occurrence of variation was not explained.
3 . Darwin did not differentiate between somatic and germinal (non-heritable and
heritable) variation.
4. All useful variation are heritable.
5 . Individuals were considered as units of evolution .
6. Darwin had no idea about genes, genome and gene pool.
7. Natural selection operates through survivaI of the fittest.
8. Did not recognise isolation and its role in evolution .
9. Genetic drifts were not known to Darwin .

240. Neo -Darwinism
1 . Theory modified by Darwin's support- ers to explain mechanism of origin of
species based on latest information .
2. Theory expla ins various factors and mechanisms for the appearance of
variation in the members of a population.
3 . According to Neo-Darwnism , variation may be heritable or germinal and
non-heritable or somatic.
4 . Only germinal variation are passed on to the next generation.
5. Populations are the units of evolution .
6. The gene pool of the population evolves.
7. Natural selection operates through differential reproduction.
8. Isolation of a population or species into smaller groups is essential for the
origin of new species.
9. Role of genetic drifts in introducing variation in the gene pool was em-
phasised in case of small and isolated populations.
• significance of genetic variability or adaptation
• natural selection, and genetic drifts
• isolation
• origin of new species
3.4 MUTATION THEORY OF EVOLUTION
Muta tion theory for the origin of new species was proposed by Hugo de Vries
(190 I). According to mutation theory, new species arise from the pre-existing
ones in a single generation by the sudden appearance of distinct and
discontinuous heritable changes. De Vries called such sudden distinct heritable
changes as mutations (L. mutare = to change). Darwin called them sports and
Bateson named them saltations or saltatory (discontinuous) variation.
Hugo d e Vries: Hugo de Vries was a Dutch botanist, professor and director of
Botanical Garden, Amsterdam, Netherlands. He was one of the three scientists
who discovered Mendel's laws in 1900. Hugo de Vries observed sudden
appearance of seven new varieties of Evening Primrose (Oenothera

241. lamarckiana) which grew wild near his garden.
These were named:
I. Oenothera gigas: The giant form with large leaves and flowers. 2. 0.
rubrinervis: The plants with red-veined flowers and fruits. 3. 0. nannella: The
dwarf variety, most attractive.
4. 0. laevifolia: Plants with smooth, style and rounded leaves. 5. 0. brevistylis:
With very short style and rounded leaves.
6. 0. oblonga: With oblong needle - like leaves.
7. 0. albida: Plants pale-white without chlorophyll.
According to de Vries , these characters originated suddenly and full-fledged and
were heritable. The plants with new characters were named mutants and the new
characters were called mutations (Gr. mutator = to change).
De Vrie s emphasised that new species arise by sudden changes and not by
fluctuating variation. De Vries called such sudden variation sports or sudden
discontinuous mutations or saltatory mutations.
De Vrie s classified these eight varieties or species into following categories: I.
Progressive Species: These species had developed some new characteristics,
e.g., 0. gigas and 0. rubrinervis. 0. giga s is a giant variety. It has a stout stem
twice the thickness of stem of lamarckiana . Its leaves are large, broad and more
in number. Its flower buds are large and stout and its seeds are large in size.
0. rubrinervis is a red-veined variety. Its fruits also contain red veins and red
streaks. The leaves are thin and more brittle.
2. Retrogressive Species: In these species, there is lossof some parental
characters, e.g., 0. laevifolia, 0. brevistylis and 0. nannella.
3 . Degressive Species: In degressive species, some of the vital characters are
lost, affecting their survival, e.g., 0. albida and 0. oblonga . In 0. albida, the
chlorophyll system is defective and O. oblonga is a weak variety with narrow
needle-like leaves .
4. Inconstant Species: These species do not breed true and often produce new
varieties . For example, 0. scintillans produces flowers of its own variety as well
as of the original variety. 0. lata produces only pistillate flowers . These are
pollinated with pollen from other varieties.
3.4.1 Salient Features of Mutation Theory

242. The main postulates of mutation theory are:
I . Mutations are sudden, large and distinct discontinuous heritable changes. 2.
All organisms have an inherent tendency to change or mutate. 3. Individuals
showing mutations are called mutant s.
4 . Mutations always appear full-fledged . So, there are no incipient stages in the
development of a character or organ .
5. Different mutations may appear in different members of the same population
or same species giving rise to several related new species.
6. Mutations are indeterminate, i.e., they appear in all directions and may be
useful or harmful.
7. Mutations are subjected to natural selection. Beneficia l mutations are selected
and favoured by nature and harmfu l or
lethal mutations are eliminated by the
death of mutants.
8. Mutations may appear again and again
generation after generation. Thus, they
provide more and more chances of
selection by nature .
9. Mutations are large and distinct. They
are different from minor, fluctuati ng
directional variation, which according
to Darwin produce new species .
10. Evolution by mutations is discontinuFIG. 3.9: Short-legg ed Ancon Sheep.
ous and a jerky process.
I I. Mutation theory can exp lain the occurrence of vestigial and even
overspecialised organs .
12. Though rare, mutations are of common occurrence.
3.4.2 Evidences to Support Mutation Theory
After de Vries discovery of mutations, mutant forms were observed in severa l
organisms.
Examples:
I. Appearance of a short-legged Ancon sheep to normal long-legged parents in
1871.
2. Appearance of horn less Hereford cattle from normal homed parents in 1889
in sing le generation.

243. 3. Birth of hairless cats, dogs and mice from normal parents.
4. Formation of seedless variety of banana.
3.4.3 Merits of Mutation Theory
Mutation theory has following signific ant contributions:
• The universal occurrence of mutatio ns explains that mutations are important
for evolution or for the ori gin of new species.
• Mutation theory registers the importance of mutations as raw material for
evolution.
• It explains the cau ses of differences between related species.
• Mutations along with natural selection form the most important creati ve force
for evolution.
3.4.4 Criticism of Mutation Theory
De Vries work was exposed to severe criticism. Some objections were :
• Mutation theory can not explain the discontinuity among indi viduals of
different species.
• Mut ation s do not provide sufficient opportunity for all specialised
adaptations.
• Mutations cannot explain the presence of flightless birds on oceanic islands.
• Mutation s cannot explain occurrence of vestigial organs.
• Darwinists contended that evolution resulted from gradual fluctuating
inheritable differences over a long seri es of gen erations, whereas mutationists
believed in sudden appearance of species differen ces.
• Mutation theory can not explain phenomenon of mimicry (the resemblance
between mimic and model), protective colouration and the relationship between
the position of nectaries in the flowers and the length of proboscis of insect s that
pollinate these flower s. Thi s is ca lled coev olution.
• Most mutations are deleterious, harm ful or lethal , whereas evolution is based
on the usefuln ess of the mutant characters.
• Mutation rate is very low. It is one per milli on or several million gene loci .
How mutations can help in evolution at such a slow pace ?
• B.M. Davis discredited muta tion theory by claiming that Oenoth era
lamarckiana is of hybrid nature which could be produced by crossing two wild
American spec ies.
• Jeffreys also confirmed Davis view. However, extensive cytological studies

244. conducted by Gates showed that mutants of Oenothera are more than hybrid
segregates.
• Blakeslee, working on Datura and T.H. Morgan on Drosophila have shown
that origin of species as described by de Vries in Oenothera is by no means
exceptional but is a common phenomenon among plants and does occur in
animals also.
• Morgan (1909) showed that mutations are of all magnitudes in Drosophila.
3.4.5 Modern Theory of Mutation
According to modem mutation theory :
1. Mutations are universal and occur in all organisms.
2. Mutations occur in all types of cells. The somatic mutations that occur in the
body cells are non-heritable.
3. Mutations that occur in the germ cells are heritable.
4. Only heritable mutations help in evolution.
5. The various agents (chemicals and radiations) that introduce mutations are
called mutagens.
6. Mutations can be neutral, harmless, harmful or beneficial. The harmful or less
useful mutations are gradually eliminated by natural selection.
7. Useful mutations keep accumulating in the gene pool of naturally breeding
populations generation after generation and lead to their divergence.
8. When environment changes, the adaptedness of the inhabitants is disrupted.
The harmony between environment and its inhabitants can onlybe established
again by changing the genotype. The change of genotype is brought about by
mutations. Some mutants may prove useful in new environment and replace the
old normal genes of the population establishing the new adaptive norms .
9. Mutations form raw material for evolution. But alone, they can not produce
new species.
3.5 MODERN SYNTHETIC THEORY OF EVOLUTION OR THE
EVOLUTIONARY SYNTHESIS
In the last 50 years, a number of new facts have been added to the knowledge of
evolution by geneticists, systematists, paleontologists and molecular biologists.
The theory of natural selection has been reanalysed by Mayr (1980) , Smocovitis
(1996) , Dobzhansky, J.B.S. Haldane and Ronald A. Fisher. Their approach led
to 'The Synthetic Theory of Evolution'. The modern synthetic theory of

245. evolution is the merger of Darwinian selection and genetic theory of heredity
and variation. The origin of new species by synthetic theory can be discussed
under following heads, namely:
I. Genetic variability in populations (variation in the gene pool of populations)
and causes of genetic variability
2 . Significance of genetic variability
3. Isolation
4. Natural selection and genetic drift
5. Origin of species
3.5.1 Genetic Variability
Population is a group of individuals of a species living in a given geographical
area and freely interbreeding in nature. The total gene contents of all the
individuals of a population is called gene pool. The members of a population
share the same gene pool and interbreed with one another more preferentially
than with the members of neighbouring sister population s. Thus, they have a '
free gene flow'. Change s in the gene pool of a population diversify it from other
sister populations of a species.
3.5.1.1 Causes of Genetic Variation at Individual Level
Gen etic variation , i.e., changes in the genes or chromosomes of gametes
introduce genotypic differences in the individuals of a population. Genotypic
variation are essential for evolution. These are introduced at two levels:
1 . Gene Mutations: Gene mutations refer to changes in the chemical and
structural composition of genes. These changes may occur by the rearrangement
of nitrogenous bases present in a specific DNA molecule or the addition or
deletion of one or more nucleotides in a DNA segment. These changes are also
called point mutations.
• Gene mutations occur at random. These are non-specific and non-directional.
These may produce drastic changes or may remain insignificant.
• Most gene mutations are recessive to normal gene but dominant mutations also
arise. Recessive mutations are expressed only in homozygous state.
• Most gene mutations are deleter ious or harmful, but not all. A few may prove

246. to be beneficial. The harmful mutations are eliminated by natural selection,
whereas beneficial ones are preserved in the gene pool of the population.
• Usually one gene mutates in every 2,000 gametes . Some genes mutate several
times, whereas some genes do not mutate at all or mutate rarely. Some genes
may mutate back to normal. This is called reverse mutation.
• Mutation rate is influenced by changes in the external or internal environment
of the organisms.
• Mutations occur natura lly but may be artificially induced by several mutagenic
agents , such as:
(a) radiations (X-rays , UV-rays or infra-red rays)
(b) che micals (nitrous acid, mustard gas, colchicine, peroxides, phenol , certain
organic compounds, etc.), and
(c) hig h temperature.
• Mutations introduced artificially are called induced mutations .
2. Mendelian Recombination of Genes: Mendelian Recombination is the
reshuffling or mixing of genes in sexually reproducing organisms resulting in
new combinations of parental genes in the offspring. It occurs in the following
ways:
I . By Fertilisation: In sexual reproduction genes from two different parents
(male and female) come together by the union of male and female gametes (i.e.,
fertilisation). So the offspring receive equal number of genes from each parent.
2 . By Meiosis: Meiosis, at the time of gamete formation, leads to random
assortment of maternal and paternal chromosomes in the gametes.
3. By Crossing Over: Crossing over and exchange of chromosome-segments
between maternal and paternal chromosomes during meiosis results in new
combinations of maternal and paternal genes.
All these changes result in the new combinations of dominant and recessive
genes introducing changes in the genotype and phenotype of the individuals.
3.5.1.2 Causes of Genetic Variation at Population Level At population level
genetic variation are introduced by:
1 . Chromosomal Mutations or Chromosomal Aberrations i.e., Change in the
Structure of Chromosomes
(a) Changes in Number of Genes:

247. (i) Deletion or deficiency: loss of one or more genes.
(ii) Duplication: addition of one or more genes.
(b) Changes in the Arrangement of Genes:
(i) Inversion: rotation of a block of genes in a chromosome at 1800 • (ii)
Translocation: exchange of parts between non-homologous chromosomes.
2. Change in the Number of Chromosomes (Heteroploidy)
(a) Change Involving Entire Sets:
(i) Haploidy: having only one set of chromosomes, i.e., n chromosomes. (ii)
Polyploidy: each set of chromosomes is represented more than twice:
Triploidy (3n) , Tetraploidy (4n), Pentaploidy (5n).
(b) Changes Involving the Number of Chromosomes in One Set of
Chromosomes (Aneuploidy)
(i) Monosomic: loss of one chromosome from one set, i.e., 2n 1 (ii) Polysomic:
addition of one or more chromosomes to one set: 2n + 1 or
2n + 2.

249. (iii) Nullisomic: loss of both the chromosomes of a pair: 2n 2.
3. Hybridisation: Hybridisation is the intermingling of genes of two populations
of a species or between individuals of two species which are otherwise
separated. This can occur either by migration or by artificial cross breeding by
man. The hybrids share genetic material from two different species. For
example:
(a) Mule is a hybrid of horse and donkey.
(b) Raphanobrassica is a hybrid of Raphanus (radish) and Brassica (cabbage).
(c) Pomato is a hybrid of potato and tomato.
All the above factors produce genetic variation (These have been discussed in
separate chapters).
3.5.2 Significance of Genetic Variability or Adaptation
The success in survival and reproduction d epends on the characteristics of the
individuals. Organisms with certain characters or combination of certain
characters (i.e., certa in genotypes) may prove to be more successfully adapted
than other genotypes. For example, a rice variety from Central India, Oryza
nivara , was found to be resistant to grassy stunt virus infection and survived,
whereas other varieties of rice crop were all destroyed due to infection in early
1930s. Similarly, during servere drought, plants and animals which can
efficiently minimise the loss of water and maximise water absorption have better
chances of survival. It means adaptations play a major role in survival. In the
struggle for existence, only those organisms manage to survive that are well
adapted to the environment. In other words, natural selection selects and favours
the multiplic ation of those genetic variation which are of adaptive value to the
organism or to the population by encouraging survival and reproduction of
individuals with such gene combinations.
3.5 .3 Isolation
Isola tion or segregation of individuals of a species into several populations or
groups under psychological, physiological or geographical influence is
considered to be one of the most important factors responsible for evolution.
Geographical isolation includes physi cal barriers like mountains, rivers, oceans
and long distances, which prevent interbreeding between related forms.
Physiological ba r r iers help in mainta ining the individuality of the species,

250. because these isolations do not permit interbreeding among the individuals of
different species. All these factors lead to reproductive isolation.
3.5 .4 Natural Selection and Genetic Drift
Natural Selection: It includes all those kinetic forces introduced by physical and
biotic factors, which determine how and in what direction an organism is going
to change. Natural select ion plays no favouritism, but is obvious that the
organisms which are more suited for the environmental conditions will survive
overpowering the force of competition and produce more offspring. Thus, the
natural selection is a creative process which uses the variation and mutations as
the raw materials from which better adapted individuals with more chances of
survival are obtained. How natural selection acts in nature can be exemplified as
follows :
In any physical environment at a given time, a certain proportion of individuals
in the popu lation carry normal genes while others represent the mutants. These
individuals have mutant genes combined in such a manner that the individuals
carrying them differ from normal parents. If the gene pool of sach a population
achieves stability, i.e., there are no more changes in the genotype of individuals
of the population, such a population will exhibit following conditions:
(a) mutational equilibrium
(b) random mating
(c) equal chances for all genotypes to live and reproduce.
But a population is never stable and constant. The changes occur due to gene
mutations, chromosomal rearrangement and recombination of genes. Due to
unequal opportunity of mating and inadequate chances of survival in every case,
the individuals with changes of survival value survive and perpetuate
introducing more offspring to the population. The natura l selection operates
through differential reproduction and competitive reproductive success always
exerts a selective influence. As a result, certain mutational changes or variation
establish themselves in the line. This process is known as non-random
reproduction or differential reproduction.
Genetic Drift: In small populations, natural selection has less role to play in
fixing a gene or a gene combination. In small populations, gene frequencies
fluctuate purely by chance. This sudden drift or change in gene frequency to one

251. or other side is called genetic drift. This was proposed by Sewall Wright and is
also called Sewall Wright effect. According to this concept , a mutation arising
in a small population is either fixed or lost just by chance irrespective of its
adaptive value . The fixation or loss of a mutation is due to tendency of
populations to become homozygous. For this reason , in small populations
sometimes unfavourable characters are also fixed. Individuals homozygous for
deleterious genes die deleting the harmful genes from population's gene pool.
When a small population gets isolated from the bulk of large population, it
gradually becomes genetically diversified from parent species because of
fluctuation and fixation of some characters and loss of others in small isolated
population.
3.5.5 Origin of New Species
The populations of a species, when present in different environments and are
separated by some of the above mentioned barriers, accumulate different
mutations independently and become morphologically and genetically so
different that they become reproductively isolated and form new species .
3.6 NEUTRAL THEORY OF EVOLUTION
Investigations at the molecular level , at which finer distinctions can be made
between genetic make ups, have led to the controversial, 'Neutral Theory of
Evolution'. Kimura proposed this theory and suggested that speciation is not due
to selection of advantageous genotypes but elimination of deleterious alleles and
random selection of neutral alleles.
100
~
Evolutionary Biology
Accord ing to Neutral Theory of Molecular Evolution most mutations or genetic
variation at molecular level are of neutral or nearly neutral value . They lack
adaptive significane and are fixed in the popul ation or are eliminated by genetic
drift. It means, except a few, almost all the mutations are alike in adaptive value.
It is only chance or random drift which delineates a novel collection of mutants
into a group , divergent from the parental population.
K imura 's neutral theory has received support from data collected by Jukes and
King. They found a large number of ' hid de n mutations' or 'sllent nucleotide
substitutio ns' in proteins of closely related species that are not retained in the

252. population for any especial advantage .
Kimura 's neutral theory was based on the following observations:
• Ther e is a far greater rate of mutat ion in regions of coding and non-coding
DNA that contribute to change in function as compared to the rate of mutations
in regions of DNA for functional area of proteins .
• Rate of neutral mutations is constant. For exam ple, rate of amino acid
substitution in a particu lar protein is almost identical in species closely related
as well as widely divergent.
• The number of substitu tions in a-chain of haemoglobin in mouse, horse and
man is about the same.
• The amino acid substitution in the above cases was random in kind as well as
in position.
• Many important proteins are polymo rphic, but carry out the same function .
Neutral theory recognises that most morphologic al, physiological and
behavioural features of organisms evolve chiefly by natural selection and are
based on base pair substitution s. It also acknowledges that many mutations are
deleterious and are eliminated by natural selection. But it holds that most of the
variation at molecular level have little effect on fitness all the becaus e all the
differences in base pair sequence are not trans lated into difference s at the
protein level.
• Acquired Characte rs
• Darwinism
• Genetic drift
• Isolation
• Neutral theory of evolution
• Pangenes
• Struggle for existence
KEY TERMS
• Adapta tions •
• Gemmules •
• Genetic variability •
• Lamarck ism •
• Natural selection •
• Sexual selection •

253. • Survival of the fittest • Artificial Select ion
G ene mutations
Germplasm
Nee-Darwinism
Origin of species
Somatoplasm
Synthetic theory of evolution
REVIEW QUESTIONS
1 . Discuss salient features of Lamarckian theory of inheritance of acquired
characters. Critically examine this concept in the light of modern information .
2. What is the law of inheritance of acquired characters? How does it explain the
elongation of neck in giraffe?
3. State Lamarck's theory of evolution. Explain it with some suitable example.
4. List the main factors on which Lamarck based his theory of biological
evolution .
5. Give Weismann 's theory of germplasm. What light does it throw on
Lamarckism ?
6. Give an account of Darwin 's theory of evolution. Discuss the theory in the
light of modern researches.
7. Write an essay on Darwin's theory of natural selection and orig in of species.
8. Write a short essay on "Natural Selection ".
9. Explain Darwin 's theory of natural selection with a note on its present status.
10. Describe Lamarcki sm and Darwinism under followin g heads:
(a) Explain essential points of evolutionary theorie s of Darwin and Lamarck .
(b) How could evolution of long neck of giraffes be explained in terms of each
of these two theorie s.
(c) List the weaknesses of each of these theories .
II. (a) Who proposed the theory of orig in of species by natural selection? (b)
Explain how natura l selection differs from artificia l selection in terms of
organic evolution.
(c) Give one example each of natural and artificial selection .
12. Write short notes on:
(a) Neo-Darwinism
(c) Modern synthetic theory (e) Alfred Russel Wallace (g) Theory of use and
disuse.

254. (b) Indu strial melanism in peppered moth (d) Sexual selection
(f) Galapagos Island s
1 3. List three mechanisms/factors by which variant genotyp es are produced in
nature.
14. How does natural selection differ from artificial selection in terms of organic
evolution?
15. ' Industrial melanism in peppered moth is an excellent example of natural
selection'. Justify this statement.
16. What observation on Galapagos Islands affected Darwin's think ing for
evolutionary explanation?
17. (a) What is the concept of natural selection?
(b) What were the sources that led to the development of concept of natural
selection?
18. What is the pangene sis theory and why did Darwin proposed this theory ?
FURTHER READINGS
I. Bowler, PJ. , 1989. Evolution: The History of an Idea, University of Californ
ia Press, Berkeley.
2. Browne, EJ ., 2002 . Charles Darwin: The Power of Place. Vol. II of a
Biography, Alfred A. Knopf, New York.
3. Browne, J., 2006. Darwin s Origin of Species, A Biography. Douglas and
Mclntyre, Vancouver, Canada .
4. Darwin c., 1845. The Voyage of Beagle (Originally publ ished as a Journal of
Research).

256. 102 ~ Evolutionary Biology
5 . Darwin c., 1859. On the Origin of Species by Means of Natural Selection (Or
the Preservation of Favoured Races in the Struggle for Life). Murray, London.
6. Desmond A. and 1. Moore, 1991. Darw in, Warner Book, New York.
7. Dobzhansky, T., Ayala, F.J. Stebbins, G.L and Valentine, J.M., 1977.
Evolution San Francisco: W. H. Freeman and CO.
8. Eiseley, L., 1958. Darwin s Century, Garden City, New York: Doubleday and
Co.
9. Lerner, I.M. and Libby, W.J., 1976. Heredity, Evolution and Society. San
Francisco: W H. Freeman and Co.
10. Mayr, E., 1982. The Growth ofBiological Thought: Diversity, Evolution and
Inheritance. Harvard University Press, Cambridge , MA.
II. Mayr R., 1978. Evolution, Scientific American, September, pp. 46-55. 12.
Moore, R. and Editors of Time-Life Books, 1964. Evolution , New York: Time-
Life Books.
13. Oldroyd, D.R., 1980. Darwinian Impacts: An Introduction to the Darwinian
Revolution. Open University Press, Milton Keynes, Oxford, England.
14. Osporat, D., 1981. The Developm ent of Darwin s Theory: Natural History,
Natural Theology, and Natural Selection , 1838- 1859. Cambridge University
Press. 15. Richards, R.J., 1992. The Meaning of Evolution: The Morphological
Constru ction and Ideolo gical Reconstruction ofDarwins Theory. The
University of Chicago Press, Chicago.
16. Scott, E.C., 2005. Evolution vs. Creation ism: An Introduction, University of
California Press, Berkeley CA.
17. Thompson , K.S., 1995. HMS Beagl e: The Story ofDarwin sShip. WW
Norton, New York.
DOD

258. UNIT-II
Mechanisms of Evolution
Chapter 4. Variation
Chapter 5. Gene Mutations

260. Chapter 6. Chromosomal Aberrations
Chapter 7. Variation in Chromosome
Number (Heteroploidy)
Chapter 8. Reproductive Isolating Barriers

262. 4
Variation
4.1 VARIATION AND VARIABILITY
Darwin phasised that the members of a population differ in their physical,
behavioural and functional activities. He called these differences in the
organisms as variation. Most populations are polymorphic exhibiting different
expression of the same trait because genes and the environment interact to
determine the expression of traits of every individual.
Gunter Wagner and colleagues (1997) have drawn an important distinction
between variation and variability.
While variation are differences among closely related organisms within a
population, variability represents the tendency or propensity of individuals or
their phenotypes to exhibit variation.
4.2 NATURE OF VARIATION
Variation may influence any character of the individuals. These may be:
I . Morphological Variation: These are seen in shape, size, colour or pattern of
body parts of the organisms.
2. Physiological Variation: These affect the physiological processes and
functioning of organs.
3. Psychological Variation: These include changes in the mental traits.
4. Ecological Variation: These are produced by the influence of environment and
are usually non-heritable.
4.3 TYPES OF VARIATION
Variation can be categorised into five contrasting sets as follows :

264. 106 Iil Evolutionary Biology
l. Set I (a) Heritable variation
(b) Non -heritable variation
2. Set II (a) Meristic variation
(b) Substantive variation
3. Set III (a) Continuous or fluctua ting varia tion (b) Discontinuous variation or
mutation
4. Set IV (a) Determi nate variation
(b) Indeterminate variation
5. Set V (a) Somatic or somatogenic variatio n (b) Germinal or blastogenic
variation
4.3.1 Heritable and Nonheritable Variation
I. Heritable Variation: The se variation are passed on to the next generation.
They arise due to changes in the genetic material of germplasm or gametes and
are also called genetic or ga me tic variation. Only these variation have
evolutionary significance.
2 . Nonheritable Variation: Such variation are acquired by the organi sms during
their life time due to the influence of environment. They are also called somatic
or somatoge nic va r iation . They are lost with the death of the organism and do
not have evolutionary sign ificance.
4.3.2 Meristic and Substantive Variation
I. Meristic Variation: The meristic variation include vanation in the number of
parts of an organi sm, as for example, occurrence of six digit s in hand or foot
instead of normal five, thirteen ribs in man instead of twelve, presence of six
arms in starfish while norm ally it has five, or variation in the number of
scutellar bristles in Drosophila.
2. Substantive Variation: The substan tive variation are variation in form , size,
shape or colour of an organism or its parts, e.g.. the eye colour, hair colour,
shape of the nose, ear and eye, the height of the body or plant and shape of the
leaves, etc.
4.3.3 Continuous and Discontinuous Variation

265. I . Continuous Variation: The continuous variation are minute variation which
occur in graded serie s. These fluctuat e on either side of the average condition
and differ only slight ly from one another. Since the continuous variation
fluctuate on either side of the average, they are also described as minus and plus
variation. The characters showing continuous variation are quantitative
characters, being controlled by polygenes . They show bell-shaped distribution
curve.
EXAMPLE: Changes in height or colour in man or colour of wheat kernal are
continuous variation.

267. Darwin regarded continuo us variation
as fluctu ations and described them to be
significant for the evolution and origin 1500
of species. The fluctuating variation
occur constantly in nature . The larger
1250the number of individuals exhibiting
a particular variation, the less is the
deviation from the mean . Converse ly,
1000
the greater the deviation from the mean,
the lesser is the number of individual s 750
exhibiting it. According to Darwin,
the progressive accumulation of these
500fluctuation s in one direction leads to
directional evolution and formation of
new species.
250
2. Discontinuous Variation: The
discontinuous variation are sudden and 54 56 58 60 62 64 66 68 70 72 74large. They deviate to a great
extent Height (inches)from the normal and do not fluctuate
FIG. 4.1: Continuous distribution of
variation in
from the average but appear as totalheight among 3995 women.ly new. These are also
described as
sa ltations or muta tions. These are mostly stable and heritable, being transmitted
to succeeding generations. The discontinuous variation are rare and without any
definite periodicity.
EXAMPLES:
Discontinuous variation are presence of six fingers or toes in man,
hornless calf, tailless kitten, thornless rose and long-tailed Japanese cocks . Hugo
de Vries described that these variation play an important role in the origin and
evolution of higher taxa.
4.3.4 Determinate and Indeterminate Variation
I . Determinate Var iati on: Determinate variation are confined to some specific
lines and occur in a specific direction. These may be called directional. These
are generally in the direction of adaptations.
EXAMPLES: Determinate variation are:
• In leaf-eating beetle, Diabrotica soror, the change from one colour pattern to

268. other (Ke logg, 1906).
• In Biston betularia, the abundant.
dark coloured moths became more and more
• Gradual reduction in the number of digits in the evolution of horse.
2. Indeterminate Variation: Such variation occur in any conceivable direction.
The se may be called fluctuating variat ion. These do not confine to any law.
108 [i] Evolutionary Biology
4.3.5 Somatic or Somatogenic and Germinal or
Blastogenic Variation
I. Somatic Variation: Somatic variation are caused by the direct influence of the
environment upon the body of the organisms. These are local changes in the
organisation of organisms and are neither inherited from the parents nor
transmitted to the offspring, but are lost with parent's death. These can be
described as acquired variation or non heritabl e variation.
EXAMPLES: Better developed muscles of athletes, loss of any body part due to
some accident, small feetof Chinese ladies and sun-burnt complexion of
Europeans in tropics, etc. Eggs of Funbulus fish, when subjected to sea water
having more quantity of magnesium chloride, develop into larvae with a
cyclopean eye.
Lamarck in hi s theory of inheritance of acquired characters laid emphasis on the
transmission of acquired variation from one generation to other, but Weismann
proved that acquired variation are nonheritable.
2 . Germinal Variation: The germinal or blastogenic variation occur in the
germplasm of organisms. The germ cells or gametes develop from the
germplasm and on fusion form the zygote which develops into the adult. The
variation from the germplasm and the gametes are passed on to the offspring.
Such variation are also called constitutiona l or cong enital or heritabl e variation
. These develop because of new combinations of genes either at the time of
gamete formation or at the time of fusion of male and female gametes.
EXAMPLES: Occurrence of supernumerary digits in man, horse, cat, etc.
The differences between bla stogenic and somatoge nic variation are not distinct
and in certain cases somatogenic variation after repeated generation s may
produce blastogenic effect. Boag (1983) studied heritability of several characters

269. in populations of Darwin's finches in Galapagos Islands by measuring banded
adults and their offspring. He concluded that most of the phenotypic variation
have a genetic basis.
4.4 SOURCES OF VARIATION
Th e sources of somatogenic and blastogenic variation are different.
Somatogenic variation are caused exclusively by environment such as heat or
cold, presence or absence of particular vegetation, scarcity of food, presence or
absence of enemies, etc. The blastogenic variation are produced by genetic
variability.
4.4.1 Causes of Somatogenic Variation
I . Environment: The environmental conditions directly influence the organisms
and introduce various changes in their organisation . The heat or cold, presence
or absence of vegetation , scarcity of food, presence or absence of enemies,
climati c conditions, etc., influence individuals. For example, puppies from ' pure
breed ' parent may all have practically the same genotype, yet each varies in
phenotype from the litter mates. The variation may be very distinct or
insignificant. One of them may
be a runt, much smaller than the rest. But it also has the same genotype as others
of the litter. The runt's phenotype is developed probably because it could not
receive proper food while in the mother's womb, and even after birth the litter
mates compete so strongly that this weakling could not get plenty of mother's
milk and could not grow to the extent other mates of the litter. If it breeds with
one of its other mates, the runt's phenotype does not appear in any of the
resulting offspring.
Tower and Agar conducted several experiments to show the effect of lowering or
raising of the temperature on the growth of rats and mice. In majority of cases,
mice reared in a warm room at about 21°C, were found to differ considerably
from those reared in a cold room at 5°C. The shape of larvae of sea-urchins and
frogs is modified when lithium salts are added to water in which they live. When
tadpoles of frog are fed on thyroid of mammals, they pass through all the stages
of metamorphosis very rapidly but remain small in size, whereas tadpoles fed on
ordinary diet, grow to normal size, but take much longer time to complete
metamorphosis.

270. 2 . Endocrine Glands: The hormonal secretions of the endocrine glands which
influence the development and differentiation of various physical and mental
characters are also responsible for causing various somatogenic and blastogenic
variation.
4.4.2 Causes of Blastogenic or Genetic Variation
(Cytological Basis of Heritable Variation)
The factors responsible for genetic or heritable variation on which evolution
depends are:
I. Changes in the number of chromosomes : Heteroploidy.
2. Changes in the structure of chromosomes: Chromosomal aberrations.
3. Changes in the structure/composition of genes: Gene mutations.
4. Changes in the genic composition of organisms by Sexual reproduction.
5. Changes in the genic composition by hybridisation.
4.5 VARIATION IN NUMBER OF CHROMOSOMES (HETEROPLOIDY) The
organisms are usually diploid (20) i.e., they possess two sets of chromosomes .
But rarely the number of sets of the chromosomes may change.
Variation in the normal diploid chromosome number is termed ploidy. It may be
haploidy or monoploidy, polyploidy (euploidy) and heteroploidy (aneuploidy).
4.5.1 Changes Involving Entire Set of Chromosomes
1. Monoploidy: Only one set of chromosomes (n) i.e., haploid number of
chromosomes is present, i.e., each chromosome of the set is represented singly.
2. Polyploidy: The complete set of chromosomes is represented more than twice.
It may be triploidy (3n), tetraploidy (4n), pentaploidy (5n), hexaploidy (6n),
heptaploidy (7n), octaploidy (8n) or decaploidy (IOn).
11 0 ~ Evolutionary Biology
I
Haploidy or
Monoploidy
I
Autopolyplo idy
I
Allopolyplo idy
I

271. Amphipolyploidy
I
Aneuploidy
I
Monosomies
I
Polysomies
I Nullisomies
Single monosom ies
I
Double monosomies
Trisomies
I
Tetrasomics
FIG. 4.2: Different types of changes in the number of chromosomes
The Table 4.1 shows multiplication in the number of same set of chromosomes.
These are called autopolyploids because in them single genome is repeated more
than twice. Autoploidy is common mode of evolution in many plant groups,
including mosses, apples, pears, bananas and tomatoes, etc.
Po lyploids having multiple copies of different sets of chromoso mes and
produced from the cross breeding of different species are called allopolyploids.
For examp le, fusion of a gamete of species A and species
chromosome sets of the hybrid are doubled
AABB chromosomes are formed (Fig. 4.3).
B produces diploid hybrid AB. If in the offspring, allotetraploid with
'M:jl::t!11 Changes in the Number of Chromosomes Involving the Entire Set s
Examples
5. Eup lo id Type No.
1. Monoploid or haploid
2. Diploid (Normal ) 3. Triploid
Nu mber of ho mo logous chromosomes in entire set
1n (One set)
ABC (Three chromosomes)
2n (Two sets) 3n (Three sets)

272. 4. Tetraploid 4n (Four sets)
5. Pentaploid 5n (Five sets)
6. Hexaploid

274. 6n (Six sets) MBBCC (6 chromosomes) or 2 x (ABC) AM BBB CCC (9
chromosomes) or 3 x (ABC)
AAAA BBBB CCCC (12 chromosomes) or 4 x (ABC)
AAAAA BBBBB CCCCC
(15 chromosomes)
AAAAAA BBBBBB CCCCCC
(18 chromosomes)
Both auto and allotetraploids have led to the origin of new species in nature.
However, polyploidy can be induced by exposing the dividing cells to chemicals,
such as colchicine which interferes with the formation of spindle and with the
segregation of chromosomes at anaphase of cell division.
Evolutionary Role of Polyploidy in Speciation of Plants
Polyploidy has played a significant role in the speciation in plants because:
• Polyploidy contributes characteristics of intrinsic value such as large flowers,
firmer texture and large seeds and fruits .
• Polyploidy fixes heterozygous gene combinations particularly those derived by
hybridisation between races of the same species.
• Polyploidy introduces heterozygosity.
Species ..A. Species
~
2n = AA 2n = BB Gamete A Gamete B
~
F1 Sterile 2n =AB hybrid
Chromosome doubling
Allotetraploid 4n = AABB
FIG. 4.3: Formation of an allotetraploid.
Heterozygotes, usually, have an adaptive advantage .
• Polyploidy acts as a conservative process and stabilises interspecific hybrids.
• Polyploidy facilitates gene exchange between distantly related species .
• Polyploid evolution is irreversible.
Evolutionary Role of Polyploidy in Speciation of Animals In animals polyploidy
could not get much success in the origin of new species. The reasons for its

275. failure in speciation in animals are:
• Polyploidy disturbs sex chromosome mechanism so that the successful
polyploids are either hermaphrodite (planarian genus, Dugesia) or
parthenogenetic as in brine shrimp, Artemia, the moth Solenobia and weevils of
family Curculionidae.
• White (1973) has emphasised that cross fertilisation in animals is the main
cause for failure of polyploidy. A newly arisen polyploid animal is unable to
reproduce by itself.
• Polyploidy disturbs the delicately balanced developmental process.
• Allopolyploids can not be formed in animals because they suffer from
developmental sterility, developmental deformity or hybrid weakness.
4.5.2 Changes Involving the Number of Chromosomes in a Set (Aneuploidy)
1. Monosomics: Loss of one chromosome from the karyotype, i.e., 2n I.
112 [i Evolutionary Biology
2. Polysomies: Addition of one or more chromosomes to the karyotype of
organism i.e., 2n + I or 2n + 2.
(a) Trisomic: 2n + I
(b) Tetrasomic: 2n + 2
3. Nullisomics: The loss of both the chromosomes of a pair.
Changes in the Number of Chromosomes in a set
Type of Aneuploid
1. Normal diploid
2. Monosomic
3. Double monosomic
4. Nullisomic
5. Trisomic
6. Double trisomic
7. Tetrasomic
8. Pentasomic
Number of Ch romosomes Examples
2n MBB CC
2n-1 MBB C
-
2n-1 -1 M B

C_

276. 2n - 2 MBB
-
2n + 1 MBB CCC
2n + 1 + 1 MBB~CCC 2n + 2 MBB CC CC 2n + 3 MBB CC CCC
4.6 CHROMOSOMAL ABERRATIONS
Chromosomal aberrations are changes in the structure or appearance of a
chromosome. These are also called changes in the phenotype of the
chromosome. Chromosomal aberrations include changes both in the number of
genes or in the arrangement of genes in a chromosome . Therefore ,
chromosomal aberrations can be of the following types:
4.6.1 Aberrations due to Change in the Number of Genes These involve loss or
addition of genes in a chromosome causing change III its length. These are:
(a ) Deletions or Deficiencies: These include loss of chromosome segment which
can be either at the end of chromosome (ter minal deletions) or within its length
(intercalary deletions or deficiencies).
A change from ABCDEFG either to ABCDE or CDEFG is terminal deletion and
to ABCFG is intercalary deletion.
(b) Duplication: It is repetition of a block of genes. This may be due to terminal
duplication or intercalary duplication.
(i) Terminal duplication occurs at the beginning or end of a chromosome, so that
ABCDEFG becomes either ABABCDEFG or ABCDEFGFG. (ii) Intercalary
duplication occurs within a chromosome's length in which ABCDEFG changes
to ABCDCDEFG.
4.6.2 Aberrations Due to Change in the Arrangement of Genes In these
aberrations chromosome length remains unchanged but arrangement of genes
changes. Such aberrations are of following two types:
I. Inversion: Inversion is the reversal of gene order in a gene block within a
chromosome due to the rotation of a chromosome segment by 180
0
so that
ABCDEFGH becomes ABEDCFGH. Inversion may be pericentric (when
centromere is included in the inverted portion) or paracentric (when centromere
lies outside the inverted part of the chromosome).
2 . Translocation: It is shifting of a chromosome segment to a different

277. chromosome. In homologous translocation, the chromosome segment is shifted
to a new location within the same chromosome. This is also called transposition.
In heterologous translocation, the chromosome segment is shifted to a
nonhomologous chromosome.
The tran slocations may be reciprocal, if equal-sized segments are exchanged
between two non-homologous chromosomes. In nonreciprocal translocation,
there may be no exchange between nonhomologous chromosomes but only a
shift from one linkage group to other linkage group takes place .
G G
H H
segment
H
I
I
I

278. A
m------A B
rn
AA
B
B
C B C
+--Break
C

D
+--Break
D

FC
E
E
E F
F
D

279. G +--Break m~c. Inversion (Middle piece of
chromosome fallsDeleted
out and rotates through 180·
and then rejoins)segment
A. Deletion (Middle piece of
chromosome falls out) A

280. B
rn
A
A
B C +- Break
C D
D E
DB
C E F EF
Chromosome 1
FD G
Break-. E
H
F

I
W W
w

G F
X X
X
H G
y y y
I H
Z Z
Z
~ I
Chrom osome 2
Duplicate

282. segment D. Nonreciprocal Translocation (Piece of
B. Duplication (An extra length of chromosome-1 breaks off and joins
chromosome-2 ; chromosome joins a homologous chromosome chromosomes 1
and 2 are not homologous) FIG . 4.4 : Different types of structural chromosomal
changes.
114 [iJ Evolutionary Biology
3 3 4 -5 b
~ a rac~ n t ri c Cd

283. Inversion e 1\
Translocat ion

284. Op l
h 9 f
x ( 1)
5 -4
Karyotype of Karyotype of
D. virilis D. ezoana
FIG. 4.5: Origin of karyotype of Drosophila ezoana from primitive D. virilis by
translocation and inversion (haploid se t of chromosomes of D. virilis and D.
ezoana are shown).
4.6.3 Evolutionary Significance of Chromosomal Aberrations
Chromosomal aberrations are an impo rtant source of variation in a population
partly due to position effect and partly due to the formation of chromosomes
with different number and arrangement of genes . Translocations produce new
linkage groups. Inversions help in preserving gene sequences in different popul
ation s, while duplications produce gene familie s having several copies of the
same gene providing more chances for mutations and phenotypic variation. For
detail s refer Chapter 8. Fig 4.5 shows how the karyotype of Drosophila ezoana
has arisen form Drosophila virilis by inversions and translocations. Similarl y, in
Fig 4.6 shows how human chromosome No. 3 has evolved by the translocations
and inversions in six steps. From cretaceous period onward several
translocations and inversions in rodent-like chromosomes have occurred to
produce primate chromosomes.
4.7 GENE MUTATIONS OR POINT MUTATIONS
An individ ual is the product of interaction of his total genotype. Every gene
plays a part in the process of deve lopment and differentia tion of the organism.
A singl e gene may affect more than one characteri stics (pleiotropism) or many
gene s or a group of definite number of genes may regula te one characteristic
(polygenes). Whatever may be the case, genes control the characteristics through
the formation of enzymes. All the living proce sses in the embryos as well as in
adults are catalysed by the enzyme s which interact in an intricate fashion. The
information for the synthesis of these enzymes are located in the DNA molecules
in the form of sequences of nitrogenous bases. A combination of three
nitrogenous bases code s for one amino acid of the polypeptide chain of the
protein or enzyme and is known as a codon or triplet code. Several codons

285. arranged in a definite sequence dictate the formation of a particular enzyme and
finally the development of a particular characteri stic. Any
change in the arrangement of nitrogenous bases or addition or deletion of
specific nitrogenou s base s in the DNA changes the entire base sequence and the
reading frame work of genetic message encoded there in. It mean s gene
mutations or point mutation s affect single genes and the prote ins synthesised by
them . Gene mutations may be of the followin g types:
4.7.1 Substitution Mutations
Substitution mutations involve substitution or replacement of one nitrogenous
base pair by other base pair. These can be transition or transver sion.
I . Transition: It invol ves replacement of one purine by another purin e or one
pyrimidine by another pyrimidine or replacement of a purine-pyrimidine base
pair by another purine-pyrimidione base pair (i.e., A = T pair replaced by G ==
C pair) or a pyrimidine-purine base pair by another pyrimidine-purine pair (i.e.,
C == G pair replaced by T = A pair) .
2 . Transversions: It involves replacement of purine by a pyrimidine or a
pyrimidine by a purine , i.e., replacement of purine-pyrimidine base pair by
pyrimidine-purine base pair (i.e., A = T replac ed by C == G).
Step 1 Step 2 Step 3 (translocation) (fus ion) (inversion) 8 11 4

286. +
~"- :~
R
~ ~ ~~A
T A +
J
A
S A
R
Step 4 StepS Step 6 (inversion) (fus ion) (inversion) T

287. J-
T T
A
A A A S S S
R
R
R R T T
T
S S S
Human chromosome no.3
FIG. 4.6: Evolution of third chromosome of man from translocation between
rodent-like chromosomes 8th and 11 th followed by fusion with 4th rodent
chromosome and some inversions.
Original molecules
DNA
ITAe CAT TAG GAG cee ATTI
mRNA IA UG GUA AUe yliT Tc: \iii T T.!n (Jl
0,... III III
~- . s~
3 III . ~(\) c . s.l s.l. ~ .
" ?(\)
_~~ ~~
Protein
(\)10 c
(\)

(\)
19
~
~
(\) 9.'2"
.'5 g
o
o
A
"2..
~
~
g
Mutation
DNA

288. mRNA
Protein B
FIG. 4.7: The effect of insertion or deletion of a nitrogenous base in a segment of
DNA. This changes the codons on mRNA and message encoded in DNA (frame-
shift mutations).
4.7.2 Frame-Shift Mutations
These mutations are caused by insertion or deletion of one or more nucleotides
in a molecule of DNA. The reading of the encoded genetic message is shifted
beyond the point of insertion or deletion. The frame-shift mutations are of two
types :
1. Insertion or Duplication: It is produced by the addition of one or more
nucleotide pairs in the original nucleotide sequence .
2. Deletion: It is omission or loss of one or more nucleotides from the original
nucleotide sequence .
Single gene mutations may confer the ability to use new substrates as nutritional
sources or to detoxify harmful substances. This may increase or decrease the
range of ecological adaptability of the mutants .
4.8 MENDELIAN RECOMBINATION OR SEXUAL RECOMBINATION
Recombination refers to the mixing or reshuffling of the genes in the offspring,
which they receive from the parents. In sexually reproducing organisms , there
are

290. two sexes male and female. Individuals of each sex produce germ cells or
gametes (males produce sperm or pollens and females produce ova). At the time
of gamete formation (meiosis) the two sets of genes segregate in a manner which
results in
haploid gametes (possessing one of the two sets of genes) . When the gametes of
two parents unite during fertilisation, the two sets of genes form pairs again .
During the process of gamete formation and fertilisation, a great array of
recombination of pairs of genes occurs because of segregation, independent
assortment and crossing over.
Recombination is important for three main reasons:
• It brings about new combinations of genes contained in the parents thereby
resulting in the mixing of a particular allele with a series of genes not associated
with it earlier.
• The number of recombination of genes is infinitely number of gene mutations.
Although, mutations are variation, most new types in a population arise by
recombination, i.e., by the new combination of old genes . This process could
produce more and more varied types at a faster rate than mutations can, and
recombination rather than mutation is the immediate source of variability in
population.
• The cumulative results of recombination may lead to postadaptation in case of
same existing environment or preadaptation in the new environment. For
example, if a heterozyguous oganism that produces large number 'of offsprping
of the constitution Al A2B IB2 is crossed with another of the same constitution,
the offsrping produced will be:
larger than the possible the ultimate source of
A) A) B) B) ; AI A) B) B
2
; A) A) B
2
B
2;
A) A
2
B) B);
AI A
2
B
I
B
2;
AI A
2
B
2
B
2;
A
2
A
2
B) B
2;
A
2
A
2
B
2
B
2;
A
2
A
2
B) B)
Although, each of the above genotypes could arise by mutation, but the changes
of their occurrence in one generation are remote. The number of diploid
genotypes that can be obtained from following formula:
any number of alleles of a gene is calculated by the
g=
r (r + I) 2

291. where, g = the number of diploid genotypes, and
r = the number of alleles at a given locus .
If two different loci , located on two different chromosomes, are taken into
consideration so that these can combine freely (independent assortment) the total
number of possible diploid genotypes for the two genes considered together will
be:
r(r+l) r(r+l)
gA x gB = ---'------' x
---'------'
2 2
[r(r+ I)J
2

293. If it is presumed that there are two alleles at each of the two loci, then
Maternal chromosomes
and, gA x gB = ( r =2
2
(22+ 1))2 9
Parent cells
Paternal chromosomes
Recombination by itself does not produce any change in the information but
uncovers arrangements of gene s on which selection can act by favouring some
and eliminating others . The importance of recombination can be further
illustrated by the following example:
Suppose, there are two populations each homozygous for every gene, but one
reproduces sexually, and other asexually. If gene a. has mutated to az in each of
the populations then in the asexual population, all the offspring of the single
individual with mutation will be heterozygous (a t az), while other members of
the group will be homozygous (a. a.) . On the other hand in a sexually
reproducing population, three genotypes will develop :
a. a. ; at az; «. az because once the mutant allele is formed, it will combine at
random in gametes. From one mutation the sexually reproducing population will
be three times more variable. Thus, recombination of genes is a means of
producing a continuous supply of variables to be 'tried out' by natural selection.
Gametes Offspring

294. Parent ,en,©
1\

295. Gametes~~
Offspring
FIG . 4.8: New combinations of parental genes in the gametes as a result of
independent assortment.

297. 4.9 RECOMBINATION DUE TO EXCHANGE OF GENES BETWEEN
HOMOLOGOUS CHROMOSOMES OF A PAIR
Crossing over or exchange of genes between h omologous chromosomes occurs
during meiosis when one member of a pair of hom ologous chromosomes
exchanges parts with its fellow. The crossing over and chiasmata formation
occur regularly in the chromatids of paternal and maternal chromosomes
(homologous chromosomes) at
each meiosis as shown in Fig. 4.7. The contribution of crossing over to variation
can be illustrated by the following example:
Suppo se, in a population, all the individual s have the genotype Aa , Bb, Cc for
three genes present on the same homologous pair of chromosomes. Since
chiasmata are formed at almost every meiotic division, gametes with new gene
combinations are produced. Crossing over speeds up the mixing of new mutant
alleles with the genes already present. In the absence of crossing over these
alleles will remain together, only two types of gametes with parental
combinations are formed . But as a result of crossing over new combinations of
alleles are formed. Chiasmata may be formed at any point on these chromo
somes and each point of chiasmata formation doubles the number of kinds of
gametes, the chances of gene distribution are increased man y fold. Moreover,
new combinations of alleles produced as a result of crossing over may prove
more beneficial and may be favoured by natural selection (See Fig. 4.9).
-...--=--~
Homologous chromosomes of a pair
Sister chromatids

299. y
I
FIG. 4.9: Crossing over and exchange of segments between the chromat ids of
homologous chromosomes during meiosis leads to genetic variability.
4.10 HYBRIDISATION
Hybridis ation is the interbreeding or crossing between the individuals of two
different species so that the genes from one species are introduced into the gene
pool of another species. In other words, it is bringing together of genes from two
different gene pools.
A ccording to Stebbins (1979) 'Hybridisation is crossing between populations
having different adaptive gene complexes. Such populations are either different
races or subspecies separated by geographic isolation.
4.10.1 Salient Features of Hybridisation
I . Hybridisation brings genes from two different gene poo ls together.
2. It increases the size of gene pool.
3. It brings about new combinations of genes .
4. It establishes variation in populations.
5. It leads to the formation of new varieties and species.
6. The hybrids are usually sterile, only a few are fertile. Ferti lity depends upon
the similarities in the gene pools of intercrossed populations.
7. Maj ority of hybrids have lower adaptive value than those of the parental
species in the existing environment, but may prove more successful in some
changing environment.
8. Only a few hybrids may prove to be better adapted to certain environment.
4.10.2 Types of Hybridisation
Hybrid s between different speci es are more common in plants than animals, but
natural popu lations of fresh water fishes, toads and warb lers are found to
produce hybrids under natural conditions. Hybrids may be:
(a) interspecific i.e., between two speci es, or
(b) inter generic i.e., between the memb ers of two different genera.
1 . Interspecific Hybridisation: It is crossing between two different species of the
same genera. For example, Sibley (1954) described hybridisation between two
species of red-eyed towhee (birds) in Mexico. One species, called collared

300. towhee , Pipilo ocai lives in coniferous woodlands of southern Mexico. The
other species, called spotted towhee , P erythrophthalmus, lives in oak forests of
northern Mexico.
In the mountains of Southeastern Mexico , these two species are found together
in the same area without interbreeding, and without interspecific hybrids . But in
many parts of Western Mexico , the two species interbreed resu lting in the
formation of numerous hybrids . These hybr ids are intermediate between the
two species in their appearance and are formed where original oak trees were
comp letely cleared for

302. human use. This example shows that hybridisation can produce variability
among populations.
2. Intergeneric Hybridisation: It is crossing between two genera. For example,
Karpechenko, the Russian geneticist, obtained hybrids of two different genera,
Raphanus (radish) and Brassica (cabbage). The hybrid is called
Raphanobrassica. The diploid number of chromosomes in each of the two
genera is 18. The hybrid Raphanobrassica also has 18 chromosomes, 9R from
radish and 9B from cabbage. The fertile strain of Raphanobrassica is an
allotetraploid, formed by the duplication of these chromosomes and has 36
chromosomes (18 R + 18 B).
In mammals , the hybrid between horse and donkey is the most common one. A
cross between female horse and male donkey produces mule , while a cross
between male horse and female donkey produces hinny.
3. Introgressive Hybridisation: According to Anderson, ' hybridisation that
introduces some gene s of one species into the gene pool of another species is
called introgressive hybridisation. It is the result of back crossing, i.e.,
crossing of hybrid to its parent species .
EXAMPLES: Anderson and Hubricht (1938) studied introgressive hybridisation in
two species of spiderwort plant, Tradescantia.
(a) Tradescantia canaliculata grows in full sunlight on the top of cliffs.
(b) Tradescantia subaspara grows in wood land shade on the bases of cliffs.
In some place s, where there is gradual slope between the top and base of the
cliff, the forest dwelling species (growing at the base of cliff) has extended its
range upward and vice versa the cliff top specie s has extended its range
downward. The hybrids are produced between them where the extended ranges
of two species overlap. The hybrids share the characters of both the parental
species and are successful because of the suitable environment.
When the F
I
hybrids breed with cliff top species , some genes of the forest
dwelling species are introduced into the cliff top specie s. Similarly, some genes
from cliff top species are introduced in the specie s growing on the base of cliff.
The hybrids are described to function as 'go between' the parental species .
The introgressive hybridisation brings about flow between two different gene

303. pools, increasing genetic variability.
4.10.3 Hybridisation and Evolution
Hybridisation promote s the origin of new characters and variability. The new
combination of genes may increase rate of mutations. Increased genetic
variability provides background for natural selection to operate and helps in
evolution. However, hybridisation has following limitations :
1 . Hybrids are mostly sterile .
2. New combination s of genes need to be established in the population.
3. New environmental niches are needed for new hybrids.

305. 122 ~ Evolutionary Biology
• Allele
• Autopolyploidy
• Continuous variation
• Discontinuous variation
• Gene families
• Pericentric inversion
KEY TERMS
• Allopolyploidy
• Base substitution
• Deletio n
• Duplica tion
• Inversion Hybridisation
• Point mutation
• Aneuploidy
• Blastogenic variation
• Determinate variation
• Euploidy
• Paracentric inversion
• Polyploidy translocation
REVIEW QUESTIONS
I. Explain what is meant by 'variation ' ? Describe various kinds of variation and
discuss their relative importance in the phenomenon of evolution.
2. Write an essay on variation.
3. Write down the definition of variat ion. Describe the kinds and causes of
variation in detail.
4. How is structural rearrangement in chromosomes brought about ?
5. Explain that the variation are the raw materials for natural selection.
6. Explain what contributes to the progress in evolution, citing suitable examples
to the discontinuous variation.
7. Differentiate between somatogenic and blastogenic variation. Which of the
two are important from evolutionary point of view?
8. Describe various sources that introduce variation in the living organisms.
9. Explain what is meant by the term introgressive hybridisation?

306. 10. Write short notes on the following:
(b) Determinate variation and indeterminate variation (d) Geographic variation
(a) Meristic and Substantive (c) Polymorphic variation II. Explain the follow ing
: (a) Frame-shift mutations (c) Deletion
(b) Substitution mutations (d) Duplication
FURTHER READINGS
I . Carroll, S.B., 1. K. Grenier and Molecular Genetics and Evolution Malden,
MA .
S.D. Weathebee, 2005. From DNA to Diversity. of Animal Design. 2nd ed.
Blackwell Publishing,
2 . Dobzhansky, Th ., 1937, 1941, 1951. Genetics and the Origin of Species.
Columbia University Press , New York
3. Dobzhansky, Th., and O. Pavlovsky, 1953. Indeterminate outcome of certa in
exper- iments on Drosophila Populations. Evolution, 198-210.
4. Dowling, T.E., and C.L. Secor, 1997. The role of hybridisation and
introgression in the diversification of animals. Ann. Rev. Ecol. Syst., 28, 593-619.
5. Falconer, D.S. , and T.F.e. Mackay, 1996. Introduction to Quantitative
Genetics, 7th ed. Longman, Harlow, Essex , U.K .

308. 6 . Gerhart, J., and M. Kirschner, 1997, Cells. Embryos and Evolution: Towards
a Cellular and Developmental Understanding of Phenotypic Variation and
Evolutionary Adaptability. Blackwell Science, Malden, MA.
7 . Hallgrimsson, 8., and 8.K. Hall (eds.), 2005. Variation: A Central Concept in
Biology. Elsevier/Academic Press, Burlington, MA.
8. Hey, J.W.M. Fitch and F.J. Ayala (eds.), 2005. Systematics and the Origin of
Species on Ernst Mayr 's lOath Anniversary. The National Academies Press,
Washington DC.
9. Mayr, E., 200 1, What Evolution Is. With foreword by Jared Diamond, Basic
Books, New York.
10. Niklas, K.J., 1997. The Evolutionary Biology of Plants. University of
Chicago Press, Chicago.
II. Pagel, M., (ed. in chief), 2002. Encyclopaedia of Evolution; 2 volumes,
Oxford University Press, New York.
12. Rolf, D.A., 1997. Evolutionary Quantitative Genetics. Chapman and Hall,
New York.
DOD

310. 5
Gene Mutations
5.1 DEFINITION
The term 'mutation' was introduced by Hugo de Vries, one of the three scientists
who independently rediscovered Mendel's Laws of Heredity. The term was used
by de Vries for large, sudden and spontaneous inheritable changes in
morphology, survival and behaviour, etc., which appear suddenly in naturally
reproducing populations. After the development of molecular genetics , H.J.
Muller suggested that the term mutation be restricted to change in a gene from
one form to another form or from one allele to other allele. In molecular term,
gene mutation is an alteration in the sequence of nucleotides in a DNA, whether
it introduces a phenotypic effect or not. Change in the sequence of nuc1iotides is
also called point mutation.
Acc ording to recent definition a mutation is any heritable change in the
genetic make up of an individual other than that which may be caused by
the simple recombination of genes.
Mutatio n may include change in the gene structure or composition (the gene
mutation or point mutation) or a change in the chromosomes either in structure
or in number (chromosomal mutation). More frequently the term mutation is
used for gene mutations .
5.2 HISTORY

312. Spontaneo usly occurring inheritable changes were known to plant and animal
breeders long before anything about genes was known. In late eighteenth
century, a male lamb was born in a flock of sheep of a New England fanner, Seth
Wright. It had very short legs and was unable to jump over the fence like the
original breed. The lamb passed on the new characteristic to its offspring and
became the sire of a short-legged race of sheep. Another lamb with similar legs
was born from normal parents in Norway some thirty five years later. In 1853, an
abnormal girl was born
to Negro parents . She had black skin splotched irregularly with white patches.
When married to a black man, about half of the children of this girl also
developed same splotched skin.
Darwin was aware of such sudden heritable changes and called them "sports".
But he considered them too rare to be of much importance in evolution. He
considered them monstrosities than new types.
An opposite view was taken by Bateson (1894) and especially by Hugo de Vries
(1848-1935). In his nutation theory, de Vries maintained that evolution proceeds
by large, descrete and sudden changes and called them mutations. Blakeslee,
working on Datura and T.H. Morgan on Drosophila have shown that origin of
species as described by de Vries in Oenoth era is by no means exceptional but is
a common thing among plants and does occur in animals also.
Morgan (1909) showed that mutations are of all magnitudes in Drosophila.
Today, mutations are observed among bacteria, bacteriophages and viruses as
well as in man, and all other living organisms. With increased knowledge in the
field of molecular biology, it has become clear that de Vries was fundamentally
correct in stressing the significance of mutations to the evolutionary process.
However, mutations alone cannot account for evolution. These provide the raw
material on which other evolutionary forces act to bring about the evolutionary
change.
5.3 CHARACTERISTICS OF MUTATIONS
• Mutations are changes either in the genes or in the chromosomes and are called
gene mutations and chromosomal mu tations respectively.
• Mutations may occur at any stage in the development or life cycle of an
organism. These may be somatic or germinal. The somatic mutations arise in

313. somatic cells of the body and produce local phenotypic changes. These are
nonheritable. Somatic mutations may be represented by patches of modified
tissues in the normal body giving mosaic pattern. Malignant tumors are regarded
to arise by somatic mutations.
• Germinal mutations appear either in the mature gametes or in the cells of sex
organs or during the process of maturation of gametes. These are passed on to
the next generation, i.e., these are heritable.
• If a mutation occurs in one of the mature gametes, it will appear in the single
individual of the progeny only if it is dominant.
• Mutations may be dominant or recessive. The dominant gene mutations are
detected at once , while the recessive ones may remain hidden for generations.
• More frequently mutations are recessive and deleterious. Some of them may be
lethal even.

315. • Mutation is the only known method by which different alleles of a gene may
arise. Multiallelic genes or multiple alleles of a gene arise by mutations.
• Mutated gene may be pleiotropic, affecting more than one character. For
example, white eye mutant in Drosophila not only changes the eye colour
126 ~ Evolutionary Biology
from red to white but also causes transparency of the testicular envelope , and a
change in spermatheca shape, a lowered viability, longevity and fertility.
• Mutations are of all magnitudes but those with slight effects are probably more
frequent than tho se with marked effects.
• Mutations are never directional and do not always involve the same characters.
• Contrary to de Vries concept, mutations do not produce new species.
• Mutations usuall y arise during rep lication of DNA , which usually occur s
during cell di vision .
• More than five times more new mutations are introduced in the population
via sperm than via eggs because more cell divis ions occur in the germ line
Leading to sperm atogenesis than the oogenesis.
• Mutational changes can affect any organ of the body and any kind of traits
both physical, physiological or bioch emical.
• Mutation s are universal, occurring from viruses and bacteria to man or any
plant.
• Mutations in nature occur at random without any concern to their usefulness.
They are neither need-based nor directional.
• Mutations arise due to error in base pairing during DNA replication or
misrepair
of damaged DNA. The process of mutation is neither directional nor adaptive
but a consequence of unrep aired or misrepaired damage.
5.4 KINDS OF MUTATIONS
Mu tational changes are classified according to the nature of tissue, the nature of
genetic material , the stage in the life cycle when mutation occurs and the mutag
enic effect of mutation.
5.4.1 Types of Mutations according to the Nature of Tissue

316. In u nicellular organi sms, any mutations that occur are passed on to the daughter
cells when cell divides . In multicellular organism s, mutations are of the
following two types:
1. Somatic Mutations (Gr. Soma = body): The se mutations occur in the somati c
cells of organisms. They produce local phenotypic change in structure and
function s of the organ where mutations have occurred. Only the descendents of
mutant cell express that chang e. Hence , these mutation s are insignificant and
localised to few cells only.
S omatic mutations are nonheritable and are lost with the death of organism.
However, if somatic mutations occur during early developmental stage or during
embryonic stage , they might show phenotypic effect and may even be
transferred to germ cells .
I n plants, mutant progeny can be obta ined by vegetative propagation like
budding, grafting, layering or cutting, etc. Some exampl es of mutant variet ies
in plants raised by man by artificia lly inducing mutat ions are:
• Emperor seedless grapes
• Golden delicious apples
• Horticultural varieties of garden plants
2. Germinal Mutations or Germ Line Mutations: These mutations occur in
germplasm, germ cells or gametes. These are heritable, are expressed in the next
generation and are established in the populations. Germinal mutations are
established in the gene pool of the populations.
5.4.2 Types of Mutations according to the Nature of
Genetic Material
1. Gene Mutations or Point Mutations: These are sudden changes in the structure
or function of genes or both.
2. Chromosomal Mutations: These are changes in the structure of chromosomes
involving either change in the number of genes or in the arrangement of genes in
a chromo some .
5.4.3 Types of Mutations based on the Stage in Life Cycle 1. Gametic Mutations:
Such mutations are introduced in the gametes at the time of gamete formation .
These are heritable mutations.

317. 2. Zygotic Mutations: These occur in the zygote and are also heritable.
5.4.4 Types of Mutations based on Mutagenic Effect
1. Dominant Mutations: These mutations introduce dominant changes, i.e., a
normal gene becomes recessive and mutant gene becomes dominant. The
dominant mutations express themselves immediately in the cells in which these
are present, whether homozygous or heterozygous. These are easy to be
identified . But dominant mutations are very rare in nature.
2. Recessive Mutations: These mutations produce recessive effect, i.e., a mutant
gene is recessive to normal and is able to express itself only when the mutant
gene is present in both the homologous chromosomes. The mutant character is
not expressed immediately, but takes several generations to become
homozygous.
5.4 .5 Types of Mutations According to their Significance 1. Beneficial
Mutations: When present these are useful to the organisms.
2. Lethal Mutations: These mutations produce a visible effect when
heterozygous but are lethal only in homozygous condition. In such cases, normal
or wild type character is recessive. Sickle cell anaemia trait in human beings is
an example of lethal mutations.
3. Detrimental Mutations: These are recessive mutations that affect viability only
in homozygous condition. The heterozygous individuals are normal.
4. Neutral Mutations: The mutated allele that does not produce a visible effect or
does not affect the fitness of the organism in its natural environment is called a
neutral mutation. Such mutations do not affect or modify the functioning of
proteins encoded by the mutated genes.
Neutrally mutated alleles are neither favoured nor eliminated by natural
selection. They may be lost or their number in the popu lation may increase with
time , purely by genetic drift. Accumulation of neutral alle les in a population
over long time, provides genetic variation in the gene pool of the population and
helps in the divergence of gene pools of closely related populations after they are
isolated.
5. Conditional Mutations: Some mutations produce their phenotypes only under
certain restrictive conditions only. Such mutations are cond itional mutations.

318. For example, many conditional mutants are temperature sensitive and express at
a permissive temperature (around 30°C) because of the presence of temperature
sensit ive enzyme.
6. Unconditional Mutations: The mutations can produce their effect under all
conditions whether perm issive or restrictive.
7. Biochemical Mutations: Biochemical mutations affect metabolic reactions and
change the intermediate metabolites or the end products. In living systems, the
various steps in the metabolic pathways are controlled by enzymes. Usually,
there is one enzyme for each step. Enzymes are proteins and proteins are
synthesised by genes. Normally, there is one gene for the synthesis of one
enzyme . When a gene undergoes mutation, it may fail to produce the specific
enzyme or produces a modified enzyme. In the absence of specific enzyme, the
particular substrate, on which it acts, is not converted into the products. This
leads to the accumu lation of specific substance in the body and causes serious
comp lications that are expressed in the form of a disease . These are called
biochemical disorders and such mutations as biochem ical mutations. (F ig. 5.1)
For examp le, in human being s, ph enylketonuria and alka ptonuria are two
metabolic disorders caused by mutation in genes controlling metabolic pathway
of phenylalanine.
5.4.6 Types of Mutations Based on the Method of Introduction
1. Spontaneous Mutations: Naturally occurring mutations are known as spo ntan
eous mutations. These appear in the progeny of parents which were not treated
or induced with any know n mutation-producing substance. They are known as
spontaneous because the exact cause of their appearance is not known . After the
discovery of mutagenic effect of X-rays, other similar radiations and certain
chemical substances, it was presumed that spontaneous mutations may be caused
by cosmic rays and other high radiations occurring natura lly in the atmosphere.
2. Induced Mutations: Mutations caused by mutagenic agents are known as
induced mutations. It has been shown that mutation rate can be raised well above
Phenylalanine
--.
Tyrosine-
~

319. Phenyl pyruvic acid
- - •• 3,4-Dihydroxyphenylalanine
3 Gene mutation + Albinism
(D Gene mutatio n+ Phenylketonuria Melanin Hydroxyphenyl pyruvic acid
~
2-5, Dihydroxyphenyl pyruvic acid
~
Homogentisic acid
Gene mutation + Alkaptonuria Maleylacetoacetic acid
1
Fumaryl acetoacetic acid

320. 1
Fumaric acid
+
Acetoacetic acid
co
,
+ H
1
,O
FIG. 5.1: Biochemical pathway of metabolism of amino acid phenylalanine and
metabolic disorders caused by biochemical gene mutations (1), (2) and (3).
the spontaneou s rate by exposing to various mutagenic substances. Number of
mutations is in direct proportion with the dose of radiation but is independent of
intensity. For example, a dose of 5,000 rontgens (unit of radiation) will cause the
same number of mutations whether received over a period of 20 minutes or 20
months . Chromo some breaks are presumed to be proportional to the dose.
5.5 CAUSES OF MUTATIONS OR MUTAGENIC AGENTS
Mo st mutant genes arise spontaneously in nature . But many other factors are
also responsible for mutations. Mutations arise when DNA or nucleic acid
undergoes replication at the time of cell division . The copied gene may differ
from the original gene becau se of base substitution. A number of mutagenic
agents have been discovered that increase gene mutation frequency. These are:
5.5.1 Radiations
X-ray and other ionising radiation s such as n , 13 and y-rays, neutrons , protons
, etc., induce mutations and chromosomal aberrations by disrupting chemical
structure of chromosomes. Ultra-violet rays, which are nonionising radiations,
are also mutagenic but these are less effective in breaking chromosomes .
5.5.1.1 Types of Radiations
Radiations may be natural or man-made:
1. Natural Radiations: Natural radiations arise from cosmic rays and radioactive
elements. These occur in small amounts in the environment and are known as
background radiations. These are respons ible for spontaneous mutations.

321. 2. Man-made Radiations: Man-made radiations are produced by X-ray machines
and nuclear power plants.
5.5.1.2 Biological Effect of Radiations
Biologica l effect of radiations is based on the penetration and ionising power of
rays. Hence, radiations are of two types:
1. Ionising Radiations and their Effects: The ionising radiations are X-rays,
gamma rays, alpha and beta rays, neutrons , protons and other fast moving
particles. These are of high energy. These are present in cosmic rays and are also
released by radioactive isotopes, such as 32P, 35S, 238U.
(a) u and 13 rays do not penetrate beyond human skin and do not produce
germinal or heritable mutations.
(b) Gamma and X-rays collide with the molecules of the cells at high speed and
eject electrons from the outer shells of atoms. The atoms, therefore, become
positively charged free radicles or ions. The ejected electrons move at high speed
and, in tum, knock other electrons freed from their respective atoms. When
energy is dissipated, the free electrons attach to other atoms which become
negatively charged ions. These ions then undergo chemical reactions to
neutralise their charge to reach a stable state and during these chemical reactions,
the ionising radiations produce their mutagenic effects.
Ionisin g radiations produce breaks in the chromosomes and chromatids causing
chromosomal aberrations and abnormal mitosis in irradiated cells. These breaks
are presumed to involve the sugar- phosphate backbone of the polynucleotide
strands. The breaks then lead to the loss of chromosomes, chromosome
segments, deficiencies, duplicatio ns, translocations or inversions. These changes
result in the formation of abnormal chromosomes and abnormal chromosome
number in the daughter cells and abnormal functioning. This may lead to cell
death. Often various cancers (specially blood cancer or leukemia) are caused by
radiations.
2. Nonionising Radiations and their Effects: The nonionising radiations and
ultraviolet light rays have longer waveleng ths and carry much lower energy.
Their penetratio n power is much less than X-rays. In human beings, the
ultraviolet rays are often absorbed in the skin and the gonads remain unaffected.
However, these affect microorganisms, pollens of plants and gametes of animals.
Noethling and Stubbe have worked out the effect of ultraviolet rays on pollen
grains of Snapdragon (Antirrhin um). Knapp studied the effects of UV radiations

322. on the spermatozoa of liverwort and Stadler and Uber on com pollen.
The ultraviolet rays are absorbed by nucleic acids and cause alterations in the
bond characteristics of purines and pyrimidines. The bases so altered are known
as photoproducts. Pyrimidines are more prone to such changes. Two adjacent
pyrimidines of the same DNA strand are found to form covalent bonds forming
dimers. The thymine dimer is formed most readily. Dimerisation interferes with
the proper base pairing of thymine with adenine and may result in the pairing of
thymine with guanine (see ionisation responsible for transition). This results in
substitution of T = A to C == G. Similarly, cytosine dimer causes mutations by
becoming deaminated to uracil dimer and produces substitution of G == C to A
= T (transition).
5.5.2 Chemical Mutagens
Several chemicals are strongly mutagenic . Auerbach (1949) found that mustard
gas acts as a powerful mutagen in Drosophila. Formaldehyde and urethane when
mixed with food on which Drosophila larvae grow produce mutations. Triazine
dioxide, caffeine, phenol and some cancer producing compounds are also
mutagens . Demerec (1952) discovered that manganese and ferrous compounds
act as mutagens in bacteria. Organic peroxides are also mutagenic in bacteria.
The mutagenic agents in combination with each other or with other agents
produce a variety of modifying effects. Infrared radiation alone is not mutagenic
but pretreatmemt with infrared followed by X-ray radiation raises chromosomal
aberrations. On the contrary, cells exposed to ionising radiation under conditions
of anoxia exhibit reduced rate of chromosomal aberrations.
5.6 MOLECULAR BASIS OF SINGLE GENE MUTATIONS OR POINT
MUTATIONS
Point mutation s are changes in the number or arrangement of a single base pair
in DNA. These are introduced at the time of DNA replication due to errors in
pairing of complementary bases. Though the process of DNA replication is very
accurate and cells have proof-reading mechanism also, some mistakes in base
pairing do occur due to some internal or external factors . The smallest change in
the structure
-o-~ o ~~o
H~O~O)C_~H2~o.....e-ti''+-(' ~<=O /' ·~oo H CH, UV

323. radiation
-o-p..O THYMINE • -o-~o 0 ..... ~~CH ,6

o
~
=0
H,¢~ o)e-~c=o H
2.~ CH,-~
H c(CH,
,
THYMINE DIMER , THYMINE
Fig. 5.2: Thymine dimer formed by ultraviolet radiation which causes formation
of cyclobutane ring between two thymine residues .
132 ~ Evolutionary Biology
5-Bromouracil (BU)
2-Aminopurine (AP) Replaces thymine by pairing with
guanine
Replaces adenine by pairing with cytosine
Nitrous acid Deamination
,
strand crossing over
Hydroxylamine
___+• NH,OH _____-+-_+ Hydroxylation of cytosine
Ethylmethane-sulphonate4----+
CH.-CH,-O-SO,-CH,---f--...,.Alkylation of purines
Ethylethane sulphonate ~_-_ CH,-CH,-O-SO,-CH,-CH, HC CH.
Acridine orange
'I N I Ho/Nyy"yyN'CH, . ~
Frame-shift mutations by intercalation
Ultraviolet rays (UV) 254 nm wavelength Pyrimidine dimers repair errors
X-rays
5 nm wavelength Breakage of single and double-stranded DNA
FIG. 5.3: Some mutagens, the ir structure and their mode of action .
Gene Mutations
I Substitution Mutations
..
Transition
(Replacement of a purine by another . purine or a pyrimidine by another
pyrimidine in a polynucleotide chain)
Frameshift Mutations
Transversion
(Replacement of a purine by a

324. pyrimidine or vice versa)
+ +
Deletion Mutations Insertion Mutations (Loss of nucleotides) (Addition of
nucleotides)
FIG. 5.4: Types of gene mutations.
of g ene includes gain! loss or the substitution of a single nucleotide , which may
lead to the mutation of one allele of a gene into another allele. Point mutation s
or gene mutations are also called micromuta tions.
The point mutations or gene mutations may be of the following two types : 1.
Substitution mutations; 2. Frames hift mutations
5.7 SUBSTITUTION MUTATIONS
Sub stitution mutation s arise by the replacement of a nitrogenous base by
another base in a codon , i.e., a nitrogenous base of a triplet codon of DNA is
replaced by another nitrogenous base, changing the codon . The altered codon
may code for a different amino acid and may produce a protei n molecule with a
single substitution of one amino acid or it may lead to the termination of trans
lation of a polypeptide in between producing a nonfunctional protein or may
cause no change . The altered protein may produce an altered phenotypic effect
in case the mutation has occurred in a gene that codes for ribosomal RNA ,
transfer RNA, nontranslated regulatory sequence (like enhancer) or protein-
coding region of DNA.
In classic genetics , a mutation that changes a sing le gene locus is called point
mutation. But in modem usage , point mutation represents single base pair
substitution in a nucleoti de sequence of a DNA molecule.
Lewis and John have identified following types of base substitution mutations:
5.7.1 Missense Mutations
Missen se mutation s arise due to the substitution of one or more amino acids for
another one in a protein molecule. This results in altered gene product producing
minor to drastic or lethal phenotypic effect. An example of missense mutation is
sickle-cell anaemia. It is caused by a single base substitution in the sixth codon
in the gene that codes for
~-chain
of haemoglobin. In norma l haemoglobin-A, the

325. sixth codon in DNA for
~
chain of haemoglobin is CTT and in mRNA GAA
which codes for glutamic acid. In
~-chain
of haemoglobin-S of sickle cell persons ,
the sixth amino acid is valine (not glutamic acid) . The substitution of glutamic
acid by valine is due to substitution of thymine (T) by adenine base (A) in the
sixth codon in DNA which becomes CAT in DNA and GVA in mRNA. The
change of codon from GAA to GVA in mRNA codes for amino acid valine and
the changed
~-chain
is responsible for sickle cell trait in man. (Fig. 5.5)
Something More
• Loss of function-mutations are also called null mutations or knock out
mutations.
• Gain of new function mutations are called neomorphic mutations.
• Missense mutations are nonsynonymous mutat ions.
• Silent mutations are synonymous mutations .
• Antisense mutations are nonsense mutations.
5.7.2 Nonsense Mutations
In nonsense mutations, the substitution of a nitrogenous base in a functiona l
codon changes it to stop codon or termination codo n. As a result, a termination
codon is

327. 134 ~ Evolutionary Biology
Wild-type haemoglobin
Wild -type haemoglobin DNA
3' 5'
5' 3'
Sickl e-cell haemoglobin
M utant haemoglobin DNA
3' 5'
5' 3'
Template strand of DNA
mRNA
5' --"'--=-""---3'
mRNA
5·L.__-"'---""-""O"A 3'
G
mRNA
Normal haemoglobin
r;-----''''lI
Sickle-cell haemoglobin Polypeptide chain
A part of 13-chain of normal haemoglobin
A partA partchain of sickle cell haemoglobin
FIG. 5.5: Effect of a substitution mutation in gene coding for pchain of
haemoglobin. Note the codon (sixth codon) of norma l gene is mutated to CAT
by base substitution and codes for GUA instead of GAA in mRNA. This has
resulted in the substitution of glutamic acid by valine in sickle cell haemoglobin.
in troduced in the middle of a functional sequence of nucleotides and causes
premature termination of polypeptide chain synthes is. The incompl ete
polypeptide chain fails to form a functional protein and a norma l dominant gene
unable to produce functional protein mutates to recessive allele.
5.7.3 Synonymous (Silent) Mutations
In synonymous mutation, the substitution of a nitrogenous base in a functional
co- don changes the codon but the changed codo n also codes for the same

328. amino acid because of redund ancy of genet ic code. As a result, the synonymous
mutation does not change the amino acid inW the polypeptide change and has no
effect on the phenotype. Such mutations are also called silent mutations or same
sense mutations.
5.7.4 Types of W Substitution Mutations The substitution mutations can be of
the following two types: I. Transitions 2. Transversions
5.7.4.1 Transitions
T ransitions are changes that involve replacement of one purine in a polynucl
eotide chain by another purine (i.e., A
~~
G) or the replacement of one pyrim
idine by another pyrimidin e (i.e., C
~~
T). The transition substitutions are
described as cop y error mutations because they are introduced during DNA
replication. As a result of transition T = A base pair is replaced by C == G pair
or C == G pair is replaced by T = A pair.

330. Gene Mutations !iJ 135
Wild type
DNA template strand 3' T
A ~C
T T C A A.A·C:C G A ·.T .T4 S' S' 3' mRNA S'
IA U G A A G U U U G G C U A A! 3' protein ) Met H Lys H Phe H GIY ~
Amino end
Carboxyl end T instead of C
3' S' S' 3'
Missense A instead of Gmutation
ING C U A N ymRNA SI A U G
A A G U U U
IMet H Lys H Phe H Ser I~
Altered protein due to changed codon
A instead of T
3' S'Nonsense
S' A T G 11'u11 A G ·T T T G G C T A
3'
mutation
U instead of A
G'0' A G U U U G G U U A AIYS'IA U
IMetI¥
A instead of G
3' S'Silent or 3'
Synonymous S'
mutation U instead of C S' IA U G A A G U U U G G l
UI U
A A I3'
IMet H Lys H Phe HGly I ¥
Premature
termination of
transcription due to stop codon
No effect because changed codon also codes for the same amino acid
FIG. 5.6: Different types of mutations produced by the substitution of a single
nitrogenous base and the changes introduced by them .

331. Transitional substitution s are introduced due to following factors present in the
cell's internal milieu which interfere with the correct base pairing at the time of
DNA replication:
• Tautomerisation •Ionisation
• Base analogs •Deamination
1. Tautomerisation: Nitrogenous bases (adenine, cytosin e, guanine , thymine
and uracil) exist in more than one chemical forms. These alternate forms are
called tautomers and this phenomenon as tautomerisation. It is caused by the
shifting and rearrangement of hydrogen atoms from their normal position. These
are called tautomeric shifts. Due to tautomeric shift, the amino group (-NH
2)
of
cytosine and adenine is converted into imino (-NH) group and likewise keto
group (C = 0) of thymine and guanine is converted to enol group (C-QH).

333. 136 /j) Evolutionary Biology
In its rare or tautomeric state, a nitrogenous base cannot pair to its normal
partner. Rather, a tautomeric adenine pairs with the normal cytosine and
tautomeric guanine with thymine. Similarly, tautomeric thymine pairs with
normal guanine and tautomeric cytosine with adenine. Such pairs of nitrogenous
bases are known as 'forbidden base pairs' or 'unusual base pairs'.
The rare bases can introduce mutations during DNA replication. For example, if
adenine in the chain of a parent DNA is in rare state, the complimentary new
chain formed from this chain will contain cytosine. At the time of next
replication, this cytosine (introduced into the complementary strand due to
tautomeric shift in adenine) would pair with guanine. This will produce a
substitution of A = T base pair by G ;: C pair. Similarly, a substitution of G ;: C
by A = T pair can be produced if cytosine is in tautomeric state.
The copy error situation is not stable because at the next replication the
tautomeric base returns to normal state and pairs with thymine.
2. Ionisation: Transitions may also be introduced by ionisation of base at the
time of DNA replication . Ionisation involves the loss of hydrogen from No. I
nitrogen of a nitrogenous base. For example, in its ionised state, thymine pairs
with normal guanine and ionised guanine links with normal thymine. Similarly,
ionised adenine pairs with normal cytosine and ionised cytosine pairs with
normal adenine.
3. Base Analogs: Certain chemical compounds have molecular structure similar
to nitrogenous bases present in DNA nucleotides . These are called base analogs.
These are usually derivatives of nitrogenous bases of DNA and occur as natural
as well as artificial base analogs. Some of the natural base analogs are 5-methyl
cytosine (in wheat and grasses), 5-hydroxymethyl cytosine, 5-glucosyl
hydroxymethyl cytosine (in E. coli), 5-hydroxymethyl uracil (in certain viruses)
and 6-methyl purine (in some bacteria) .
The artificial base analogs are 5-bromo uracil (5-8U), 5-iodo uracil (5-IU), 2-
bromo and 5-methyl-cytosine. The former two are base analogs of thymine and
,/
r>
, ,'
L....,'---:-i
- T""'I
";'i----rb---:
- ',
i-,-'-·'----.--"
A

334. i'i
-b,.....;.
IIIi T G
G
T
~
i
~ ! I~
i
G i ~ I~
III
~
Original DNA
I
i
I:
I
i
I
:
I
I
c
I
I
I I
I
7
~ 2 ) 3
I
4 ) 5 6 7 8
' - -'
~
'-
'

335. 1
I r I : I I
A
:
C
:
G G T
II i III i III III II
I ,
Transition
T : G : C C AI I I ' I
i , i

,
I I
7I , I

I
~
2 ) 3
~
4 )
' - -"
5 6 7 8
'-'
FIG. 5.7: Transition mutations are caused by substitution of purine by a purine or
pyrimidine by a pyrimidine in a DNA
molecule.
latter those of cytosine.
(a) 5-Bromouracil: 5-bromouracil (5-8U)
(5-methyl uracil). Its chemical structure

337. is a structural analog of thymine is very muc like that of thymine.
Common State
Amino group
IPurines
l Tautomer form
Imino group
),
,.-----.
)..--==..-,
N " I
\._ N
N"
H I"/ <, :
,p---) \,\H_C'~~h""-c-L~---: \ .. H_C7'
"C~6\ NH --',';\8 9 II S ... 6\, ; ::w:=~ \N- -C
II \
\N=C=C
-
(Amino form) / \ 3
2
}---'H/
\1Aden ine N-C4 '-, 1N '"
H
N
=C
\
H H
Adenine \ (Imino form)
Keto group
Enol group
t :::...-,
~
-,-':':':_-J N
',' 0 " ,
~ N,,: ,'
C/):\\\87' 7"":: Ii: \" ..
H
-;><, :1
H
-C
/

/

:6\C: !
--==~
\
II
C-!T-
9
- C~
If .. \~--. S \ -/ N- -C " 1N :Guanine N- ", 1NH
1------'
I
(Keto form)
/\ 3 2 !----'H /
\'
C
N= \H N=
NH
2
NH
2
Guanine (Enol form)

338. (Amino form) \ 1 2
I Pyrimidines I
Amino group
Imino group
,.r;
:::::
.
....:
r--' A"-{ ...
:
:
H ::
" : I -,\ :
.
H #\ I
\ --.LJ
N- H \.
;C
s
\:~ )
"
..
..
Cytosine H-C6 ~7---'
/
N-C
H
~
0
'; S\:\,. JJ
H-C6 '~ NH-,"
"
\
1 2/
-
N-
C
Cytosine /
~
(Imino form) H 0
Keto group ::-------..-..
~
He ,{ 0
-';cs
: '1/
...~

339. Thymine
H
-C6
\ NH

,'
f'...,
3 \ "
C ) ..
(Keto form) \ 1 2 ;------'
Enol group
H3C\ :.-f:
~~
1---'t',\..
); s
~C
\:~ "
H-C6 -,
3N
1 2j-----"
ine (Enol form)

341. /
N-C
\
N- -C Thym
~
/
~
0H 0 H
FIG. S.8: Forbidden base pairs resulting from tautomer
isation (Levin. 1969).
138 ~
Evolutionary Biology
Tautome~ ~ }
Tautomerisation0T / "
A C
Normal base pair
Tautomer A pairs with C and notT
~ ~ ~} (forbidden base pairing)
"" 1 B Mutant base pair
-,~ } Norm: 1
replication A = T-. G = C
/ rn
}
Normal replication

342. m C/UJ Mutant base pair
C
"<.
A T
II IG= C-. A= T I
Tautomerisaiton 0 G }
m~ III Normal pairTauto
C pairs with A and not with G C
FIG. 5.9: Mechanism of substitution of A = T pair by G '" C and G '" C pair by A
= T by tautomerisation .
Adenine
(Imino form)
F Orbiddenl
base pair
A=C
Base-pair changes
I~
' ~Ii Mutational step at the time
.... of DNA replication Mutational step at the time
o f DNA replication
~ lonised A ~ pairs with C
Ionised T pairs with G ReP
(C-strand
l ica tionL~
- G Replication
template) ~__~ (G-strand

344. template) A B FIG. 5.10: Forbidden base pairs of adenine with cytosine and
thymine with guanine result ing from the ionisation of No. 1 and No. 3 nitrogen
respectively (After Levine , 1969).
If bacterial cells grow in a medium containing 5-BU, it is incorporated into
newly replicated chain of DNA in place of thymine. 5-BU can also be found in
two tautomeric forms.
Its keto form is more common while the enol state is rare or it may occur in
ionised state. Its normal keto form pairs with adenine and the rare enol form
links with guanine. The enol form is short-lived and soon changes to normal keto
form . In enol form BU pairs with G. Whenever such mispairing between G ==
BU occurs and DNA undergoes further replication, G == BU is replaced with G
== C. Keto BU pairs with A which on next replication pairs with T. Thus , G ==
C pair is replaced by A = T pair.
000
II II II
/
HN 3
4 5 C 3 4 5 C 3 4 5
C~
~ HN
/C~
--@8J HN
/C~
C--{8]
1
2 611
-:?'

1
2 6 11
1
2 6 11
o
~C<,1 / C- H C<,1~- H
0
# C<,1 / C- H
NO
N
/ N
I I I
H H H
5 Bromouracil Thymine Uracil FIG. 5.11: Base analog 5-bromouracil is
structural base analog of thymine. NH,
N
~;~~~5:~NUN~
N N
N H ~N
{
H

345. Adenine
6-Methylaminopurine
2-Aminopurine 2, 6-Diaminopurine
H~116
~
CH 3
o o

HNy HN HN I
H ' OH
a'l'~NJl HH H

346. o o
:jLHN
a'l'~ H
H
y
H a'l'~ N ~H a'l'~ N H a'l'~N H
Thymine
Uracil 5-Bromouracil 5-Fluorouracil 5-Hydroxymethyl uracil o o o

347. ~]c
f~
H
:X~~
HN
:.x
N):ill
HN
, H ~ N HN~N
,
' H Gaunine Hypoxanthine (-inosine) 8-Azaguanine
NH,

348. N~H C
H
N3=CH,oHI N~cf'~~~H N3:,1
cf'l N H cf'l N I H cf'lN)lH
H
H H H
Cytosine 5-Methylcytosine 5-Hydroxymethyl cytosine 5-Bromocytosine FIG.
5.12: Some base analogs of aden ine , thymine, guanine and cytosine.
140 [j] Evolutionary Biology
(b ) Aminopurine: Aminopurine is an artificial base analog of adenine . It can
substitute adenine as well as can pair with cytosine. This cau ses substitution of
A = T pair by G == C pair as shown in Fig. 5.14. The incorporati on of base
analogs like BU, AP, etc., into DNA chains can be chemically detected.
4. Deamination: Certain chemical substances like nitrous acid (HN0
2) ,
hydroxylamine (NH
20H)
, diethyl sulphate (DES), ethyl methane sulphonate
(EMS) , ethyl ethane sulphonate (EES), nitrosoguanidine (NTG) and
nitrosomethyl urea (NMU), change the base sequence in DNA by a series of
chemical steps. Some of them like nitrous acid and hydroxylamine cause
oxidative deamination of nitrogenous bases by replacing amino group (-NH
2)
by
hydroxyl group (-OH). Th e deamination of cytosine leads to the
Pairing of 5-Bromouracil with Adenine
Keto group H
l.
-,
Br
,/
O
-\- -H-N
" II
/
N
\
C---l-C '
,/ \
C-C
/ 7 ::::::::'8CH
1/
5

349. :
'
~
X '
II
6
5
~
I 9 N HC 6 3 N H----- N 1 4 C-\ \
1 2 / \ 2 3 /
N-C HC=N
dR
dR
/~
0
5-Bromouracil
(Normal keto state)
Pairing of 5-Bromouracil with Guanine
E nol group •• --',
Br
> 'O[8}~ - - - - o
N
\ //;// ~
C-C
/ 7~8/ C H
1/
C----!.C .
3 NHC 5 4,\,," /
- -H
-N
1
6 5~
6 3 N 4 C \ \ 1 2 / \ 2_3 / dR
/
NC C-N
~
O - - - - -H-N
/
dR
5-Bromouracil \ H
(Rare enol state) Guanine
FIG. 5.13: Base pairing of 5-BU with aden ine and guan ine in keto and enol
forms respectively.

350. A
/
A} Normal
~
replication
II
T
~ T~~}
II T

351. <. AP
Nor'!1al
pair
IA =T-+G~C I
C
I-----rn
G
I
III Mutation C pair
FIG. 5.14: Mechanism of mutation caused by the substitution of G == C for A =
T by base analog aminopurine.
formation of uracil, deamination of adenine forms hypoxanthine (H) and of
guanine forms xanthine. Hypoxanthine exhibits bonding similarity with guanine.
At the time of DNA replication, uracil pairs with adenine and hypoxanthine pairs
with cytosine. This leads to the substitution of A = T for G == C and G == C for
A = T.
• Substitution mutations may arise spontaneously through copying errors during
DNA replication by forbidden base pairing or unusual base pairing between
adenine and cytosine or guanine and thymine.
• Some chemically reactive by-products of cell metabolism alter the base-pairing
property of some nitrogenous bases by ionisation and taut omerisation.
• Base substitutions may occur by the action of mutagenic agents like nitrous
acid, alkylating agents and mustard gas. These chemicals are responsible for
deam ination and depurination.
• Deamination and. depurination are the most frequent chemical reactions known
to create substitution mutations.
• UV radiation from sunlight promotes covalent linkage between two adjacent
thymidine molecules forming the thymidine dim er.
• It is estimated that a total of about a trillion (10
12
)
purine bases (A and G) are
lost from DNA in the cells of our body by depurination just in a second.
• Replication mechanism is estimated to make approximately one error per 10
7
nucleotide copied.
• Fortunately, the cell's ' DNA mismatch repair system' corrects upto 99% of
these errors.

352. • The overall accuracy of DNA replication comes down to one mistake in 10
9
nucleotides copied.
The transitions introduced by nitrous oxide and hydroxylamine etc., are one-way
transitions. These transitions do not undergo reversion.
5.7.4.2 Transversions
Transversions are changes that involve replacement of a purine in a
polynucleotide chain by a pyrimidine (i.e., A
~~
C ) or of a pyrimidine by a
purine (T
~~
G). As a result of transversion, T = A may be replaced by A = T or
by G == C base pairs and G == C pair may get replaced by C == G or T = A
base pairs.
Transversions are introduced by certain alkylating agent s, like ethyl methane
sulphate (EMS) and methyl methane sulphate (MMS) which cause depurination,
i.e., removal of a purine nitrogenous base from DNA polynucleotide strand.
These chemicals alkylate the purine bases in the nitrogen at the sixth position in
guanine and adenine. The alkylated purine then moves out of DNA strand
(Depurination).

354. The removal of a purine from the strand of DNA (depurination) leaves a gap at
that point (Fig. 5.19). At the time of replication, any of the four bases can
possibly get inserted at this place in the complementary strand. If the base is
replaced by a pyrimidine, it is transition and if by purine then it is transversion
(Fig. 5.19). In
142 ~ Evolutionary Biology
I
A. DeamlNltlon of adenine
A. DeamlnatJon of cytosine \
N
\ /

\
C-C
r:
C-C ,, 5 4~ Nitrous-e,,5 4\--e , ~ 6 3N
\ \ 1 2
/
/ /
N-C
'\
I
Hypoxanthine
I
dR 0
I dR 0 ICytosine I Uracil I
B. Palring of hypoxanthine with cytOSIM 8 . Pairing of uradl with adenln.
Ic.Subatttutlon of A_T palf1lfor Gil CIC SubstiutJon of CIiiG pairs for T_A

356. Repl
(A-slIand
Gication
L
lemplate) I~=~ I
FI G. 5.15: Mechanism of mutation caused FIG. 5.16 Mechanism of mutation
caused by by nitrous acid by deamination of adenine to nitrous acid by
deamination of cytosine to
hypoxanthine and its pairing with cytosine. uracil and its pairing with adenine.
the next cycle of DNA synthesis , a DNA molecul e is form ed which contains
the complete transversion. These transversion s are nonreversibl e.
The occ urrence of tran sver sion was first postu lated by E. Freese In 1959. A
lkyla
ti
ng agents can introdu
ce:
I I I r I I I I
A T G C T G G T 1. Trans ition by substituting a purine
II II III III II III II I II
Original
DNA for
a
purine
or
a
pyrimidi
ne
fo
r
a
T A C G A C C A
I I I
I I I
I
I pyrimidine or
2 3 4 5 6 7 8

357. 2. Transversion by substituting a purine 1111
for a pyrimidine or a pyrimidine for
( ,
-
)
a p
urine
.
r
I
I
I
(
:
I II II I I I:
A T G C A:: C:: T:: G
II II III III :
I Transversions I
5
.8
FRAMESHIFT MUTATIONS
I I I I I I IIG::11 :: 11 1:
T
A
C
G
:
11:: 11 1::
T:: A::C:
J II
I
I I
I
I I iii i Ii I
2 3 4
~..~ : ~..~ : ~!) t..~) Th e mutations caused by the additi on
FIG. 5.17 Diagram to
show transversion
or deletion of nitrogenou s bases in the type of substitution
mutations.
DN A or mRNA are known as frameshift mutations, because these shift the
reading frame of codons from the site of change onward . One deletion may be
neutralised by one addition or vice versa, provided they occur at same place or
very close to each other. Three deletions or three additions may disturb only a
few frames.
Frameshift mutations in which reading of genetic message is altered beyond the
point of change result in the formation of a completely altered polypeptide chain.

358. 5.8.1 Types of Frameshift Mutations
The frame-shift mutation can be of the following two types:
I. Deletion Mutations: These mutations are caused due to the loss or deletion of
one or more nucleotides.
2. Insertion Mutations: These mutations are caused by the addition of one or
more extra nucleotides in a DNA molecule at one or more places.
5.8.2 Mechanism of Origin of Frameshift Mutation
Acridine dyes have been found to cause deletion or insertion of a single base
pair. Acridines like 5-aminoacridine and proflavin become intercalated between
I
A. Alkylation of Guan ine IIC2 Hs i. I
N
\
0
HC /
1
8~7 '"
C-C
/
N9 J 5 6~
- C 4 1 N
dR \
N3 2!
\
NH
2
Guanine
0 e-Ethylguanine (OeEtG) B. Pairing of 0 e-ethylguan lne with thymin e
IC2 Hs i
...-o::::: N
\
0 0
I
CH
HC ;:::::'-- -, / ~ 3
C-C
" IDepurinationIdR N=CIN CH
\
\
HN-H-O
I
N _!I

359. C-C
I ~
/
\
'\\
H-N c-l
II \N
° e-Ethylguanine
Thymine
FIG. 5.18: Mechanism of depurination caused by alkylating agent. The alkylated
guanine pairs with thymine and leaves the DNA strand .
144 Iil Evolutionary Biology
Strand I Strand II
A=T A=T
c
o
:
-
- - -~ :
~:;:;
Depurination
I
.. c
A=T.oro
- - ------,
<
tIJ·~
~!;I=G
C==G
Parent
strand Parent strand A=T
-1
f- A = T

II
C~

~=T
C==G~ tC G
Unusual substition
Normal replication
A=T
A=T~ C_ G

360. FIG. 5.19 Substitution of A = T by C '" G as a result of transversion.
two adjacent purines and increase the distan ce between them from 3.4 A° to 6.8
A
o .
At the time of DNA replication, the dye molecule moves out and the gap is
either filled by the introduction of a purine or pyrimidine base pair or the gap is
deleted with the loss of a base pair. The former adds a base while latter leads to
its loss. Introduction of an ethylated base pair removes one base pair from DNA
and causes frameshift deletion mutation.
5.9 MUTATION RATES
Mutations are unavoidable. They are the properties of genes or more correctly of
alleles. Normally, mutation rate is the rate at which an allele will change by the
substitution of a pair of nucleotides each time the DNA is replicated i.e., each
time a cell divides. The average mutation rate or nucleotide substitution rate
per replication is one in 10 billion i.e., 10-
10
• In eukaryotes the rate of mutation
in the entire genome in the life time may vary from 0.1 to 100 mutations per
genome per generation. The rate of mutations in functional genes is close to
0.003 mutations per generation . But all loci or all regions within a locus are
equally mutable . Mutation
Gene Mutations Iil 145
Extra A
Template
DNA
Insertion of one nucleotide pair
mRNA
Frameshlft causing immediate nonsense (1 nucleotide-pair Insert ion)
mRNA
5
'
'4. .
Lyl..!!!issinJlj 3·
Mutant Met ~ Lys H Leu H Ala roopolypeptide
Frameshift causing extensive missense
(1 nucleotide-pair deletion)
3'''
Met ~ Phe

361. . missing ,3=
H Gly I~
No frameshift , but one amino acid missing (3 nucleotide-pair deleti on).
Deletion of one nucleot ide pair (all amino acids in the polypeptide chain changed beyond the deletion) shift
of codon reading frame
Deletion o f a codon
(3 nucleotides) causes
one amino acid missing. A 3 nucleotide pair insertion (not shown) would lead to an extra amino acid in the
polypeptide chain
FIG. 5.20: Frameshift mutations cause shifting of reading frame of genetic
message beyond the point of insertion or deletion.
r ates are found to be different at both phenotyp ic and molecular levels, among
and within the loci.
Th e nonsynonymou s substitutions occur faster than the synonymous
substitutions. Mutati ons occurring in functional genome that code for proteins
are identified while mutations occurring in junk DNA ar e functionally neutral
and remain silent. Sim ilarl y, gene s for morphological traits mutate less faster
than genes that control life history traits.
Eyre -Walker and Keightiey (1999) studied amino acid composition of 46
different proteins in humans and chimpanzee to discover changes that occurred
during the period of their divergence. These were estimated to be 4.2 amino
acids changes by missense mutations each generation.
/--{.:::::::~ Nitrogeno us bases
)----:e..-/. Acridine dye molecule DNA strands
A B
FIG. 5.21 : A. Normal DNA molecule ; B. DNA molecule with acridine dye
molecule inserted between purines.
ALT T
I I
First repl ication Second replication

362. A -H A nucleotide TLA
~ A is introducedI- cii{ GL
~
t
I
6.8 AO

X
Ie ----!"GGap
3.4 AO I Addition
...
A ---I T
I
T

363. G
------t-'
Acridine Gf-e
Segment,
dye
lost'" T molecule
A I
rT
:A -----Yei
rG~ e iI
-----jG e
~
e
De
letion
I
G rI I
FIG. 5.22: A. Addition of a base pair with the help of acridine dye; B. Deletion
of base pair produced by acridine dye.
Mutation rate for spontaneous mutati ons is much less as compared to induced
mutations. The mutation rates of induce d mutati ons are enhanced by polym
erase enzyme that replicates DNA as in the case of infectiou s agents like
viruses, (herpes simplex, rubella and chicken pox).
5.10 EFFECTS OF MUTATIONS ON FITNESS
E ffects of new mutat ions on the surviv al and reproduction directl y influence
the fitness of the populations or species. According to their fitness effects,
mutations are classified into following categori es:
• Lethal mutations kill the individuals that carry them and are highl y
disadvantageous.
• Deleterious mutations reduce fitness or survival ability of individuals but do
not kill them .
• Neutral mutations neither reduce nor enhance fitness, i.e., they are neither
harmful nor beneficial to the individuals carrying them. They are most
numerous.
• Advantageous mutations increase fitness of the posse ssors and are favoured by
natural selection.

364. This classification of mutations is context-dependent, because:
• A mutation may prove to be advantageous in one environment but may be
neutral or deleterious in a new or changed environment.
• The fitness of the mutant will depend on the interaction of whole genotype of
the individual and not on one individual gene .
5.11 RANDOMNESS OF MUTATIONS Mutations to some extent are random
because:
I . It cannot be predicted that out of a large number of gene copres of the
individual which one will undergo mutation.
2. Mutations do not arise in response to environmental needs. It means
environment cannot induce adaptive mutations.
"The adaptively directed mutation does not occur", is one of the fundamental
tenets of modem evolutionalry theory. Mutations are adaptively random rather
than directed was demonstrated by Joshua and Esther Lederberg (1950).
By using replica plating technique, they showed that in Eschirichia coli,
mutation for penicillin resistance arose spontaneously and was already present in
the bacteria before they were exposed to penicillin. Initially bacteria were grown
on agar plate without penicillin. It contained numerous colonies ofE. coli, each
being derived from a single cell (step I). These colonies were tested on the
master plate for penicillin resistance (step II). They pressed a velvet cloth against
the plate to transfer bacterial cells from each colony to the velvet (step Ill). They
then touched the velvet to a new agar plate with penicillin. This transferred some
bacteria from master plate to this replica plate in the same order as present on the
master plate (step IV). Only a few colonies appeared on the replica plate (step V)
which had penicillin-resistant mutant cells . This proved that some bacterial cells
were already penicillin-resistant and this mutation had occurred before bacteria
were exposed to penicillin.
Utility of a new mutation is accessed by natural selection in the existing or in
changed environment. The advantageous mutations are probably the rarest type
of mutations and neutral or synonymous mutations are the most numerous ones .
They add to the genetic diversity in populations.
II Each colony is tested, and some colonies prove to be resistant to penicillin Colonies of E.coli derived
from a single cell are grown
on an agar plate

366. FIG. 5.23: Lederberg's experiment with E.coli bacteria that mutations
are not induced by the environmental need.
5.12 MUTATIONS AND GENETIC POLYMORPHISM
Occurrence of different a lleles (more than two alleles) on a gene locus in a popu
lation represents multip le allelom orphism or genetic polymorphism. It is
estimated that about two thirds to three quarters of all loci in many species are
polymorphic, and an average individual is expected to be heterozygous for about
one quarter to one-third of all its loci. This means in Drosophila species with an
estimated 2,000 gene loci, a single fly is presumed to be heterozygous for about
500 gene loci or even more ; and in human being s with an estimated 25,000
gene loci, an individual can be heterozygous for above 6000 gene s or more .
Britten (1986) estimated that of the three billion nucleotides in the haplo id
genome per human, one person differs from another on an average of above five
million nucleotides.
Genetic polymorph ism is essential for evolution. It allows popu lations to
confront new environmental cha llenges and som e of their mutations may prove
to be advantageous in the new environment.
5.13 MUTATIONS AND EVOLUTION
At present , mutations along with variations are consi dered to be the raw
material for evolution. Mutations constantly generate new hereditary variants.
These variants accumulate in popu lations leading to geneti c polymorphism.
Other forces of evolution like natural selection, genet ic drifts, isolation, etc..
operate on mutations to bring about genetic divergence in the naturally
interbreeding populations.
Most mutations are more or less disad vantageous to their possessors but may be
neutral and rarely advantageous too. Classical mutants obtained in Drosophila
usually show deterioration, breakdown or disapp earance of some organs.
However, all the mutations are not harmful. Useful mutati ons are also known .
Also mutations are normally recessive and have their ill-effect only when
homozygous. Such neutral and deleterious but recessive mutations accumulate in
the gene pool of population without serious , harmful or deleterious effects. They
furnish a reservoir of genetic variability for evolutionary force s to opera te upon
.

367. The organisms are very delicately adju sted to the environment in which they
live and any change will natura lly be in disharmony. But if these organisms are
placed in the environment other than that in which they usua lly occur, their
mutations might prove to be advantageous. This has been accounted by
Dobzhansky by presum ing that the mutat ions which improve adaptedness of
the species to its normal environment were tried out by natural selection, and
have been incorporated into the "normal" genotypes.
For example, expo sure of insect popu lations to DDT has led to a widespread
increase in the frequency of DDT-resistant insects because of any of the
following changes:
• an increase in lipid content that allows the fat soluble DDT to settle down in
the body tissue
• release of enzyme s that break down DDT into relatively less toxic products
• chang es in the permeability of insect cuticle to DDT absorption
• reduced toxic response of nervou s system to DDT
• behavioural response to reduce contact with DDT
Wh en environment change s, the adaptedness of the organism is disrupted and
the harmony between environment and its inhabitant s can only be established
again by changing the genotype. Some mutant s prove useful in new
environment and replace the old normal genes of the population forming the new
adaptive norm.
Mut ations are very important for the survival of the species. A living species
that would suppress mutation process might gain a temporary advantage in an
unchanged environment. But when the environment changes and some of the
mutants are found to be better fitted than the normal , a nonmutable species
would be the looser and the mutated species with the accumulation of new
mutation s will be favoured by the natural selection.
• Back mutation
• Deamination
• Ionising radiation
• Mutagens
KEY TERMS

368. • Base analog
• Frame-shift mutation
• Induced mutation
• Mutations
• Nonsynonymous mutations · Point mutations
• Spontaneous mutations • Synonymous (silent)
• Transversion mutations
• Conditional mutations
• Forbidden base pair
• Missense mutations
• Nonsense mutations
• Substitution mutations
• Tautomerisation
• Transition
REVIEW QUESTIONS
I. What are mutat ions? Discuss the importance of mutations in evolution.
2 . What is mutation? How can you detect mutation in an organism? Describe its
role in evolution.
3. Define mutations. What is their role in specia tion?
4. Briefly amplify the role of gene mutations on the basis of evolution.
5. Describe molecular basis of gene mutations.
6. Give an account of the role of mutations in organic evolution.
7. Give an account of gene mutations and chromosomal mutations.
8. Give an acco unt of the types of mutations and factors responsible for their
introduction in living beings.
9. Write short notes on:
(a) Gene mutations
(b) Spontaneous and induced mutations
(c) Gene mutations and chromoso mal mutations
(d) Mutagenic agents

370. (e) Chemical mutagens
(f) Radiations .
10 . How can we discriminate whether mutations arise randomly without regard
to their adaptive or non-adaptive effects (preadaptively) or whether they arise as
adapt ive responses to specific selective environmental effects (postadaptively)?
II . Can gene mutation rates account for the prevalence of gene mutation s in
population s.
FURTHER READINGS
I. Carroll, S.B., J.K. Grenier, and S.D. Weatherbee, 2005. From DNA to
Diversity. Molecular Genetics and the Evolution of Animal Design, 2nd ed.,
Blackwell Publishing, Malden, MA.
2 . Fox, C.W., J.B. Wolf, 2006. Evolutionary Genetics: Concepts and Case
Studies. Oxford University Press, New York.
3. Futu yma, 0.1., 2009. Evolution (2nd Ed.). Sinauer Associates, Inc. Publi shers
Sunderland, Massachusetts USA.
4. Hall, Brian K., and B. Hallgrimsson, 2008. StrickbergersEvolution: The
Integration of Genes, Organisms and Populations. Fourth ed., Jones and Bartlett
Publishers, Sudbury, Massachusetts, Boston, USA.
5. Hey, J., WM . Fitch and F.1. Ayala, 2005. Systematics and the Origin of
Species. On Ernst Mayrs IOOth Anniversary. The National Academie s Press,
Washington, DC.
6. Houle, D., and A.S., 2006. Mutation. In e. W Fox and 1.B. Wolf (eds.)
Evolutionary Genetics, pp. 32-48. Oxford University Press, New York.
7. King, M.e. and A.e. Wilson, 1975. Evolution at two Levels: Molecular
Similarities and Biological Differences between Humans and Chimpanzees,
Science, 188, 107-116.
8. Li, WH., 1997. Molecular Evolution, Sinauer, Sunderland, MA.
9. Lederberg, J. and E.M. Lederberg, 1952. Replica Plating and Indirect
Selection of Bacterial Mutants, 1. Bacteriology, 63: 399-406.
10. Leigh, E.G. Jr., 1973. The Evolution of Mutation Rates, Genetics Suppl.,
73:1-1 8. II . Long, M., E. Betran, K.Thorntom, and W Wang, 2003. The Origin
of New Genes: Glimpes of the Young and Old. Nature Rev. Genet., 4: 865-875.
12. Pagol, M., 2002 Encyclopedia of Evolution. 2 volumes, Oxford University
Press, New York.
13. Roff, D.A., 1997. Evolutiona ry Quantitative Genetics, Chapman and Hall,

371. New York.
0 0 0

373. 6
Chromosomal Aberrations
6
.1
CHROMOSOMAL ABERRATIONS
The visible changes in chromosome structure are called chromosomal
aberrations or alterations of chromosome structure. These include changes either
in the number or in the arrangement of genes in the chromosomes. These are
also called structural aberrations or the phenotype of the chromosome.
6.2 ORIGIN OF CHROMOSOMAL ABERRATIONS
Chromosomal aberrations are produced due to chromosome breakage and
irregularities in the rejoining of broken pieces. Chromosome breakage occurs
spontaneously in low frequency in almost all animals . Cosmic radiation,
nutritional deficiencies and environmental conditions are responsible for
spontaneous chromosome breaks. Their frequency is altered by several factors,
such as age, oxygen availability, temperature and metabolic condition.
Breaks in chromosomes can be induced by radiations (X-rays, gamma rays, p
rays), chemical mutagenic agents (i.e., alkylating awgents, insecticides,
pesticides, herbicides, fungicides and base analogues) and by several viruses.
Some genes are also known to induce chromosome breakage .
6.3 TYPES OF CHROMOSOMAL ABERRATIONS
Chromosomal aberrations can be of the following types:
1. Change in the number of genes in a chromosome
(i) Loss of genes from a chromosome (Deletion or Deficiency) (ii) Addition of a
block of genes (Duplication)
2. Change in the arrangement of genes in a chromosome

375. (iii) Altered sequence of genes due to rotation of a block of genes by 180
0
(Inversion)
152 !il Evolutionary Biology
3. Change in the arragement of gene s
(iv) By the exchange of genes between nonhomologous chromosomes or transer
of genes from one linkage group to other (Translocation)
6.4 DELETION OR DEFICIENCY
D eletion or deficiency is the loss of chromosomal material due to breakage of a
chromosome segment. The loss of chromosome segment occurs when a portion
of chromosome gets detached and the broken segmen t without centromere is
lost and digeste d by enzymes. The portion of the chromosome carrying the
centromere, functions as a genetica lly deficient chromosome. Depending upon
the length of lost segment from the chromosome, the loss of genes may vary
from one to severa l genes .
Al though , deficiency and deletion are synonymous, but a distinction is often
made. The loss of terminal segment of chromo some is often described as
deficiency, whereas the loss in the intermediary position of a chromosome is
designated by deletion.
6 .4.1 Types of Deletions
Deletions are of two types:
I . Terminal Deletion: When the lost segment is terminal with telomeric end, the
deletion is terminal.
2. Intercalary or Interstitial Deletion: In interca lary deletion, the lost segment is
without telomer ic ends and is intercalary.
The chromoso me with the loss of telomeric end becomes unstable and tends to
fuse with a similar damaged end. Thus, deficiencies which have estab lished in
the populations are either intercalary or terminal deletions, capped by very small
telomeres.
T·''''-IIEiiil Summary of Different Types of Chromosomal Aberrations
Aberration Type of Aberration Change in Chromosome
I. Deletion
(AIB IC~FIGI H)
!

376. Adeletion removes a chromosomal segment
(AIB lcIE}{FIGIH)
1 . Terminal deletion Loss of a segment from a chromosome.
Lo ss of terminal

378. segment (having telomere). Effect on
Chromosome and Phenotype
(i) Produces
pseudo-dominance of the recessive
alleles.
( ii) Rare.
(iii) Lost segment forms a ring chromosome.
2. Interstitial deletion
II. Duplication (Repeat)
~~ B I C+
01 E):{FI G I H)
!A duplication repeats a segment (AIBlcIBlcloIE~
1. Tandem duplication
2. Reverse tandem Loss of an
intercalary segment of chromo-some from the region
between telomere and centromere. Presence of same block of genes
more than once in a chromosome.
The repeated block of genes located
just next to the
original seq-uence; gene sequence similar to the original sequence.
The repeated block of genes located
next to the original block but sequence of genes in the
repeat is reverse. (i) Provides additionaI genetic material for mutations .
(ii) Increases
genome size
(iii) Causes
evolution of new genomes.
3. Displaced duplication
4. Translocation duplication III. Inversion
1. Paracentric inversion

379. 2. Pericentric inversion (A I B I c I 0 I E):{ F I GIH)
t
l
t
An inversion reverses a segment within a
chromosomal
(A I0Ic I BIE )::IF I GI H)
(A IBIc I GIF 1=0=EI0 IH) Repeated segment is located in the same
chromosome but away from the original sequence .
Additional segment is located in a non-homologous chromosome.
Chromosome with altered sequence of genes due to the rotation of a gene block
by 180·.
Inverted segment without centromere . Inverted segment of chromosome has the
centromere.
(i) Helps in
deve-Ioping reproduc
tive isolation and origin of species .
(ii) Results in a di-centric and an acentric chromo-some at anaphase-I in
inversion hetero-zygote .
(iii) May change the shape of chromosome.

381. IV. Translocation 1. Simple
translocat ion (AI B

382. t
~~:~~('11A1"ns,ocat'on movesa segment from one
ch romosome to
nonhomologous chromosome. In a reciprocal translocation
nonhomologous chromosomes exchange fragments
(MINlolcIDIE~
0Blp1Q~
(b) In a nonreciprocal
translocation a chromosome
transfers a fragment
but receives none in return.
(¢IDIE~
(AI B1M INlol~]Q~ 2. Shift translocation 3. Reciprocal
translocation
Tr ansfer or
exchange of
genetic material in non-homologous chromosomes .
1. Segment from a chromosome is added to the end of some
non-homologous chromosome.
2 . An interstitial segment of one chromosome is inserted within some non
homologous
chromosome . (ii) May cause
trans-location duplication .
3 . Exchange
of reciprocal segments
between nonhomologous chromosomes.
6.4.2 Behaviour of Chromosome with Deletion during Meiosis (Cytological
Detection)

383. The deletions can be identified in individuals heterozygous for deletion during
meiosis:
I. By the presence of some ring-shaped chromosomes on the equato r during cell
division. The broken ends of intercalary segments jo in to form rings. These are
left on the equator becaus e they lack centromere.
2 . By the presence of bu ckle or loop formed by the unpaired segment of the
normal chromo some in case of an intercalary deletion heterozygote. When a
chromosome with intercalary deletion pairs with its normal non-deleted
homologue during meiosis, the counterpart of deleted segment of the normal
chromosome buckles out.
3 . By the presence of an unpaired segment in the normal chromosome in the
case of termin al deletion heterozygote. (Fig. 6.1 D)
Creighton and McClintock observed above mentioned configurations in maize.
Same can be observed in the polytene chromosomes of Drosophila.
(i) Leads to the formation of new linkage groups.
(iii) Chain or
ring of four chromosomes is formed at meta-phase spindle.
6.4.3 Genetic Effects of Deletion or Significance of Deletion Deletion was the
first chromosomal aberration, detected by Bridges in 1917, while studying X-
chromosome of Drosophila. Its genetic effects are :

385. I Intercalary Deletion I
If
-
~ Los t
~ ?Wj,segment
(i) Original chromosome
A
(i
i)
W
ith i
ntercalary delet
ion
( I
BC
.' I
A
I Terminal Deletion I
Lost segment
(
A
_ I I
BC DE
'
1 1
,) (i) Original chromosome
(
A B CD E
0
, _, I I I )\7 (ii) With terminal deletion B
/ LOOP Normal
Pairing in
~
F....-chromosomes
A1-' BC D E F )
Pairing in
I I _ I I I I ) heterozygote
heterozygote
( ,-, "-..... Chromosome /'"
C
with deletion
D
FIG. 6.1: Different types of deletion : A. Intercalary deletion ; B. Terminal
deletion; C. Pairing of a normal chromosome with a chromosome having
intercalary deletion (the normal chromosome develops a loop); D. Pairing of a
normal chromosome with
a chromosome having terminal deletion .
1. Deleterious Effect: Since deletion involves loss of genetic material, it is bound
to have some deleteriou s effect on the organism. The deleterious effect depends

386. upon the amount and nature of genetic material lost. A deletion might be small
enough not to cause any detectable morphological change in the chromosome
and does not cause any harmful effect. But deletion s of large size are usually
detriment al for the organi sm . Homozygosity for deletion irrespective of its size
is likely to be lethal. Similarly, deletion of functional gene/genes can be quite
harmful to the diploids and haploid s, but may not be harmful in polyploids
which have extra copies of these genes present on extra chromo somes.
2. Recessive Mutants: Deletion s act as recessive lethal mutations because
deletion heterozygotes with small deletions are visible but the deleted genes are
not able to produ ce their phenotypic effect.
3. Pseudo-dominance: Deletion heterozygotes have only one copy of those genes
which are absent in the chromoso me with deletion. In case, their recessive genes
are present in the normal chromoso me, they express them in all their offspring
as if they are dominant. This phenomenon is called pseudo-dominance.
4. Absence of Crossing Over: Crossi ng over is completely absent in the region
of deletion because of the genes present in hemizygous condition.
ABC D E F G H
I I I I I I II I I I 11 '1A
I II I I I II I I I 11 '1
B
E
C
FIG. 6 .2: Behaviour of deficiency chromosome during meiosis. A.
Pairing of two normal homologous chromosomes ; B. Pairing of deletion
chromosome with a normal chromosome showing deficiency of C and D genes;
C. Pairing of deletion chromosome having deficiency for 0 and E genes with a
normal chromosome .
6 7
Normal--+ chromosome
Chromosome with deletion of
- gene loci from 2nd to 12th loci
BIC 12/D
FIG. 6.3 Pairing of bands in two homologues of a polytene chromosome of
Drosophila in which a chromosome with deletion pairs to a normal homologue .

387. Note the looping out of those genes that arepresent in normal chromosome but
lost in the chromosome with deletion .
6.5 DUPLICATION OR REPEAT
If a particu lar block of genes is represented more than once in a chromosome, it
is known as repeat and the phenomenon of repeat is called duplication .
Th e repeated section comes from another chromo some which suffered a
deletion . Duplication s are more frequent and less deleterious. These do not
lower the viability, but do result in abnorm alities of structure or function .
6.5.1 Types of Duplication
Based on the location of duplicat e segment in the chromosome, duplication is of
following four types:
1. Tandem Duplication: The repeated segment lies next to the original sequence
and has genes in the same sequence.
2. Reverse Tandem Duplication: The repeated segment lies next to the original
sequence but with reverse gene sequence in the duplicate segment.
3. Displaced Duplication: The repeat ed segment lies in some other part of
chromosome. It may be:
(a ) Homobrachial displaced duplication: The duplicate segment lies in the same
arm of the chromosome which has the original gene sequence .
(b) Heterobrachial displaced duplication: The duplicate segment lies in the arm
different from the arm having original sequence.
4. Translocation or Transposition Duplication to a Nonhomologue: The repeated
chromosome segment lies in some nonhomologous chromosome.
6.5.2 Origin of Duplication
Duplication can arise in following three ways :
I. By un equal breakage of homologous chromosomes with exchange and
reunion of broken segments.
2 . By unequal crossing over between nonsister chromatids of two homologous
chromosomes that are not perfectly aligned. Recombination then results in ta nde
m duplication in one chromosome and deletion in the other. Most unequal
crossing over occurs between sequences that already have tandem repeats
because duplicate regions can pair out sequence, producing further duplication.

388. 3. By transposable eleme nts (TE) which produce copies of genes that can move
to any new place in the genome.
They may carry along gene s that lie close to them . These contain genes for
transposition (movement) and insertion. They lead to translocation duplication,
where repeat segment of chromosome come s to lie in some nonhomologous
chromosome.
Normal Sequence:
Centromere
(abc- lde l~fghi
\1
g
I
Fragment added to the gene sequence below:
Types of Duplicat ion :
abcdede f9h
A•
abc dee d
•
f 9 h
i
Reverse Tandem B
adebcde f9h j
1 • ) Displaced homobrachial C (on same arm)
abc d e f 9 h del J
Displaced heterobrachial
,i 1 ) (on different
arms) D
kim n 0 P q d e r s t Transposition
- )(to a nonhomologue) E d e Extra chromosome
FIG. 6.4 Types of duplication.
6.5.3 Behaviour of Chromosomes with Duplication or Cytological Detection of
Duplication
I. In polytene chromosomes of Diptera, like Drosophila, duplications can be
observed under the micros cope where repeated band s of same type can be
identified.
2. Large duplications are visible as loops, formed by the duplicated segment in a
dup lication heterozygote during pachytene stage .
6.5.4 Genetic Effects of Duplication
Duplications do not produce a drastic or lethal effect as produced by deletion s.

389. But some duplications produce harmful phenotypic effect, e.g., bar eye character
in Drosophila and Down's syndrome in human beings .
Duplic ation s provide additional copies of genes. They form gene familie s of
similar or identical genes. The extra copies of gene s can modify through
mutation s and assume new functions . The se provide opportunity for gene s to
get modified into tolerable or beneficial ones. Duplications can overcome the
effect of deletion.
A. Normal pairing B. Mispairing c.Unequal crossing ovef). Results of crossover
2 copies
•-. • • •. (normal)• •1 copy
•
(deletion) 3 copies
(duplication) Two gene copies
•
2 copies (Tandem duplication) (normal)
FIG. 6.5 Origin of tande m duplication due to unequal cross ing over between
two homo logou s chromosomes dur ing meiosis.
Effect of Transposable Elem ents (TE)
By transposition and unequal crossing over, TEs can increase or decrease the
number of genes and can alter genome size. About 10 per cent of the human
genome has more than one million copies of a retrotransposable element (RE)
called Alu.
• Recombination between copies of TE located in non-homologous sites with
same sequence polarity can cause del etion of the region between them and
recombination between two copies of TE with opposite polarity inverts the
region between them (Fig. 6.6).
• Insertion of retrotransposable element (RE) into or near control region
interferes with or alters gene expressio n by changing rate/time/amount of
transcription.
• Insertion of a TE into a coding region, changes or destroys the function of
protein by causing frameshift mutation.
• Insertion of REs increases mutation rate of host genes.

390. Markers~
Transposable
1

2
element
2 3 4
1--+1
I

I \
1--+ I
I I
1+--1
I
5' 3' 15' 3' 5' 3' 5'13' 5' Recombination ecombination

391. 2
9
2 l1 2 4
seq
uence .....~ ~ 1--+1
I
1+--1
I
deleted_ _ B--.
1--+ 1
A
6.5.5 Evolutionary Significance of Duplications
1. Complementary and supplementary genes, duplicate genes and multiple
factors, etc., are presumed to have evolved through duplication.
2. Numerous gene families of similar or identical genes are found in many
species. This shows that duplications are common in evolution. A few examples
of gene familie s, which have arisen by duplication are:
• Ribosomal RNA gene copies: In eukaryotes ribosomal RNAs are transcribed
by clusters of long tandem arrays of duplicate genes.
• Copies of haemoglobin genes: The genes producing different haemoglobin
proteins show that duplic ated genes have evolved on different pathways to
produce different haemoglobin proteins.
3. Ohno (1970) has sugges ted that the increase in DNA content per cell and
origin of new genes with distinct function s have occurred due to duplication . 4.
Evolution of LDL Mutant in Human Beings: The low density lipoprotein (LDL)
gene in humans has evolved by unequal crossing over between two normal gene
copies ofAlu repeats (Fig. 6.7). The exons (functional segments) of LDL gene in
human beings are three in number and are separated by two Alu gene repeats
present in the intron region.
The unequal crossing between Alu 1 and Alu 2 results in the formati on of two
chromosomes, one with an extra copy of exon 5 and anoth er one without exon
5. This deletion is due to unequal crossing over.
Normal LDL gene Alu repeat 1
Alu repeat 2
Exon\ ~
~
S' I' 4 II....I ...I-T-I--.I""="'slr-T---r-l-;-.....".,.--

392. I I I I t9.1
Homologous Normal LDL gene
chromosomes ~-r-T--r-r-..,.. S'~ I I I IslJ
Out-of-register pairing of two Alu repeat
sequences leads to unequal crossing over
••ow.
mutant LDLge",
1
Unequal crossing overr-:::-_':"":':"'"__.,.,.,....-----.,

393. 1==
3' and has only one AluI
One of the resulting genes lacks exon S §
s,=C:I=1 I I I I
Alu 1 Segment gene (not found
in human pOPulatior
The other, with I I I [6'1 3' duplicated exon S
FIG. 6.7: Formation of a mutant LDL gene because of unequal crossing over
between two Alu gene repeats. This causes deletion of exon 5 from one
chromosome and addition of exon 5 to another chromosome of the homologue.
(The numbered boxes are exons 4, 5 and 6).
Duplication of a gene followed by random and natural selection may lead to two
genes with different functions. In the absence of gene duplication, a mutation
would be lethal if the mutant gene produces an aberrant protein. If there is gene
duplication, the same mutation that would otherwise be lethal is not selectively
disadvantageous since the original normal gene continues to produce normal
protein.
160 Iil Evolutionary Biology
Selective advantage of gene dupli cation Normal gene Normal gene
Mutation Mutant gene ~ Mutation
~
G
ene duplication and mutation AMutation Abnormal pro
tein

394. 1
Select io n against defective gene Normal protein Abnorma l protein (
Inconsequential)
~
Additional mutations and selectio n lMutant gene with
Normal gene three mutations
Two genes with different functions
FIG. 6.8: Selective adva ntage of gene duplication.
6.6 INVERSIONS
6.6.1 Definition
Sometimes , the number of genes in a chromosome is not changed but the
sequence of genes is altered by the rotation of gene block within a chromosome
by 180°. It means inversions are reversals of gene order in a gene block within a
chromosome.
6.6.2 Origin of Inversion
Inversion arises by two break s in a chromosome and reunion of interstitial
segment after it has rotated by 180°. Inversion can also arise when a
chromosome becomes folded on to itself and the two breaks may be located
close together. The two broken ends thus produced undergo reunion with
exchanged ends . (Fig . 6.9)
6.6.3 Types of Inversion
Inversions are of two types:
1. Paracentric Inversion: When both the breaks in the chromosome during
inversion occur on the same side of the centromere , the inversion is called pa
racentr ic. The inverted segment of chromosome is without centromere. If
paracentric inversion

396. occurs singly , i.e., on one side of the centromere alone, it is called intraradial or
ho mobrach ial inversion. On the other hand, when two paracentric inversions
occur
one on either side of the centromere, the inversion ab c d e 9 h is called
interradial or heterobrachial inversion.
(I I I I I I
2. Pericentric Inversion: In
pericentric
einversion, the inverted segment contains the
centromere, i.e., it involves one break on either
side of the centromere.
In version occurring in a single chromosome
is called chromosoma l inversion , whereas that
occurring in both the members of a homologous
pair is called allelosomal in version. Allelo somal
inversions occurring in the homologous arms of
ethe two chromosomes are called allelobr ac
hial
and those occurring in nonhomologous arms are
called hetero som al inversion .
6.6.4 Behaviour of Chromosomes
having Inversion
I . In inversion homozygote s, a chromosome
a b 9 e d ch i
pair with identical
inversion undergo normal
I I I I I
meiotic pairing and get separated normall y FIG.
6.9: Origin of inversi on. into the gamete s. All the gametes are normal
except for some disarrangement of genes . Therefore, inversion homozygotes are
at a selective advantage over heterozygotes.
2 . Even in organi sms heterozygous for inversion , the inversion has no
detectable phenotypic effect in the individuals. But at the time of gamete
formation , the normal pairing is disrupted and abnormal cytological
configurations are produced. At the time of pairing, the normal chromosome
forms loop-like configuration to maximise the pairing of homologous regions.
The chromosome with inversion twists in the inverted region. The products of
their separation in paracentric and pericentric heterozygotes are different.
1. P a r a c en tric hetero z ygou s inversion with one chiasmata in the region of

397. inversion results in the formation of an acentric chromatid lacking centromere,
and a d ic en tri c c h r om a t id

399. Original
ABCDE FGHI Chromosome

400. r r
Chromosome with
AB C F E D G H

Paracentric
c:t:::nb=i~::::EIb Inversion
Pairing
ABC G HIIn
Heterozygote
FIG. 6.10: Paracentric inversion and pairing of inverted chromosome with the
normal chromosome during meiosis.
co nnecting two chromosomes. At the end of meiosis there are two noncros s
over chromatids, one acentric chromatid and one dicentric chromatid. (Fig.
6.12.)
Th
e a
centric
a
nd dicentric
c
hro
Original
ABC D E F G H Chromosome I I I !X! I I I matids are inviable, so only the
noncross over chromatids form
the viable products of meiosis
in paracentric inversions of
heterozygotes.
2. Crossing over in a pericentric
heterozygous inversion results
in the formation of one normal
and one inverted chromosomes
and two chromosomes which

401. 1
Chromosome with
ABCF E D GH
Pericentric
I I I I IX! I I
Inversion
Pairing A B C G H in
Heterozygote
f9 h
ha ve duplications for certain
genes and deficiency for others.
The gametes receiving chromo
some with deficiency are rarely
F IG. 6.11: Pericentric inversion and pairing of inverted chromosome with
normal chromosome during meiotic prophase.
v iab le. Fig . 6. 13 Therefor e,
number of viable gametes produced in pericentric heterozygotes is low. So the
inverted heterozygotes semisterile.
6.6.5 Consequences of
B
:
C
:
E
:
F
:Inversions B :E FC D
Si nce the number of ge nes In an
A

a d c
b
e
in vert ed chromoso me remains th e ;§~8i§3~~§~~~Sg5 ;
same , the effects of inversion are not
a d c b e
so drastic as are the effects of
deletion
C
and duplication. The consequences of
inversions are as under:
I . Pericentric inversions,
In
nor .
mal course, lead to changes
In
th e appe arance of a chro mo
some. For exampl e, a chromoABC D E F
some
wi
th m

402. ed ian
c
entromere
1 C O • •
BC
• •
d
•
a
• ' 1
A
(V-s haped) may change into
•e
a hook-shaped or rod shap ed
e

•
f
form. Conversely, in pericentric
inversion a hook-shaped chromoFIG. 6.12: Results of a single chiasma in a
some
may change into Vshaped
paracentric heterozygous invers ion . (A) Pairing
c
hromosome.
b etween a normal and a paracentric inverted chromosome. (B) Single cross ing
over in the 2. Effectsof inversion on phenotype inversion. (C) Results of
separation of cross may be produced becau se of
over chromatids.
positio n effect. A gene in one location on a chromosome does not nece ssarily
have the same action in another position.
3 . Inversions lower recombination frequency within the inverted sequence and
tend to retain the original combination of genes. Such a nonrecombinant block is
called a supergene. This help s in preserving a superior arrangement of genes.
4. If two groups of organi sms are separa ted for a long period, they might
accumulate so many inversions of different types that these may lead to the
formation of new species by the splitting of parental stock. Thi s ha s been
supported by the study of chromosomes in different species of Drosophila.
~}NOrmal
c hromosome } Chromosome
with pericentric 9 inversion
1~!:
2~
g; !
.

403. 3 Pericentric inversion
4
a e d c b
~ A
Pairing and cross ing over C D
E
1
F
2
3
4
a
1
9
11
A B C D E F I I I (I I I I A B C D
e
a
2-4( I I I I) I I I I a e d c b f 3' I I I
( )
I I I I G F E d c b f 4.2< I I I I I
( J
I I I
c
9
I
9
I
6.6.6 Significance of
Inversions
I. Inversions help in the origin of
G
G I
new specie s by changing gene arrangement in chromosomes. 2. Inver sion s
provide proof for the occurrence of crossing over and
s upport the view that only two of the four chromatids undergo cros sing over.
3 . Inversion s, sim ilar to translocation, help in establi shing species relation in
Drosophila.

404. 4. As a result of inversions chances
FIG. 6.13: Results of single chiasma in pericentric inversion loop producing
gametes with duplication and deficiency. A. Pairing between a normal and a
pericentric chromosome; B. Single crossing over in the region of pericentric
inversion; C. Results of separation of cross-over chromatids to form four types
of gametes.
D
~
Pericentric inversion H
of crossing over are reduced and,

406. F G H
therefore , the recombination of
genetic loci is restricted. Therefore,
inversions are considered as cross
ove r repressors.
c B E ~
~tCJD~S::::::~
FIG. 6.14: Change in the form of chromosome due to pericentric inversion.
68 69
70
FIG. 6.15: Salivary gland chromosome of Drosophila showing loop formation
due to inversions.
6.6.7 Inversions and Chromosomal Differentiation in
Species of Drosophila
Cy totaxonomic differences between closely related species in Drosophila have
arisen through the fixation of about one hundred paracentric inversions, three
centric fusion s, a peri centric inversion and a few structural changes in V-
chromosome . Of these more than 31 different paracentric inversions are known
in chromosome III.
The karyotype of Drosophila virilis is the most primitive of all Drosophila
groups. It has five pairs of acrocentric rod-shap ed chromosomes and one pair of
dot-like chromosomes . From virilis karyotype have diverged two hypothetical
primitive karyotypes.
A. Primitive II Type
Th e North American forms of Drosophila, D. novam exicana, D. americana and
D. texana have evolved from primiti ve II karyotype by following chromo somal
changes due to six inversions:
1 . D. novamexicana karyotype has arisen by six inversions in primiti ve 11
karyotype.
2. D. americana karyotype has resulted from the centric fusion of second and
third chromosomes and of fourth chromosome with X-chromosome; duplication
of V-chromosome (YI, Y2) and a large number of inversions in chromosomes.

407. 3 . D. texana has arisen by the fusion of second and third chromosomes and
inversion in fourth and fifth chromosomes.
D. americana and D. texana are regarded to be geographical races or subspecies
because of similarity in their karyotype.
B. Primitive III Type
This karyo type has resulted from a pericentric inversion in second chromosome
that changed the acrocentric chromosome into a metacentric chromosome.
y
x/
D. ezoana D. Xu littoralis / D. lx vexana
1.0 1, 0 0 0 0o
~ ? ~,a ~ c0
Xy
~ l' Prim
XY
Prirnltive III '\ r \0T~"'
CD 00),#0 0 A A0

408. "o .a BC"0B
0~~t~ ~~t~ 0~~t~0
XY XY XY D. virilis Primitive I D. novamexicana
FIG . 6.16: Chromosome phylogeny of virilis group of Drosophila. Capital
letters designate inversions occurring for the first time and small letters
designate inversions which appear thereafter (After Stone, 1962). Chromosome
numbers are shown in circles

410. and invers ions by alphabets.
T he two old world species of Drosophila. D. ezoana and D. littoralis have
arisen from primitive III karyotype by a pericentric inversion in Y-chro- mosome
which also becomes metacentric and a large number of inversions in other
chromosomes including X-chromosome. In D. littoralis, there is fusion of third
and fourth chromosomes.
In North America, montana group of Drosophila that includes montana .
borealis. lacicola and jfavomontana has also arisen from primitive III karyotype
after many additional inversions.
In obscur a group of Drosophila, D. pseudoobscura and D. persimilis also
represent geographical races differing in the presence of:
(a) an inversion in the second chromosome, and
(b) an inversion in the left limb of X-chromosome.
The male hybrids between D. pseudoobscura and D. persimilis are sterile but
female hybrids are fertile. They also exhibit nearly identical morphological
details .
6.7 TRANSLOCATION
6.7.1 Definition
Translocation is a kind of chromosomal rearrangement in which a block of genes
fr om one linkage group is transferred to another linkage group.
6.7.2 Origin of Translocation
Translocation occurs as a result of interchange of chromosome segments in
nonhomologous chromosomes. The phenomenon of translocation can be
explained by supposing that the two chromosomes having genes AB and CD
exchange segments and produce chromosomes AD and CB. Translocation is thus
different from crossing over, which involves interchange between homologous
parts of homologous chromosomes.
6.7.3 Types of Translocation
Depending upon which part or parts of nonhomologous chromosomes become
detached and reunited, the translocation can be:
1. Reciprocal Translocation : It is the exchange of parts between nonhomologous
chromosomes, as for example, exchange of segments between second and third
chromosomes of Drosophila is reciprocal translocation. Such translocations are

411. the most frequent ones and are produced
by single break in each of the r"TA B-.,...--.,

(
two nonhomologous chromosomes. c.....--LI
I
A D
)
--L..---J
Reciprocal translocations are of
two types:

413. ) )
c D c B
(a) Ho mozygous re cip r ocal in
which both the homo logous
FIG. 6.17: Reciprocal translocation
chromosomes of one pair exchange parts with the two homologues of another
pair.
Translocation
I
Nonreciprocal Multiple
,.-__1__-,
translocation
I I I
Homozygous Simple Shift
translocation translocat ion transloca tion
(b) Heterozygou s reciprocal in which only one member of each of the two
homologous pairs exchange parts .
2 . Nonreciprocal Translocation: In this type of translocation, a block of genes or
a chromosome segment shifts from one chromosome to some other
nonhomologous chromosome. This may be:
(a) Simple translocation: In such cases a sma ll segment of both the homologous
chromosomes is added to the end of both the chromosomes of some other
nonhomologous pair. There is no exchange as in the case of reciprocal
translocation. This type of trans location is very rare in nature and is caused by a
single break in one chromosome only.
(b) Shift translocation: In shift translocation an interstitial segment of one
chromosome is broken off and is inserted within the break in another nonhomo
logous chromosome. It involves three breaks, two in one chromosome and one in
nonhomologous chromosome.
3 . Multiple Translocation: In multiple trans location more than two pairs of
nonhomologous chromosomes exchange parts. This has been studied in
Drosophila and Gena/h era.
6.7.4 Behaviour of Translocation Heterozygotes
1. Homozygous Translocations: These can not be identified cyto logically
because they show normal pairing and form regular bivalents at pachytene stage

414. during meiosis . Genetically, these may show influence on the phenotype due to
position effect on functional genes. These gene combinations are usually
nonlethal and do not influence the viability of gametes. It means homozygous
trans locations do not result in sterility.
2. Pairing of Chromosomes in a Reciprocal Translocation Heterozygote: Two
norma l and two interchanged (translocated) chromosomes pair to form a cross -
shaped configuration at pachytene stage of meiosis, whereas the normal
chromosomes form a bivalent. The occurrence of crossing over and chiasmata
formation in each arm of the quadrivalent results in the formatio n of a ring or
circle of four chromosomes at the diakinesis and at the metaphase of first
meiotic division. The ring may be either

416. A B C 0 E F
I I I I I I

417. I I I I I I II I iIG H K
Nonhomo logous chromosomes
Recip roca l Nonrecproca l Transloca tion Transk>cation
I Homozygous I
A B C 0 K
! ! !

!
I
,
A B C 0 E A B ,C 0 K L
! ! ! I !
! !
,
G H J E F
! ! ! !
G H
G H , E F
I I
I ! ! !
I
L
ISimple TranslocationI F K L
! I
K L
c:!:::::::!:. c:!:::::::!:.
Break
IHeterozygousI
I Shift Translocation
A, B C 0 K
!
! !

!
, E, FB C 0
A
! ! I
! ! ! !
B C 0 E
,
F
, G H C, , K, L G
,
H
I
·
E
! ! ! F
! ! ! !
G H K
! ! !
G K L
I
I I
,
L

A B
,
H C
,
0
,
E
,

418. I
! !
FIG. 6.18: Homozygotic and heteroz ygotic type of reciprocal translocation and
simple and shift type of nonreciprocal translocation.
twisted or open. If th e chiasmata are not formed in one arm, the ring is
transformed into an open chain . Such chains were first observed and interpreted
by Belling in Datura, maize, peas and wheat. If more than two pairs of
nonhomologous chromosomes are involved in translocation, ring of 6, 8 or more
chromosomes is formed.
6.7.5 Chromosome Disjunction
The anaphasic movement of chromo somes towards the poles takes place in
anyone of the three different ways given below:
1. Alternate Segregation: The alternate chromosomes of the ring go to the same
pole, the normal chromosomes on one pole and the translocated chromo somes
on the other pole. Therefore , all the gametes receive full comp lement and are
fully viable.
2. Adjacent-I-Segregation: In open ring configuration, if one normal and one
translocated chromosome reach one pole and one translocated and one normal
chromo some to the other pole, it is called adj acent-I -seg re ga tion.
3 .Adjacent-2-Segregation: In some open ring configurations, the two
homologous chromo somes (one normal and other translocated i.e., I-I ') go to
one pole and the other two homologues 2-2' go to the other pole. This is called
adj acent-2-segregation.
Six types of gametes are formed as a result of alternate, adj acent-I and adjacent-
II ty pes of segregation, as follows:

420. P 0 R S T U Origin al chromosomes (
I 10 I IA B C 0 E F G (
.. .. .............. I II 10 I I I I I
......... ...... ..
A B C 0 E F G S T U P 0 R S T U Chromosomes with trans
location I '0' I I I I I I I
) (
I 101
) (
I I
A
:g:
C 0 E F
Pairing of chromosomes

421. In a heterozygote ~ : : : :
r,t
A. Homologous translocatIon (transposition)
A
B C 0 E F P 0 R S T U Original chromosomes I 101 I I 101 I I I
Chromosomes with reciprocal
A B C 0 P 0 Rt F G
translocation
I 101 I I 101 I I Pairing of chromosomes A in a heterozygote I
B. RecIprocal translocations
A B C 0 E F J K L M N P 0 R S T U
Origina l chromoso mes
I 101 I I I >C I I() I
I I
) I 10 I I
I I
A B
C+M N J K L 0 E F
P
0 R S T U
First modification I 10 I I I I 101 I
I
I
I
101
I
I I
M
N K L S T U
P 0 R
0
E F 101 I I I
I 10 I I I I
Second modi fication
Pelring of A B
chromosomes
In e heterozygote
C . Successive multiple reciprocal translocatlons FIG. 6.19: Pairing between chromosomes
In translocation heterozygotes having different
types
of translocations.
(i) With ABC D and E F G H } I
Complete gene complement(ii)
With A B GHand EFeD
II (iii) With A B G Hand E F G H
(Iv) WithABC D and EFeD
} I',romp'ete ge"" rompleme'tIII (v)
With A B G H and ABC D
(vi) With EFeD and E F G H
Out of these six types of gametes on ly two types of gametes have complete gene
complement. The remaining four types of gametes possess certain genes in
duplicate and are deficit for others. Such gametes are usually inviable making
the organisms infertile.

422. 6.7.6 Significance of Translocation (Fusion and Fission)
Fusion: Trans location may cause change in the morp hology or appearance of
chromosomes. Centric fusion between two nonhomologous acrocentric
Robertsonian

424. I
Reciprocal Normal Transl",::.:

425. ~"\z z-~I
-"~)~~~~~Nonhomologous~- ~
!~'I7
~ ~ ~H~ ~ irr\ G
chromosomes Pairing in
translocation (1
~~~~~)
heterozygote III \1
FIG. 6.20: Pairing and segregation in a translocation heterozygote with
reciprocal translocation. The meiotic products (the gam etes) have incomplete
gene complement.
chromosome segments (A and B) leads to the formation of a metacentric
Robertsonian chromosome (Fig. 6.21) with two arms A and B. Such
chromosome fusions have occured in the course of evolution. For exampl e, two
acrocentric chromo somes found in Chimpanzee and Gorilla are fused to form
metacentric human chromo some number 2, resulting in one less pair of
chromosomes in human genome.
Fission: Fission in the centromere of a metacentric chromosome produces two
telocentric chromosom es. A break in a metacentric chromosome and the recipro
cal fusion of these segments with a dot 0 like chromosome produ ces two acroce
ntric chromosomes (Fig . 6.22).
• Translocations lead to chang e in the number of chromosomes. The
A
B
A B )
Fus ion of two nonhomologous acrocentric chromosomes during reciprocal
translocation forms a V-Shaped metacentric chromosome
FIG. 6.21: Translocation due to centric fusion of two nonhomologous
acrocentric
chromosomes produces one V-shaped metacentric and one dot 0 like
chromosome. This metacentric chromosome formation is called Robertsonian
translocation .
s pontaneous rate of origin of new chromo somes is quite high but only a small
fraction of such changes are fixed in populations and cause karyotyp e

426. divergence.
• Translocation s introduc e genetic polymorphi sm in the populations of a spec
ies by formin g new linkage group s and play an important role in the formation
of new genotypes leading to the origin of new varieties and new species.
6.7.7 Translocation and Formation of New Species
Translocations are of common occurrence and have played an important role in
the introdu ction of genetic polymorp hism in the popul ation s and spontaneous
origin of

428. r eproductive isolation as shown in Fig. 6.2 1. The reproductive isolation leads to
sympatric speciation.
Tran slocations have been studie d in Drosophila, Oenothera and Maize. In man
translocat ion between 15 and 2 1 chromosomes results in 2 1 trisomy and causes
Down syndrome, and translocations in 13, 14 and 15 chromosomes cause minor
phenotypic defects.
In Drosophila, the karyotypes of different species have arisen by translocation s
between different chromosom es of the ancestral species, Drosophila virilis. The
karyotype of Drosophila virilis represents the ancestral complement consi sting
of five pairs of rod-shaped and one pair of dot-like chromosomes.
• The translocation between the X-chromosome and 5th pair of autosomes
Break in
A
entromere " IFission'1
--.
A
6PJ>o
~
Dot
2 Telocentr
ics
Metacentric 2 Acrocentrics B
FIG. 6.22: A. Fission in centromere produces two telocentric chromosomes; B.
Fission of a metacentric and a dot-like
chromosome and fusion of the ir segments produces two acrocentric
chromosomes.
ha s given rise to the chromosome complement found in D. pseudoobscura and
D. pers imilis having a pair of Vshaped X-chromosomes and three pairs of rod-
lik e and a pair of dot-lik e autosomes.

429. Metacentric In a fission/fusion heterozygote, the metacentric synapses with
acrocentric chromosomes I and II as a "trivalent"
II ( II
Balanced (euploid) Unbalanced (aneuploid)
In meiosis , some gametes get a complete chromosome complement, but others
may not
FIG. 6.23: Pairing of fusion-heterozygote chromosome with parental acrocentric
chromosome and their segregation in meiosis may yield euploid (balanced) or
aneuploid (unbalanced) complements of genetic material.
D. melanogaster D. willistoni
FIG. 6.24: Origin of chromosome complem ent of different species of
Drosophila by translocation and fusion of chromosomes of D. virilis having six
pairs of chromosomes.
• The karyotype of D. willistoni evolved from pseudo-obscura by two more
translocations and fusion between 3rd and 4th and 2nd and 6th chromosomes
which resulted in the combination of two of the autosomal rods into another
Vshaped autosome.
• The karyotype of D. melanogaster evolved from the ancestral type by two
translocations between 2nd, 3rd, 4th, 5th chromosom es with the conversion of
four rodshaped autosomes into two Vshaped ones. Therefore, it has two Vshaped
autosomes, two sex chromosomes (XX or XY) and two dot-like chromosomes
(four pairs of chromosomes instead of six pairs).
Among plants, Blakeslee and his collaborators have studied evolution of races of
Datura stramonium, the Jimson weed.
KE Y TERMS
• Aberrations
• A/u gene
• Displaced duplication
• Homobrachial
• Metacentric
• Position effect

430. • Retrotransposable element
• Telecentric

432. • Acrocentri c
• Alternate segregation
• Duplication
• Inversions
• Paracentric inversion
• Pseudogenes
• Reverse tandem
• Translocations
• Allelobrachial
• Deficiencies/Deletions
• Heterobrachial
• Karyotype
• Pericentric inversion
• Reciprocaltranslocations
• Tandem duplication
REVIEW QUESTIONS
I. Define duplication. Discuss evolutionary significance of duplications.
2. Differentiate between paracentric and pericentric inversions. How do they
contribute to genetic variations and origin of new species?
3. Discuss origin of species of Drosophila by inversion with well illustrated
diagrams.
4. What is translocation? Discuss the behaviour of chromosomes in a
translocation heterozygote.
5. Differentiate between shift translocation and reciprocal translocation. How
translocation and inversion may lead to change in the shape and number of
chromosomes in a species?
6. Differentiate between:
(a) Paracentric and pericentric inversion
(b) Allelosomal and allelobrachial inversion
(c) Homobrachial and heterobrachial duplication
(d) Nonreciprocal and reciprocal translocation (e) Tandem and reverse tandem
duplication (f) Inversion and translocation
(g) Deletion and deficiency
7. Define the following:
(a) Tandem duplication (b) (c) Paracentric inversion (d) (e) Translocation (f)
Homobrachial inversion Heterosomal inversion Dicentric chromosome

433. 8. Write short notes on:
(a) Translocation and reduced fertility
(b) Duplication
(c) Behaviour of chromosomes during meiosis in an individual heterozygous for
paracentric inversion
(d) Inversion and origin of species
(e) Bar locus in Drosophila
(f) Acentric chromosomes
9. Draw a labelled diagram to show behaviour of chromosomes with one
reciprocal heterozygous translocation during first meiotic division.
10. Why do translocation heterozygotes exhibit sterility or semisterility? II .
How does paracentric and pericentric inversions act as suppressors of crossing
over? Illustrate with suitable diagrams.
12. Write a note on consequences of inversions.
13. Describe how translocations have shaped karyotypes of different species
ofDrosophila from the ancestral type?
14. In Drosophila a part of salivary gland chromosome the bands have a
sequence a b c d elg h. The homologue with which the chromosome pairs during
meiosis has a sequence ab e d «I s h.
(a) What type of chromosome change has occurred?
(b) Draw a diagram to illustrate the possible pairing arrangement and what type
of gametes will be produced from such pairing.
15 . A plant with heterozygous inversion ab c d e/a d c b e is crossed to a plant
with homozygous inversion a d c b e/a d c b e. What percentage of gametes
produced by these parents will have complete haploid set of genes if:
(a) one cross over occurs in the inverted segment between d and c (b) there is no
crossing over.
16. How do variegated position effects originate and how can they be explained?
17. A plant of Oenothera is heterozygous for reciprocal translocation between
two nonhomologous chromosomes LM.NO and PQ.RS. Draw figures to show:
(a) Their pairing behaviour at pachytene
(b) The gametes formed from them if crossing over occurs between P and Q.
18. What are the major forms of variation in chromosome structure?
19. What structural chromosome changes can lead to changes in chromosomal
number?

434. FURTHER READINGS
1. Brian K. Hall and B. Hallgrimsson , 2008 . Evolution: The Integration
ofGenes. Organisms and Populations. 4th ed. Jones and Bartlelt Publishers,
Sudbury, Massachusetts.
2. Coyne, l.A. and H.A. Orr, 1997. "Patterns of speciation in Drosophila".
Evolution, 51, 295-303 .
3. Coyne , l .A. and H.A. Orr, 2004 . Speciation Sinaur and Associates
Sunderland, M.A.
4. Davis, A.w. and C-1. W4, 1996. The fine structure of a chromosomal region
causing reproductive isolation between two sibling species of Drosophila. Geneti
cs. 143, 1287-1298.
5. Futuyma, D.l., 2009 . Evolution-2nd ed. Sinauer Associates, INC Publishers,
Sunderland, Massachusetts U.S.A:
6. Gillespie, J.H., 2004. Population Genetics : A Concise Guide 2nd ed. John
Hopkins University Press , Baltimore, M.D.w
7. King, M., 1993 Species Evolution: The Role of Chromosome Change .
Cambridge University Press, Cambridge, Englan d.
8. Qumsiyeh, M B., 1994 . Evolution of number and morphology of mammalian
chromosomes. Jour-Hered, 85, 455-4 65.
9. White, M.J.D., 1973. Animal Cytology and Evolution , 3rd ed. Cambridge
University Press , Cambridge, England.
000

435. 7
.Variation in Chromosome Number (Heteroploidy)
INTRODUCTION
The majority of organisms (both plants and animals) are diploid, i.e., they
possess two sets of chromosomes. These are represented by 2n. The gametes
produced by them are haploid i.e., they have only one set of chromosomes
represented by n, Under normal conditions, haploid male and female gametes
fuse to produce the diploid adult and its germ cells undergo meiosis (reduction
division) to produce haploid gametes.
Changes in choromosome number involves either of the two events :
• loss or addition of entire set (genome) or sets of chromosomes.
• loss or addition of one or more chromosomes in a haploid set of chromosomes
Changes in the number of chromosomes can be separated into following
categories:
7.1 CHANGES INVOLVING ENTIRE SET OF
CHROMOSOMES (EUPLOIDY)
Euploidy or euploid variations include either loss of a complete set of
chromosomes or addition of one or more sets of chromosomes to the diploid
genome. Based on the loss or addition of one set of chromosomes, euploidy is of
the following two types:
7.1.1 Monoploidy or Haploidy
Due to loss of one complete set of chromosomes from diploid genome, only one
set of chromosomes is left. This represents haploid genome.
7.1.2 Polyploidy
Due to addition of complete set/sets of chromosomes, the genome has more than
two sets of chromosomes and may be represented by 3n, 4n, 5n, 6n, 7n etc.
,.-j Euploidy I
( Changes in the number of entire set of chromosomes)
,.
Monoploidy

436. Diploidy

437. LPolyploidy
L: Triploidy (3n)
Tetraplo idy (4n)
I-
Pentaploidy (5n
'-
Hexaploidy (6n)
I Heteroploidy I
YAneuploidy :
( Changes in the number of chromosomes in a set of chromosomes) ..-
Hyperploidy (addition)
or
Polysomy
Trisomy (2n+1) Tetrasomy (2n+2)
Y
Hypoploidy
loss)

438. l--c Monosomy (2n-1)
Nullisomy (2n-2) FIG. 7.1: Differen t types of chan ges in the number of
chromosom es in the organisms.
7.2 CHANGES INVOLVING NUMBER OF CHROMOSOMES IN A SET
(ANEUPLOIDY)
Changes involving number of chromosomes in a set is of three types:
7.2.1 Monosomy or Hypoploidy
Loss of one chromosome from the diploid karyotype, i.e., 2n I
7.2.2 Polysomy of Hyperploidy
Add ition of one or more chromo somes to the diploid karyotype of the
organism, i.e., 2n + I or 2n + 2
• Trisomy: 20 + 1 (i.e., addition of one chromosome to the original diploid set)
• Tetrasomy: 20 + 2 (i.e., addition of two chromosomes of the same pair to the
diploid genome)
7.2.3 Nullisomy
The loss of both the chromosomes of a pair.
7.3 EUPLOIDY
E uploidy is loss or addition of complete one or more sets of chromo somes.
Euploidy is very common in many groups of organisms. It is of two types: mo
noploidy and polypoidy.
7.3.1 Monoploidy or Haploidy
Individuals having only one set of chromosomes are called monoploids or
haploids and this conditio n is descri bed as monoploidy or haploidy. The
number of chro- mosomes in them is represented by n.
Ocurrence of Haploidy
Haploidy is the normal condition in lower eukaryotes and all prokaryotes. The
occurrence of haploid organisms in higher eukaryotes is very rare. Haploid
plants are obtained in the laboratory for experimental purpose. In animals only
some hymenopterous insects (Bees, Wasps and Ants) have haploid males . Their
females and sterile workers are all diploid.

439. Origin of Monoploidy or Haploidy
In Honey bee (Hymenoptera) haploid males (drone s) are found as a routine .
These arise parthenogenetically from the unferti lised eggs of diploid organisms.
Haploids may originate spontaneously in flowering plants due to
parthenogenetic development of unfertili sed eggs or can be induced
experimentally by anyone of the following methods :
• X-ray treatment
• delayed pollination
• temperature shocks
• colchicine treatment
• interspecific or intergeneric hybridisation
• culturing pollen grains
Cytology of Monoploids or Haploids
Haploid s are univalent. Their chromosomes are without any homo logue to pair
with. They produce gametes without meiosis as in Honeybee or fail to produce
the gametes because of failure of pairing during metaphase I of meiosis .
7.3.2 Polyploidy
Polyploidy is an increa se in chromosome number by the addition of one or more
sets of chromosomal genome to the diploid genom e of an organism. Organism s
with more than two sets of chromosomes are called polyploids and this condition
as polyploidy.
Occurrence of Polyploidy
Polyploidy is more common than aneup loidy or haploidy. It occurs frequently in
most families of higher plants, but rarely in animals . Many of our domesticated
vegetables, fruits and foodgrains are polyploids, such as Wheat (a tetrap loid),
Rye
(an hexaploid) and American cotton (tetrap loid). Grant (1971) has studied the
fre- quency of polyploids among eukaryote s and reported that:
• Polyploidy is rare among animals, fungi and most groups of gymnosperms
(Conifers, Cycad s and ginkgos).

440. • Polyploidy occurs regularly in angiosperm s, pteridophytes and one group of
gymno sperms (Gnetum, Ephedra and Welwitschia).
• Polyploidy is also found in some rotifers.
Reasons for Rarity of Polyploids in Animals
One of the reason s for rarity of polyploids among animals is their sex
determination mechanism. In animal polyploids , the ratio of sex chromosomes
to autosomes is disturbed and results in sterility. In those rare cases where
polyploidy has been observed as in Drosophila, the polyploids are steril e. Even
the aneuploids for sex chromosomes are steril e in Man and in Drosoplila .
Types of Polyploidy
Polyploids are classified as follows:
A. On the Basis of Number of Chromosome Sets
I . Triploids = 3n with 3 sets of chromosomes.
2. Tetraploids = 4n with 4 sets of chromosomes.
3. PentapIoids = 5n with 5 sets of chromosomes.
4. HexapIoids = 6n with 6 sets of chromosomes.
5. Heptaploids = 7n with 7 sets of chromosomes.
6. Octap Ioid s = 8n with 8 sets of chromosomes.
B. On the Basis of Source of Chromos ome Sets
Depending upon the source of additional chromosome sets, the ployplo ids are of
two types : a ut opoIypIoids and aIlopoIyploids.
1. Autopolyploids or Autoploids (Gk. autos =self)
In autopolyploid s the chromosome set or sets of the same species is added to its
diploid genome. If a diploid cell has AA genome, an autotriploid will have AAA
and an autotetraploid has AAAA genome.
Origin of Autopolyploids: The autopol yploid s could arise in any of the
following ways:
(a) by the union of diploid gametes produced in the absence of meiosis or due to
abnormal meio sis, the autotetrapIoids are formed .
(b) by somatic doub ling of the chromosomes in a diploid zygote due to failure
of cell division , the autotetrapIoids are formed .

441. (c) by the union of a haploid gamete with the diploid gamete, autotripIoids are
formed .
(d) by a cross between a tetraploid and a diploid parent autotriploids are
produced.
( e) by fertilisation of an egg with two sperm autotriploid is formed .
Occurrence of Autopolyploids: Autopolyploids are relatively rare in nature, but
these can be produced artificially. Examples of natural autopolyploid s are doob
grass (Cynodon dactylon) an autotriploid , rye(Secale cereale)an autotetraploid.
Autoploids are known in berseem, marigold, snapdragon, flex, grapes, apples,
pears, banana, tom ato es and com , etc. Autopolyploids are induce d by
treatment with heat
Diploid Genome
ISpecies AI
AA
Gametes
AA o (Nonviable)Diploid
No chromosomes chrom somes
1
.
Fertilised with 2. Fertilised with haploid gamete diploid gamete
AAA AAAA
Autotriploid Autotetraploid
FIG. 7.2: Origin of autotriploid and autotetraploid because of abnormal meiosis
at the time of gamete formation and formation of diploid gametes.
o r co ld shock or by various chemica l agent s like co lchicine durin g mito sis,
because spindle formation is prevented and cell fails to divide. Autopolyploid
are artificially induced by treating with colchicine in banana, datura, potato,
coffee, etc. Autopolyploidy is a common mode of evolution in mosses.
Cytological Behaviour of Autopolyploids: Autopolyploids possess more than
two sets of homologous chromosomes. This leads to the formation of
multivalents (trivalents, quadrivalents, etc.). Meiosis in triploids and pentaplo ids

442. is abnormal. The third homologue either fails to pair forming univalent ( I : I ) or
is randomly
Interbreeding between two Species A and B Species A Species B

443. CD CD
Formation of Unio n of Diploid Gametes
Diploi d Gamete (Irregularly formed) Fertile Allotetraploid ~

444. 10 CIDCID maD _ \ j ----Ilo.. RedUclI \
\j
----Ilo.. IrregUla;.tion \
red~c
----...division
---"... --,.. division
~
10
CID
A B C

445. aD O CD CD
Diploid Hybrid Diploid Gamete
AllotetraploidDiploid Gametes
FIG. 7 .3: Steps in the formation of allotetraploid A. Formation of diploid hybrid
by cross breed ing; B. Formation of diploid gametes from the hybrid due to
abnormal meiosis (nond isjunction); C. Formation of allotetraploid by the union
of diploid gametes; D. Allotetraploid produces diploid gametes and forms a
fertile hybrid species.
distributed to the gamet es (2 : I). The number of chromosomes in the gametes of
triploid organisms may vary from n to 2n or may form trisomies or tetrasomics.
In autotetrap loids, there are four sets of chromosomes which form quadrivalents
on pairing. Therefore, autotetraploids are semisterile.
Evolutiona ry Significance of Autopolyp loids : An autotetraploid can produce
fertile tetraploid offspring by self-pollination or by mating with other tetraploids.
But tetraploids are reproductively isolated from diploid plants of original
population, because such a cross produces triploid (3n) offspring with reduced
fertility as three sets of chromosomes segregate randomly during anaphase I of
meiosis. It means in ju st one generation, autoploidy can generate reproductive
isolation without the interplay of geographic isolation leading to sympatric
speciation.
2. Allopolyploids or Alloploids
Species A 2n = 6 Species B 2n = 4
1 . Meiotic error; chromosome number not reduced from 2n to n
Normal haploid gamete n =3
2 . Unreduced diploid gamete with 4
chromosomes
Allotriplo id Hybrid with
7 chromosomes (3n)
U nreduced triploid gamete with 7

446. chromosomes
Allopol yploids are formed by the multiplication of chromosome sets of the
hybrid produced by interbreeding of two diffe rent spec ies. An allotetraploid is
formed between two species A and B by the duplication of chromosomes their
the hybrid AB and will have sets of chromosomes of both the species
represented twice i.e.. AABB.
Origin of Allopolyploids: Allopol yploids may arise:
1. By Abnormal Meiosis: Allopolyploids arise by interspecific or intergen eric
cross. Usually, such hybrids are sterile because unrelated chromosomes fai l to
pair during meiosis at the time of gamete formation. Very rarely both the
chromosomes of the hybrid enter one gamete and produce

448. «: ~
diploid gametes . Fusion

449. of such unreduced, diploid ~
N ew species:
viable fertile hybrid (allopolyploid)
2n = 10
gametes prod uces allotetraploids.
FIG. 7.4: Origin of an
all
otriploid and an allotetraplo id by the fusion of diplo id
and haploid gametes.
2. By Abnormal Mitosis: In some cases, diploid cell of the hybrid zygote
undergoes chromosomes doubling. In such cases, division starts normally and
chromosomes divide but dividing cell fails to separate abruptly after the
duplication, resulting in the formation of allotetraploid zygote which grows into
a fertile allotetraploid adult.
3. By the Union of Diploid and Haploid Gametes: Fusion of a diploid gamete
(produced due to meiotic error in species B with the haploid gamete of other
species A, results in the formation
Variation in Chromosome Number (Heteroploidy) [i] 181
AA B B C C
Diploid
\
Diploid Diploid
AB

/
Hybrid
AAAA
BBBB
Autotetraploid A BC Autotetraploid Allotriploid
AA B B
Allotetraploid

450. 1
AAAA
B B B B AA B BCC Autoallo-octaploid Allohexaploid
FIG. 7.5: Mechanism of origin of autotetraploid, allotetraploid, autoallo-
octaploid and allohexaploid (between three species).
of allotriploid hybrid. When by chance , this triploid hybrid produces unreduced
triploid gamete and is fertilised with the normal haploid gamete of species A, it
forms an allotetraploid.
Polyploids arising following hybridisation are called amphiploids. Genome of
amphiploids can be AABB (allotetraploid) or AAAABB or AABBCC
(allohexaploids) or AAAABBBB (autoallo-octploids).
Cytological Behaviour of Allopolyploids During Meiosis: At the time of gamete
formation , the sets of homologous chromosomes in allotriploid and
allotetraploid may align on the equator as trios or quartets as shown in Fig. 7.6 A
and B. At anaphase, they may segregate in a balanced or unbalanced fashion.
This results in the formation of aneuploid gametes which are usually nonviable.
Hence, fertility of allopolyploids is greatly reduced .
In some allotetrapIoids, the chromosomes from different species align on the
equator as separate pairs and segregate normally producing balanced, viable
gametes . Such allotetraploids are fully fertile.
Occurrence of AIIopoIypIoids: Many important agriculture crops such as Wheat,
Oat, Cotton, Rice, Potato, Tobacco and a variety of fruit crops are allopolyploids.
The commonly used bread wheat, different species. Raphanobrassica Russian
geneticist, Karpechenko.
Triticum aestivum , is an allohexaploid of three is the first known allotetraploid
reported by
Evolutionary Significance of AIIopoIypIoids: The allopolyploids are fertile
when mating with each other or reproducing by self-pollination. But they can not
intebreed

452. The resulting gametes are aneuploid.
-----+
Meiosis I
All the three
homologous
chromosomes
undergo synapsis ...and unequal number of
metacentrics and Gametes
acrocentrics move to
(Nonviable)opposite poles.
A. Allotriploid
...yielding aneuploid gametes.
-----+ Meiosis II
Quartets of homologous chromosomes undergo synapsis...
...but may not
segregate two by
Gametestwo. Here they (Nonviable)segregate three
by one...
B. Allotetraploid
Fig . 7.6: Unbalanced segregation of chromo somes during meiosis in
allotriploids and allotetraploids : A. Alignment of three sets of homologous
chromosomes on equator in metapha se I and their unequal segregation during
anaphase II in an allotriploid forming nonviable aneuploid gametes; B.
Alignment of four sets of homologous chromosomes as quartets and their
unequal segregation yielding aneuploid gametes.
with either of the parent species. The allopo lyploids become reproducti vely
isolated from parent species and form new species by sympatric speciation.

454. Meiosis I
Each chromosome pairs
with a single homologue from
Gametesthe same parental species (Viable)
FIG. 7.7 : Normal segregati on of chrom osome sets in an allotetrap loid
producing balanced gametes.
7.4 ORIGIN OF NEW SPECIES THROUGH POLYPLOIDY OR
EVOLUTIONARY ROLE OF POLYPLOIDY
7.4.1 Role of Polyploidy in Plants
Both auto as well as allop loids (especially tetrap loids) have played an
important role in the evolution of new species especially in plant kingdom.
M any natural and artificial plant hybrid s are amphiploids. Our commerci al
wheat is hexaploid. Durum wheat is tetraploid . Oats, Rye, Alfalfa: Sugarcane ,
Cotton, Tobacco , Potato, Banan a, Coffee, Sweet potato, Ap- ple, Pear and
Strawberry also have pol yploid vari eti es. Sel ection for polyploid characteristic
has played a major role in the origin of many cultivated plants and cereal
varieties. Origin of few important amphiploids is discussed below:
1. Raphanobrassica: An excellent example of allotetraploids is furnished by the
work of Russian genetici st,
Raphanus
x(radish)
2n =18 R
Brass ica (cabbage) 2n =188
F,hybrid
2n =18 (9R + 98)
1Colchicine
Rapha nobrassica (Tetraploid hybrid)
4n =36 (18R + 188) FIG. 7.8: Formation of an allotetraploid Raphanobrassica
from an intergeneric cross between
radish and cabbage.
Karpechenko . He made intergenic crosses between radish, Raphanus sativus
and cabbage, Brassica oleracea. Both radish and cabbage have dipolid
chromosome

456. numb er of 18. The diploid hyb rids with 9R + 98 chromosomes are sterile bec
ause their chromosomes belong to two different species and fail to pair at the
time of meiosis. In some hybrids viable pollen and ovules having 18
chromosomes (9R + 98) are formed.
T he fusion of the se diploid game tes (2n chromosomes) produced allotetraploid
with 18 chromosomes of Radi sh and 18 chromosomes of Cabbage (4n = 18R +
188). The se hybrids had 36 chromosomes. It me ans the fertile hyb rids have
arisen by the doubling of chromosom es in the zygote which enabled them to
und ergo norm al pairing. Thi s fertile hybrid was call ed Raphanobra ss ica.
2. Gossypium:
G. hirsutum have
Th e American cultivated cottons, Gossypium barbadense and 26 pairs of
chromosomes and represent the amphitetraploid derivatives from crosses of
diploid spec ies with 13 pairs of chromosomes. The dip loid species with 13
pairs of chromosomes occur in wild state in Central America, Peru ana
Galapagos Island s (G. raimondiii and in Old World (G. herbaceum) . The Ame
rican cottons rep resent tetraploid hyb rid (G. hirsutumi between the New World
and Old World cotton species:
3. Evolution of Wheat: Th e common cultivated wheat, Triticum aesti vum used
for making flour for bread is an allohexaploid. Three different chromosome co
mpleme nt series are found in differen t species of wheat genera, Triticum . The
by the hybridisation between wild series is represented by three groups formed
wheat variety and wild goat grass.
( i) Einkorn wheat group I S represented by 2n = 14 (the primitive diploid
species) . Thi s group has Triticum
111 0 II 0 CO C C'/11 0 r
T boeotic um and T. aegilopo id es . Thi s wheat is not suitable for human
consumption be cau se the grain is tightly enclosed in the glumes.
(ii) Emmer wheat group includ e s seve n allotetraploid spec ies with
chromosome compl e

457. Gossypium
herbaceum
(Old world colton)
2n =26
(Small chromosomes)
Gossypium raimondii
(American uplandx colton or
lton)1New world co 2n = 26
(Large chromosomes)
13h + 13r \
(small large
Colchicine- --+I
Gossypium hirsutum
American cultivated colton
(Mexican colton)
4n =52
26h
+
26r
small
mes
large
chromoso chromosomes
FIG. 7.9: The origin of American cultivated colton by allotetraploidy.
ment 2n = 28 . The two mo st important species of tetraploid wheat are Triticum
dicoccum or Triticum dicoccoides (Persian emmer wheat) and Triticum durum.

459. The em mer wheat has evolved through hybridisation between wild einkom
wheat, T. boeoticum and a wild species, Aegilops (Goat grass).
(iii) Vu lgare w heat gro u p consists of 5 species of allohexaploid wheat (2n =
42). It includes the bread wheat T. aestivum T. vulgare and T. durum , and T.
compactum which has arisen through hybridisation between T. dicoccum and
another species of goat grass, Aegilops squarrosa followed by chromosome
doubling . Other species of bread wheat , T. durum and T. comp actum are also
hexaploids.
Now , it is believed that genome A, S, D are not much different. These appear to
have evolved from some common progenitor. Therefore, hexaploid wheat is
considered to be an autohexaploid rather than allohexaploid.
4. Evolution of Nicotiana-Tobacco Species: Clausen and Goodspeed synthesised
a new hexaploid species of tobacco, Nicotiana digluta from a cross between two
species of tobacco, Nicotiana tabacum (with 48 chromosomes) and Nicotiana
glutinosa (with 24) chromosomes. The hybrid was sterile. It was propagated by
vegetative cuttings. A chromosome doubling produced a fertile allohexaploid
with 72 chromosomes.
5 . Evolution of New Species of Goatsbeard, Tragopogon: In 1950, Marion
Ownbey found two fertile tetraploid species of Tragopogon , T. mirus and T.
miscellu s in USA . T. mirus was a hybrid tetraploid of T. dubius and T.
porrifolius and T. miscellus was derived from T. dubius and T. pratensis.
6. Glandularia: Another example of polyploidy is found in Glandularia . A cross
between G. pulchella
Triticum aegllopoides
Aegitops speltoidesor T. boeoticum

)(
(wild einkhorn)
(goat grass)
2n =14 AA ~2n =14 BB
T_('t,,,el
A
B
(2n=28) =AABB evolved into
Triticum dicoccum (A'A'B'B') FIG. 7.10: Origin of cultivated wheat variety T.
dicoccum by allotetraploidy.

460. and G . peruviana produced hybrid with 3% fertility. The fertility wa s reduced
because

462. of the large size of nonhomologous segments of the chromosomes. Although, the
chromosomes of hybrid paired but they fai led to produce normal gametes.
When the number of chromosomes in the hybrid was doubled, the fertility was
increased, because every chromosome was now represented twice.
7. Cultivated Potato, Solanum tuberosum: It is an autotetraploid that originated
in South America.
8. Primula: Primula kewensis is an allotetraploid of P.
18) and P. floribunda (2n = 18). Rosa = (Iris versicolor) verticil/ata (2n = is
allotetraploid of
1. setosa (n = 19) and I. virginica (n = 35) and Sparima townsendii is
allotetraploid of S. alterniflora (2n = 70) and S. stricta (2n = 50)].
9. Evolution of Cord Grass, Spartina: Spartina anglica exhibits vigorous growth
and colonisation. It is of great help in build ing up mud flats which can be
reclaimed from the sea to provide usefu l farmland.
In 1870 , a new species of cord grass was found growing in Southampton water.
It was named Spartina townsendie but now renamed S. anglica . In this locality,
two species of cord grass were known , S. maritima and S. alterniflora . S.
anglica is a polyploid hybrid of these two species.
Triticum dicoccum
x
Aegilops squarrosa (tetraploid wheat) (goat grass)
2n =14 = 28 2n A'A'B'B'
DD
Triploid hybrid 3n = 21 A'B'D
1+-- Allopolyploidy
(allohexaploid) (2n = 42) A' A'B' B'D D
FIG. 7.11: Origin of hexap loid whea t, Triticum aestivum.
Nicotiana tabacum x Nicotiana glut/nosa 2n =48
1
2n = 24
(2n
=
Hybrid
24T + 12G =36) Triploid (sterile)
Chromosome doubling

463. Nicotiana diglu ta
(2T + 2G = 2n = 72)
FIG. 7.12: Origin of a new species of tobacco, Nicotiana dig/uta from N.
tabacum and N. g/utinosa.

465. 7.4.2 Role of Polyploidy in Agriculture
Scientists have deve loped hybrid varieties of wheat, paddy, maize, cereals,
pulses, vege tables, fruits and flowers. Tobacco, Til, Potato, Tomato, Onion,
Chilli, Apple, Banana, Plum, Sunflower and Chrysanthemum all have polyploid
varieties. The polyploids give more yield, large flowers , fruits and seeds. The
polyploidy can be induced artificia lly in the laboratory.
x
Polyploidy r-:--''-=-:--l AABB
FIG. 7.13: Evolution of a new species of cord grass, Spartina anglica .
7.4.3 Role of Polyploidy in Animal Evolution
Polyploidy is much rare in animals as compared to plants beca use:
• Plants propagate vegetatively through grafting, cutting or rooting. As a result,
triploids and pentaploids can multiply and maintain their races .
• Animals reproduce sexually through gamete formation . Due to abnormal
meiosis or failure of chromosome pairing viable game tes are not formed .
• Animal polyploids face difficulty in maintaining the same proportions of X and
Y chromosomes present in diploids. A normal female has 2X chromosomes and
male XY chromosomes. A tetrap loid female would have XXXX and a tetraploid
male XXXV chromosomes. Such a male will neither be male nor female.
• Polyp loidy can be maintained in those animals which reproduce exclusively by
diploid parthenogenesis as seen in some crustaceans.
7.5 INDUCED POLYPLOIDY
Scientists have induced polyploidy by artificial methods and have evolved new
varieties or new species of plants with characters of economic importance.
Serveral methods such as chemicals, decapitation, etc., are being wide ly used to
induce polyploidy. These can be emp loyed at three different stages:
1. At the Time of Mitotic Cell Division
(i) treatment with X-rays
(ii) hybridisation artificially
(iii) treating plants at very low temperature
(iv) treating cell plants at very high temperature
(v) by using mitotic spindle inhibiting chemicals like colchicine. 2. At the Time

466. of First Zygotic Division
(i) treating the zygote at a very high or very low temperature
(ii) exposing zygote to X-rays
(iii) exposing UV-light rays
. (iv) treating the zygote or seeds with colchicine.
3. On Somatic Cells:
Duplication of chromosomes in somatic cells is also induced by above methods.
7.5.1 Artificial Induction of Polyploidy
A number of chemicals have been found to induce polyploidy in plants, the most
efficient and widely used chemical being colchicine. It inhibits the formation of
spindle fibres during mitosis but not the doubling of chromosomes. Maize and
some other plants respond to changes in the temperature. Hence, these, when
kept on high or low temperature, inhibit spindle formation and tetraploid cells
are formed. Colchicine is greatly used in horticulture because the plants treated
with 0.01-0.50% aqueous solution of Colchicine are more resistant. Decapitation
is employed in tomato plants where the bud was removed and the shoots
developing from the scar tissue were found to be tetraploids.
7.6 ANEUPLOIDY
Aneuploidy is the addition or loss of one or more chromosomes to the complete
diploid chromosome complement of an organism. The organisms with such
chromosome complement are known as aneuploids. Aneuploidy can be of two
types, hyperploidy and hypoploidy.
1.
Hyperploidy
Hyperploidy is the addition of one or more chromosomes to the diploid genome
of an organism. It may be called (a) trisomy (2n + I) when only one chromosome
is added to the diploid genome or (b) tetrasomy (2n + 2) when both the
chromosomes of a homologous pair are added to the diploid genome.
2. Hypoploidy
Hypoploid y is the loss of one or more chromosomes from the diploid genome. It
is known as (a) monosomy (2n I) when there is loss of one chromosome from
the complete diploid set, and (b) nullisomy (2n -2) when there is loss of both the

467. chromosomes of a homologous pair.
7.6.1 Trisomy
Individuals having one chromosome extra in the diploid genome are called
trisomies. In them one chromosome is represented three times (in a normal
diploid organism, each chromosome is represented twice). The extra
chromosome may belong to any one of the chromosomes of a haploid set. For
example, Datura stramonium possesses 12 pairs of chromosomes (2n = 24, n =
12). There could be 12 different trisomies.
Three chromosomes of one type
3 2 2 2
4
3
Primary trisomic
4
(Extra chromosome identical to homologous chromosomes)
3 2
2
4
Seconda ry trisomic (Extra chromosome is an isochromosome)
3 2 2 2
4
Tertiary trisomic
(Extra c hromosome has a part of nonhomologous chromosome)
FIG. 7.14: Thr ee kinds of trisom ies
7.6.1.1 Types of Trisomies
Trisomies can be primary, second ary or tertiary. In case the extra chromosome is
identical to its homologues, the trisomies are called primary trisomies. They are
called second ary trisomies when the extra chromosome is an isochromosome
(both arms are genetically identical) and tertiary trisomies when the extra
chromosome contains a part of some other nonhomologous chromosome and is
formed as a result of translocation. Trisomies may be :

468. • sing le t r isom ies with one extra chromosome, i.e., 2 + I
• double trisomies with two different extra chromosomes, i.e., 2 + I + I
• tr iple trisomies with three different extra chromosomes in one set, i.e., 2 + I + I
+ I
7.6.1.2 Examples of Trisomies
1. Trisomies in Datura: Blakselee made comprehensive study of trisomies in
Datura stramonium. T he hap loid chromosome number in this species is n ==
12. Therefore, 12 primary, 24 secondary and a large number of tertiary trisomies
can possibly be obtained. Most of the trisomies can be identified by the size,
shape and other morphological features of the fruit. (Fig. 7. 15)
2. Trisomies in Man: Trisomies are known in man also. Trisomy for certain
chromosomes produces distinct morphological abnormalities. Mongo lism
(Down 's syndrome) is caused in children having 47 chro mosomes instead of
normal 46 . In these mongo loid children 21 st human chromose is represented
thrice. These children are mentally retarded, imb ec ile and malformed. They
exhibit unusual morphological features . Some of these features resemb le
Mangoloid race. Theoretically, anyone of the 23 chromosomes may occur in
trisomic state. Two more syndromes of several deformities have been traced to
group D-chromosomes and to an extra rep resentati ve
of group E-chromosomes respectively. Nondisjunction of sex chromosomes
produces a variety of aneuploids such as males with XXV or XYY chromosomes
or females with XO, XXX chromosomes. They are not fertile.
3. Trisomies in Drosophila: Trisomies in Drosophila were studied by Bridges in
1921. Male Drosophila with an extra X-chromosome (XXY) or a male with an
extra V-chromosome (XYY) are common examples of trisomies in Drosophila.
Chromo- some IV also occurs in trisomic form .
7.6.1.3 Origin of Trisomies
Tri somies arise due to unequal segregation of homologous chromosomes during
meiosis I or meiosi s II at the time of gamete formation . Nondisjunction of
homologous chromosomes in a diploid cell at the time of gamete formation
produces gametes with n + 1 chromosomes. Such a gamete, when unites with a
normal gamete with n chromosomes, produces trisomic with 2n + I
chromosomes.

469. Trisomies may also arise due to the formation of unblanced gametes in triploid
organisms and by cross breeding between triploid females with diploid males .
7.6.1.4 Behaviour of Trisomies
A trisomic has an extra chro
mosome which is homologous

470. to one of the chromosomes of ,~.the
complement. Therefore, it ."
forms a trivalent. The pairing [JJJ 2n + em2n + []I]forms
different patterns in 2n +
primary, secondary and tertiary
111 111 11
2
trisomies . At the
time of sep
aration of chromosomes, the
gametic cells may be nand n

471. + 1 or n 1 and n + 2. ~~I 1 ~~~ ~~ ~
2 2

472. Trisomy / \2
2
2
1
7.6.1.5 Significance of1t l rU2\::::{)2Trisomies are used for
locating 1V1~ 'gen es on spec ifi c chromo-
somes.
2
7.6.2 Tetrasomy
In tetrasomic individuals ,
particular chromosome of
the haploid set is represented Three 1.2 C four times in a diploid chroFIG. 7.15:
Fruits of trisomies of Datura and different mosomal complement. The patterns
of pairing in primary and secondary trisomies
general chromosomal formula for tetrasomics is 2n + 2 rather than 2n + 1 +1.
The formula 2n + 1 + 1 represents a double trisomic, (i.e.. two different
chromosomes are represented three times) . E.R. Sears obtained a set of
compensating nullisomic tetrasomic (2n - 2 + 2) complement where addition of a
pair of homologous chromosomes would compensate for the loss of another pair
of homologues.
7.6.3 Monosomy
In monosomy , organisms lack one chromosome, i.e., their general chromosome
formula is 2n 1. If one more chromosome of some other pair is also lost (2n I
-I), these are known as double monosomics. Similarly, a triple monosomic (2n
- I I 1) has lost three chromosomes of three different pairs.
Since monosomics lack one complete chromosome , they are genetically
imbalanced and, therefore, they are either lethal or have reduced viability.
Monosomics can easily be produced in polyploids, because they possess more
than two choromosomes of each type and the loss of one or more chromosomes
could easily be tolerated. The number of possible monosomics is equal to the
haploid chromosome number. In common wheat , Sears was able to produce all
the 21 monosomics. Monosomics have also been obtained in cotton by Endrizzi
and coworkers and in tobacco by Clausen and Cameron.

473. 7.6.4 Nullisomy
Nullisomy individuals lack both the chromosomes of a homologous pair of
chromosomes. This is represented by 2n 2.
7.6.5 Origin of Aneuploids
Aneuploids could arise due to nondisjunction of homologous chromosomes
occurring during first or second meiotic division as shown in Fig. 7.16.
Translocations also
Meiosis I

475. Anapha se I sta ge
Meio sis II
Anaphase II sla ge
Gamet••
@(D@Gametes --------
1 !' + 1 n- !
Trisomic Monosomic Normal
A --------Number of chromosomes B
FIG. 7.16: Mechanism of origin of aneuploids by A. Nondisjunction of
homologous chromosomes in meiosis I; B. Nondisjunction of sister chromatids
in meiosis II
lead to the formation of trisomie s and monosomic s for certain genes or
chromosome segments.
• Allopolyploids
• Diploidy, Diploid
• Heteroploidy
• Monoploidy
• Nullisomy
• Tetraploidy
KEY TERMS
• Aneuploids
• Euploidy
• Hexaplo idy
• Monoso my
• Pentaploidy
• Tetrasom y
• Autopo lyplo ids
• Haplo idy
• Hyperploidy
• Nond isjunction
• Polyploidy
• Trisomy

476. REVIEW QUESTIONS
I . In making preparations of human tissue for chromosomal study colchicine is
usually added to the culture several hours before the smear is made on a slide. Of
what value do you think the use of colchi cine might be in the study of human
chromosomes?
2 . Why the hybrid s between two species are more likely to be fertile as a
tetraploid than as a diploid ?
3. Explain the following terms :
(a) acentric
(b) amphidiploid
(c) aneuploid
(d) autopolyploid
(e) semisterility
(f) tetrasomic
(g) Down's syndrome
(h) chromosomal imba lance
4. Differentiate between the following pairs of terms
(a) monosomic and nullisomic
(b) 2n I I and 2n 2
(c) primary trisomi c and secondary trisom ic
5. Semisterility in plants can be on accoun t of several reasons. Discus s the role
of translocation sterility.
6. Polyploidy is more easily establi shed in those plants which are vegetatively
propa- gated. Why?
7. Describe the effect when cells of onion shoot apex are treated with colchicine.
8. Why are haploid s of interest for a cytogenetici st and a cytotaxonomist?
9. Polyploidy has played an important role in the evolution of plants , whereas it
has been insignificant in case of animals. Discuss.
10. Differentiate between autopolyploids and allopolyploids.
II . Why are polyploids more vigorous in plants, but trisomie s with extra
chromosome are not successful?
12. Why is polyploidy relatively uncommon in animal s?
1 3. Down 's syndrome is caused by trisomy for chromosome 2 1. A few cases
are seen where indiv iduals with 46 chromosome s show Down 's syndrome.
Why?
14. The Am erican culti vated cotton Gossypium hirsutum has 26 pairs of

477. chromosomes. The Asiatic cotton G. arboreum has 13 pairs of chromosome s as
does an American cotton G. thurberi. When G. hirsutum and G. arboreum are
crossed, the resulting triploid s have 13 pair s of chromosome s and 13 single
chromosomes at meiosis. 13 pairs and 13 single chromosomes are also observed
in trip loids derived from a cross betw een G. hirsutum x G. thurberi. When G.
arboreum and G. thurberi are crossed, the FI individuals are highly sterile, with
chromosome pair ing being very irregul ar. What do these observations suggest
about possible origin of American cult ivated cotton?
FURTHER READINGS
I. Futuyma, 0.1 ., 2009 . Evolution 2nd ed. Sudbury, Massachu setts, Sinauer
Associates, Inc. Publ ishers Sunderland, Massachu setts, U.S .A.
2. Hall, 8.K., and 8. Hallgrimsson , 200 8. Strickberger s Evolution.. 4th ed.,
Jones and Barlett Publi shers.
3. Hallgrimsson, 8., and B.K . Hall , 2003 . Variation: A Central Concept in
Biology. Elsevie r/Academic Press, Burlington, MA.
4. Hartl, D.L., and E.W. Jones, 1998. Genetics: Principles and Analysis, Jone s
and Bartlett, Sudbury, MA.
5. King, M., Pagel, M., 1993 . Species Evolution: The Role of Chromosome
Change. Cambridge University Press, Cambridge, England.
6. Pagel, M., 2002. Encyclopaedia of Evolution. 2 Yols. Oxford University Press,
New York .
7. Qum siyeh , M.B. 1994 . Evo lution of Number and Morphology of Mammali
an Chro mosomes. Journal-Hered.. 85, 455 -465.
8. Strickberger, M.W. 1985. Genetics, 3rd ed., Macmillan, New York.
9. Strickberger, M.W. 1995 . Evolution, 2nd ed., Jones and Bartlett Publi shers,
Sudbury, Massachusets.
10. Weiss, K.M. and A.V. Buchanan, 2004. Genetics and the Logic of Evolution,
Wiley-Li ss, Hoboken, NJ.
II. White, M.1.D., 1973. Animal Cytology and Evolution, 3rd ed. Cambridge
University Press, Cambrige, England.
12. Yunis, J.J., and O. Prakash, 1982. The Origin of Man: A Chromoso mal
Pictori al Legacy. Science, 215, 1525-152 9.

479. D OD

480. 8
Reproductive Isolating Barriers
8.1 INTRODUCTION
According to 'Biological species concept ' , a species is a group of populations
whose members have the potential to interbreed in nature and produce fertile
offspring but do not produce fertile offspring with members of other such
groups.
The mechanisms that prevent interbreeding in nature among the members of the
same or different species and help in maintaining their hereditary integrity are
called isolating mechanisms. These are also called isolating barriers or
reproductive barriers (RR) because they prevent gene flow between biological
species.
8.2 HISTORY
Imp ortance of isolation (geographic isolation) was realised by Darwin, but
Wanger ( 1868) emphasised the role of geographic isolation in the formation of
species and in maintaining their genetic integrity.

482. Reproductive Isolation: A n Overview
• Isolating factors that limit hybridisation arise primarily as byproducts of
adaptive change in populations. They are central to speciation.
• Several types of factors that limit hybridisation between isolated populations
often emerge simultaneously.
• Natural selection strengthens the isolating mechanisms already operating on
populations.
• Reproductive isolating mechanisms have clear adaptive and economic value.
Mating with members of other species is expected to cause incomplete
development of hybrid embryo or produce unfit or sterile hybrids wasting
reproductive energy without contributing anything to future generations.
• Reproductive isolation generates evolutionary pressure by preventing mating
outside species boundaries and ensures production of fertile offspring to
maintain species gene pool and conserve species resources.
In 1905 , Patterson wrote, "It is a fact that each spe cies of animals has devices
which permit th e recognition and bringing togeth er of con sp ecific indi viduals
of th e opposite sex with such a degree of certainty that hybridisation occurs only
as an abnormal exception". The first systematic treatment to the subject and a
complete recognition of the various isolating factors was given by Durietz
(1930). The term isolating mechanisms was coined by Dobzhansky (1937) in his
classic work " Ge netics and th e Origin of Species".
8.3 ROLE OF REPRODUCTIVE ISOLATION
Reproducti ve isolation plays two key roles in the evolution or origin of species:
I. It segregates the members of a large, widely distributed population of a
species into smaller units to prevent interbreeding among them. Members of
such
reproductively iso lated units accum ulate gene tic variations in their gene pools
independently in response to their environment.
2. Maintenance and conservation of genetic diversity helps in the preservation of
adaptive variation acquired in the gene pool preventing disruption by the gene
flow
from nonadapted groups. This prevents genetic dilution of parental gene
complexes,
which may prove to be of great benefit in the present environment, and their
dilution

483. in the hybrids may be of great disadvantage.
Reproductive isolation prevents interbreeding among the members of different
species and limits formation of hybrids from interspecific mating .
8.4 TYPES OF REPRODUCTIVE ISOLATING MECHANISMS
In natur e, interbreeding between Mendelian popu lations is limited by different
means. There are various biologica l and behavioural characteristics of living
organism s which act to prevent or to reduce interbreeding among members of
close ly related populations of a species . Not only one but several such
mechanisms cooperate and interact together bringing about reproductive
isolation in the populations of species. A detailed classification of isolating
mechanisms was provided by Dobzhansky who broadly separated them into two
types name ly, Geographic isolation and Reproductive isola tion. The former
operati ng on geograp hically isolated populations or allopatric species and the
latter represented genotypically conditioned differences between species which
do not permit hybrid formation . Isolating mechani sms can be ext ri nsic or intr
insic.
8.4.1 Extrinsic and Intrinsic Isolating Mechanisms
I . Extrinsic Isolating Mechanisms: The geograp hic isolating mechanisms such
as distance , mountains, rivers, forests or deserts act as barriers to interbreeding .
These outside factors are called extri nsic isolating mech ani sm s. These factors
do not allow the potential mates from related populations to come in contact and
mate.
2. Intrinsic Isolating Mechanisms: These include all those internal features of the
organisms that prevent interbreeding among members of closely related
populations or
196 ~ Evolutionary Biology
species. These features can be differences in their anatomy , physiology or
behaviour and are under the control of their genetic makeup. They are classified
as ecological, temporal, behavioural and mechanical isolating mechanisms or
may involve gametic isolation, zygotic mortality, hybrid inviability, hybrid
inferiority or hybrid infertility.
8.4.2 Premating and Postmating Isolating Mechanisms

484. 1. Premating or Prezygotic Isolating Mechanisms (i.e.,before zygote formation):
The se isolating factors prevent mating between the members of different
populations or closely related species. In case prezygotic isolation is not fully
established, the males and females of different populations interbreed and viable
fertile hybrids are formed between them . It means the isolated populations have
not evolved into separate species because reproductive isolation between them is
incomplete and their gene pools are still not incompatible.
2. Postmating or Postzygotic Isolating Mechanisms i.e., after the zygote
formation): These isolati ng mechanisms operate after the zygote is formed by
the fusion of gametes from different populations or different species. They
prevent formation of fertile hybrids between the species because the gene pools
of these species have accumulated so many genetic differences that they have
become genetically incompatible and pairing bet ween their chromosomes can
not take place.
8.5 PREMATING BARRIERS Premating barriers prevent or reduce the
likelihood of transfer of gametes to members of other species. These include:
8.5.1 Geographic Isolation
In geographic isolation , two populations of the same species are separated by
some physical or geographic barrier or they occupy different geographic areas.
These barriers may be mountain ranges, deserts , thick forests, land bridges ,
water bodies , etc.
Because of the above barriers , individuals of separated populations have no
chance to meet and interbreed. This greatly reduce s the exchange of genes
between them and a single common gene poo l is splitted into two or more gene
pools. As a result , the new mutations, genetic drifts and action of natural
selection occur independently in isolated populations and they become adapted
to local conditions. This leads to genetic divergence. In due course of time, each
segregated population forms an independent species. The degree of distinctness
between these units is directly related to the effectiveness of physical barriers
and duration of separation.
Geographic isolation provides conditions for allopatric speciation in the first
place by physically separating the potential mates of isolated populations, but
can not maintain reproductive isolation between them. Given a chance to mate,
the geographically isolated population s may produce fertile, viable hybrids.

485. 1. Geographic Isolation
Species occur in different places (The first step in allopatric speciation)
2. Isolation due to Distance
3 . Ecological Isolation
Species occupy different habitats in the same area and do not come in contact
with one another
4. Temporal Isolation
Mating or flowering occur during different seasons or at different times of the
day
5. Behavioural Isolation or Ethological Isolation Species have different mating
rituals
6. Mechanical Isolation
Structural differences prevent mating or pollen transfer
7. Physical Isolation
8. Gametic Isolation or Prevention of Gamete Fusion or Gametes fail to attract
each other or function poorly

487. ....
9. Zygote Mortality
Hybrid embryos do not develop properly
10. Hybrid Inviability
Hybrid adults do not survive in nature
11. Hybrid Inferiority
Hybrid survive and are fertile but do not leave offspring 12. Hybrid Infertility
H brid adults are sterile or have reduced fertili
Fert ile hybrid offspring produced in case none of the above isolating mechanisms are operative
FIG. 8.1 : Summary of different isolating mechanisms that can prevent
successful hybrid isation between individuals of diffe rent species .
198 lil Evolutionary Biology
EXAMPLE:
1. Snails: Distribution ofland snails in Hawaian Island (Island of Oahu) provides
an example of geographic isolation. The volcanic ridges of island are separated
by valleys which have abundant vegetation at the bottom but their tops are
barren . Each valley has a different variety of land snails and the degree of
divergence between them is directly proportional to the distance between the
valleys.
2. Indian Giant Squir re l: Ratuala indica, lives in Indian forests. Its variety
living in deciduous forests of Gujarat is yellowish deciduous forests of
Maharashtra is light brown and in colour, that living in one living in evergreen
forests of Mysore is dark brown. Here different forests are acting as barriers and
isolate the three varieties of squirrels.
3. Sq uir re ls from G r a nd Canyon: Two varieties of squirrels occurring along
the south and north rims of Grand Canyon (U.S.A.) are formed by the
geographic isolation of a common ancestral populat ion that lived in the land of
Northern Arizona. Due to the formation of Grand Canyon, this population got
bifurcated about 10,000 years ago. Now, the Albert squirrel lives on the south
rim and Kaibab squirrel lives just few kilometers away on the north rim of Grand
Canyon without any natural hybrids.
4. Chatta m Islands: Chattam Islands are presently situated about 400 miles east
of New Zealand. About one million years ago, these were connected to New
Zealand by a land bridge but not now. The flora and fauna of these islands are
similar to that of New Zealand, but there are slight differences. For example,
wood pigeon of New Zealand is Carpophaga novaezealandise while that of

488. Chattam islands is C. chathmensis. Similarly, New Zealand lizard, Lygosoma
moco, and Chattam lizard, L. dendyl, are similar but slightly different. Though
quite similar, they are unable to interbreed because of ocean separating the two
land areas .
8.5.2 Isolation due to Distance
The m embers of widely distributed specie s, which occupy a large territo ry
without any natural barrier, have been found to be reproductively isolated. The
simple reason of their isolation is their inability to cover great distances. The
effectiveness depends upon the homing instinct of animals. Any possibility of
gene exchange in such isolated populations is through the occasional migrant s
or through a chain of intermediate populations.
EXAMPLES:
1. Mammals: Mammals found in Holarctic area of North America are unable to
interbreed only because they occur in distantly isolated groups and find it
impossibl e to cover such long distances.
2. Oak: The English oak, Quercus robur, is distributed throughout Europe in
areas where climate is relatively mild and oceanic. It is very similar to valley
oak, Quercus lobata of California in the USA. Both these species , Q. robur and
Q. lobata are quite similar and can hybridise and form fertile hybrids in captivity

490. but they do not interbreed in nature simply because of distance and their
geographic ranges do not overlap.
3. Elephant Seal: The southern elephant seal, Mirounga leonina, occurs in the
cold waters of southern coasts of South America, South Africa, Australia and
New Zealand. Its close relative in north is northern elephant seal, M.
angustirostris. It is found in cold waters along the coast of western North
America . Though , the two forms are very similar, they are separated by about
3,000 miles of tropical sea, and hence are unable to exchange genes .
8.5.3 Ecological Isolation or Habitat Isolation
Eco logica l isolation is seen in those populations or close ly related spec ies that
occ upy different hab itats in the same geographical area of their distribution .
Because they live, feed and mate in different habitats, they rarely venture out and
hardly interbreed. Ecological isolation is also described as habitat isolation or
environmental isolation.
EXAMPLES:
I. In India, the range of distribution of lions and tigers overlapped over
thousands of kilometers until about 150 years ago. But there were no records of
their hybrids in nature, because lions stayed in open grasslands and hunted in
groups and tigers lived in forests and tend to be solitary. Because of their
ecological and behavioural differences , they rarely came in direct contact.

492. FIG. 8.2: Distance acts as an effective barrier for reproductive isolation between
English oak, Quercus robur and California oak , Quercus tobeie , though their
leaves and acorn are quite similar.
However, tigers and lions can mate and produce hybrids in captivity, The hybrids
of male tiger and female lion are called " tiglons or tigons" and hybrids of male
lion and female tiger are called "tigers". It means no reproductive barriers exist
between tiger and lion but their hybrids are not formed because of difference in
habitats.
FIG. 8.3: Tigon (hybrid between male tiger and female lion) and Liger (hybrid
between male lion and female tiger)
2. Two ecologically distinct populations of stickleback fish (Gasterosteus) are
found in Belgium. One of them lives in freshwater round the year, whereas the
other returns to sea in winter and migrates to estuaries in spring and summer.
The latter is unable to live in one habitat due to change in chlorine contents in
the blood. Hybrids between the two forms can be obtained by artificial
insemination but in nature their occurrence is very rare.
3. Ecological isolation is illustrated by pig frog (Rana grylio) and the gopher
frog (Ran a areo/ata) . Rana grylio is aquatic and occurs in deep ponds, lakes
and marshes, lily ponds, and breeds in deep water. On the other hand, Rana
areo/ata prefers margins of swampy areas and breeds in shallow water of
isolated grassy ponds. The difference in ecological preference eliminates the
possible mating between the two species.
4 . Habitat isolation also plays an important role among plants. Arctostaphy/os
mariposa and A. patula are large bushes found in California. A. mariposa occurs
at lower elevation up to about 4,800 ft. and A. patula from 4,600 ft. upwards.
Even in overlapping belt, the former occupies drier and more exposed areas,
whereas the latter is found in more sheltered places. No pollination occurs
between two species.
5 . California valley oak, Quercus /obata, grows in the fertile soil of open
grassland on gentle slopes and valley floors. Another species California scrub
oak, Quercus dumosa grows in less fertile soil on steep slopes. If cross
pollinated, they produce fertile hybrids but their hybrids are not found in nature .

493. Disturbance of habitat results in the breakdown of ecolo gical isolation and
return to successful interbreeding.

495. FIG . 8.4: Ecological or habitat isolation in two species of oak Quercus lobata
(California valley oak) and Q. dumosa (California scrub oak). Though their
leaves and acorn are different, they are able to produce fertile hybrids.
8.5.4 Temporal (Time-related) Isolation
D ifferences in the breeding seaso n prevent mating and interbreeding among
individuals of different populations, races or of different species that occupy
similar habitat. This type of isolation is called temporal (time-related) or
seasonal isolation. Seasonal isolation is very common in plants and freque ntly
occurs in insects and certain other inverte brates .
EXAMPLE:
I. Blain (1941) found seasonal isolation in toads. American toad (BIiJo
american
liS)
and fowler's toad iBufo Jow/eri), when reproduce in captivity in
laboratory, produce hybrids. But in spite of having similar geographic
distribution, their hybrids are rare in nature, because B. americanus breeds early
in the rainy season and B. fowleri breeds late. During the overlapping period,
considerable hybridisation occurs and the hybrids resemble another species of
toads, B. woodhousii.

497. 2. Rana clamitans, R. pipens and R. sylvatica breed in the same pond in
America, but they do not interbreed because the reproductive seasons of these
species are different. Temperature of water is the controlling factor. R. sylvatica
breeds at about 44°F, R. pipens at 55°F and R. clamitans breeds at a temperature
above 60°F.
3. Two species of wild lettuce, Lactuca graminifolia and L. canadensis grow
together along roadsides in South-eastern USA. Fertile hybrids between them
can be produced experimentally but hybrids between them are rare in nature
because L. graminifolia flowers in early spring and L. canadensis flowers in
summer. This prevents interbreeding and gene exchange between the two
species.
8.5.5 Ethological Isolation or Behavioural Isolation
Eve n when populations of different sympatric species are in contact and breed at
the same time, the individuals of one species may reject or fail to recogni se
individuals of other species as mating partner. This is called ethological (Gr.
ethos = habit or custom) isolation or behavioural isolation or sexual isolation.
These species are genetically determined by different gene combinations.
A nimals have evolv ed elaborate court ship colours and mating behaviours
which can be considered as passwords between the male and female members of
the same species for the recogni tion of sex. The males of every species have
species specific courtship display s and behaviours and only females of the same
species are receptive to these displays. These specie s specific behaviours of
males toward s females are called species-speci fic recognition pattern s. The act
of mating is attempted only when there is appropriate exchan ge of stimuli.
These stimuli can be discussed under following three categories:
1. Visual Stimuli: The visual stimuli include colour-pattern s on the body of
organisms, their body form, size and their movements. These may work
independentl y or in conjecture with auditory, tactile or chemical stimuli.
EXAMPLES:
I. Difference in size prevents hybridisation between Oak toad (Bufo quercicusy
and Gulf Coast toad (Bufo valliceps). The smaller Oak toad fails to copulate with
larger Gulf Coast toad, because in either case male fails to grasp the female toad.

498. 2. The male humming birds of different species show various colour patterns ,
elaborate crests and tail feathers of different sizes and colours.
3. More than 500 species of Drosophila on Hawaiian Islands maintain their
distinctiveness by displaying different mating behaviour patterns. Their males
display complex territorial behaviour and elaborate courtship rituals.
4. Courts hip display is also seen in some birds and mammals.
2. Auditory Stimuli: The auditory stimuli include song calls and other acoustic
signals. Species specific sounds play an important role in the courtship of frogs,
toads and birds.
EXAMPLES:
I. In cricke t, Gryllus campestris, females are attracted by stridulating calls of the
conspecific males of their species.
2. Four species of leopard frogs (genus Rana) are quite similar in appearance but
produce different mating calls to attract opposite sex of their own species .
Because of different croaking sounds, the sexes of different closely related
species are not attracted. In natural habitat the hybrids between them are rare
even in the overlapping regions of their habitats (Fig. 8.5).
3. Female birds recog nise their males either by their call notes or their courts hip
display.

501. FIG. 8 .5: Range of distribution of four species of leopard frogs: 1. Rana pipens ,
2. Rana blairi, 3. Rana utricularia, and 4. Rana berlandieri. These species
closely resemble but produce different mating calls to avoid interbreeding.
3 . Chemical Stimuli: The chemical stimu li include stimuli which are detected
on contact or by the olfactory organs. Many species are known to produce
species specific odours called ph erom ones. Mammals can be specifica lly
mentioned in this category. In a polychaete, 'Gurbea clarata', the mature females
release egg protein in water which induces sperm ejection by males . Their
semen (sperm fluid) contains a sex-stuff that induces the females to spawn. This
is followed by sperm ejaculation. Such species-specific sex stuffs are the most
important isolating mechanisms in marine invertebrates with external
fertilisation.
8.6 POSTMATING PREZYGOTIC BARRIERS
These barriers prevent contact between male and female gametes even though
the mating is attempted. Either the male is unable to deposit the sperm in the
genital tract of the female or sperm are not viab le inside the female's genital
tract. These isolating barriers include :
8.6.1 Mechanical Isolating Barriers
Among animal species with internal fertilisation, the structural differences of
male and female copulatory organs may prevent copu lation and deposition of
sperm in female's genital tract.
According to Leon Durfour (1844), the genitalia in insects are deve loped on
'lock and key' principle (i.e., the male and female genita lia are so exactly fitted
to each other that even slight deviation in the structure of either, renders
copulation impossible and that interspecific crosses in Drosophila and also in
Glossina species may cause injury or even death to the fema le. No doubt,
mechanical isolation was
considered to be an effective barrier to cross breeding in some organisms, but is
not applicable to all. Even sporadic occurrence of copulation between insect
species with marked ly different genita lia has been observed.
Mechanical isolation is observed even in plants and is more common in

502. insectpollinated species. Differences in the floral structure in related species of
plants, especially with elaborate floral morpho logy, prevent pollination by the
same variety of pollinators and prevent hybrid formati on.
For examp le, flowers of Mimu/u s lewisii have broadly splayed petals for
beepollination. But flowers of M. cardin a/is have narrow, tubular corolla for
bird pollination. (F ig. 8.7)
8.6.2 Physiological Isolation
Certain species have been established only on the basis of some physiological
differences developed between them. For example, in certain species of
Drosophila, mating among the members of different species is not successful
because vaginal mucous membrane swells up after copulation. The swelling lasts
for few hours if mating is among the members of the same species (conspecific
mating) but continues for days, if the mating is interspecific (heterospecific). As
a result, the fertilised eggs of heterospecific mating perish in the parent body in
the absence of being laid down . This phenomenon is called con sp ecific p redo
minan ce.
Similarly, in plan ts, the pollen grains from some other species when fall on the
stigma of other species, fail to germinate and either perish or can not compete
with conspecific pollen .
8.6.3 Gametic Isolation or Gametic Mortality
Even if mating occur s, the sperm of one species fail to fuse with the eggs of
other species and form the zygote . This situatio n represents ga metic isolation
or gametic inco mpatibility.
The gametic mortality is seen in forms either with external or internal
fertilisation. The sperm may fail to fertilise the egg and both of them perish . A
male with

504. A BC FIG. 8.6: Copulatory organ and the posterior lobe of genital arch in males
of three species of Drosophila : A . Drosophila simulans, B. Drosophila schellia
and C. Drosophila mauritiana.
nonfunctional gonads, but with norma l sex behaviour may induce a female to
lay eggs but without fertilisation the eggs soon die.
EXAMPLES:
1. Volpe (1960) observed phenomenon of gametic mortality in toad species, Bufo
fowleri and B. valliceps.
2. Lillie found that in two closely related species of sea urchins,
Strongylocentrotus purpuratus and S. franciscanus, when gametes are released
into surrounding water, they are not attracted to each other because the proteins
on the surface of eggs and sperm can not bind to complete fertilisation.
3. In forms with internal fertilisation the spermatozoa have to pass through the
genital tract of female . The sperm from the male of different species may
encounter an antigenic reaction during their passage through female genital duct
or vagina and many get immobilised and killed before they reach the eggs.
Patterson found in cross-insemination between related species of Drosophila (D.
americana, D. virilis, D. montana and D. lacicola) that the sperm mobility is lost
rapidly in the seminal receptacles of females of foreign species.
8.7 POSTZYGOTIC BARRIERS
Postzygotic barrier s act subsequent to mating and union of heterospecific
gametes. The potential mates of two populations overcom e prezygotic barriers
and copulate but either no offspring are produced or the hybrid s have reduced
vitality or reduced fertility. These postmating mechan isms are classified into the
following categories:
8.7.1 Zygotic Mortality
The gamet es from two different species may fuse but the zygote may not
survive, or the developm ent of hybrid zygote is irregular and ends up at any
stage during development without reaching adulthood.
EXAMPLES:
1. In a cross between sea urchin, Paracentrurus lividus and Psammechinus

505. microtuberculatus, the embryos die before gastrula stage.
2. In animals with external fertilisation, spermatozoa may enter the egg but the
sperm nucleus is eliminated from the cleavage spindle.
3. Eggs of fishes can be inseminated by sperms of different species and genera,
but development does not proceed normal.
8.7.2 Hybrid Inviability
In some interspecific interbreeding, the zygote develops normall y but the
hybrids have lower survival rate than nonhybrids . Often the mortality is intrinsic
due to gene
in compatibility. The genetic programms directing development of two species
may be so different that hybrids die early in development. Even if the hybrid
comp letes the development, it fails to survive long and reach reproductive stage.
EXAMPLES:
I. Warwick and Berry showed that the cross between goat and sheep produces
norma l embryos, but they die much before birth due to gene incompatibility
(intrinsic factor).
2. In plants, hybrid inviability may be extrinsic, because in them hybrids may
have better survival in new or different habitat than in the habitats occupied by
the parent species.
8.7.3 Hybrid Inferiority or Hybrid Breakdown
In many inter specific crosses the hybrids have low surv ival value . They may
display behaviours that are mixture of both the parental species and in doing so
they are hopelessly uncoordinated. For example, hybrids between certain love
birds have great difficulty in learning to carry nest materials during flight.
In many interspecific cross es, the first generation hybrids are fully fertile , but
when they mate with one another or with their parent species, offspring of the
next generation are feeble or sterile. The weakn ess of hybrids is attributed to
some physiological or ecological differences.
EXAMPLES:
I. Leibach (192 I) described hybr id inferiority in flax hybrids. In a cross
between Unum perenne and L. austriacum, the hybrid seeds do not germinate in
their original form but the embryos on being removed from seed coat exhibit

506. luxuriant growth and the hybrid s are fully fertile.
2. It has also been noted that the hybrids of one sex, specially male ones fail to
survive or are sterile while the viability of female sex is affected little or not at
all. Occurrence of unisexual progenies is fairly common in interspecific hybrids.
3 . Strains of cultivated rice have different alleles at two loci and differ from the
common ancestor. Hybrids between these species are vigorous and fertile but
plants in the next generation with too many of these recessive alleles are sma ll
and steri le.
4 . The closely related species of Drosophila, D. pseudoobscura and D.
persimilis can be crossed in the laboratory, though no hybrids are known in
natural conditions. By experimental crossing in captivity, the F
I
hybrids appear
to be as vigorous as the parents. But F
1
hybrid males are sterile whi le F
I
hybrid
females produce numerous eggs . When backcrossed to parental species, the F
2
hybrids have low viability.
The low viability of F
2
hybrids is referred as F
2
breakdown in fitness. The
weaker hybrids will certainly be elimin ated in nature due to selection,
presumably
because of disharmonious combinations of alleles . This is in contrast to
harmonious combinations of conspecific gene composition. They are called co-
adapted.
8.7.4 Hybrid Sterility or Hybrid Infertility
-
The hybrids of certain interspecific crosses are found to be sterile or semisterile,
i .e., unable to produce normal sperm or eggs. Either the chromosomes fail to
pair at meiosis or abnormalities in the formation of spindle or failure of cell
division occur in the spennatocytes, rendering the male individuals sterile. It
means reduced fertility is an intrinsic barrier and is caused by structural
differences between the chromosomes of closely related species due to
aneuploidy or due to differences between the genes from two parents.
Hybrid sterility could be:
• Developmental Hybrid Sterility : Hybrid s are sterile becaus e gonad s develop
abnormally or meiosis break s down before completion because chromo somes

507. fail to pair.
• Segregational Hybrid Ste rility: Hybrids are sterile because of abnormal
segregation of the whole chromosome s, chromosome segment s or
combinations of genes passed on to the gametes. Gametes with excess or
shortage of chromosomes or genes may not survive and produce sterility.
EXAMPLES:
l. The hybrid offspring of a male donkey and female horse is a 'mule'. It is robust
but sterile. A ' hinny', the hybrid offspring of female donkey and male horse is
also sterile.
2. The ' liger' a hybrid between a male lion and a female tiger is also infertile .
H ybrid sterility is usually restricted to heterogametic sex (males in mammals
and insects and females in birds). This generalisation is called Haldane's Rule .
This is found to be the most consistent generalisation about speciation.
8.8 MULTIPLE ISOLATING BARRIERS
Interbreeding in closely r elated specie s is prevented not by anyone of the
aforesaid barriers or isolating mechanism s but by a whole series of ecological,
physiological , ethological and cytogenetic isolating factors. For example, two
related species may occur in different habitat s, produc e gametes at different
times of the year, have different behavioural patterns, and produce inviable
embryo on mating. But often one factor out of the series is dominant.
For example, Justin Rams ey and colleagues concluded from their exp er imental
data that more than 97 per cent of reproductive isolation between sympatric
populations of two species of monkey flower, Mimulus lewisii and
208 [j] Evolutionary Biology
FIG. 8.7: Flowers of species of monkey flower plant, A. Mimulus lewisii
adapted for bee pollination and B. Mimulus cardinalis adapted for bird
pollination.
M. cardinalis is due to pollinator fidelity. Flowers of M. lewisii are adapted for
bee pollination and have broadly spread out while flowers of M. cardinalis have
narrow, tubular corolla for bird pollination. This shows that despite other
isolating barriers to gene exchange, pollinator fidelity accounts for the maximum

508. isolation.
In evolving species , the isolating barrie rs are arranged like a series of hurdles ;
if one breaks down, another must functio n to accomplish isolation. If the habitat
barrier is broken, the individuals of two species may still be separated by their
behaviour patterns . If these also fail, the mating may not lead to the formation
of viable hybrids or the hybrids produced are infertile.
8.9 GENETIC BASIS OF REPRODUCTIVE BARRIERS OR
REPRODUCTIVE ISOLATION
Except for geographic and eco logical isolating mechanisms which are extrinsic,
all other prezygotic and postzygotic isolating mechanisms arise due to genetic
differences in the populations. These may be produced by:
8.9.1 Gene Mutations
Both premating and postzygotic isolating mechanisms arise due to changes in
gene structure and gene interactions. For examp le, differences in the
reproductive period (temporal isolation), reproductive behaviour (ethological
isolation) , copulatory organs, gametic isolation, hybrid inviability and hybrid
infertility arise due to gene differences and gene incompatibility in the genes
from two parents.

510. 8.9.2 Gene Rearrangement or Chromosomal Aberrations
Changes in the arrangement of genes in the chromosomes arise by inversions
and translocations and introduce structural alterations in chromosomes.
Heterozygosity for chromosomal rearrangements hinders with pairing between
homologous chromosomes and causes reduced fertility of hybrids (Refer
chromosomal aberrations)
Isolated populations

511. t:SQ)
E

512. F
d~
:,.'.~\\~; \,
5
Incipient
o
species
'iii
C
x
w O:::S" '~ .I i,: :
~i.t ~\\;
New species New species
::S~·t~· No barrier
~ ,,\~\\~,
Some populat ions get isolated from parental population due to extrinsic or
geographical barriers
S ome of them undergo genetic differentiation to become
reproductively isolated
and form incipient species
S ome isolated populations merge into parental one, others
reproductively isolated ones form new species
FIG. 8.8: Reprod uctive isolati on establ ished accordin g to Muller's view.
Population I
A, A, B, B,
Allele
A,A,
substitution at
A, A,
B,B, locus A
A, A,
A,A, B,B,
Population 2 Original genotype
!
A, A, B, B,
B, B,
Allele substitution
A,A, B, B, at locus BB,B,
!
A, A, B, B,

513. Reprod uctive isolation developed due to incompatibility between A, and B,
alleles
FIG. 8.9: Dobzhansky-Muller theory of allele incompatibility lead ing to origin
of reproductive isolation between two populations.
21 0
~
Evolutionary Biology
8.9.3 Change in Chromosome Number
Aneuploidy and triploidy also le ad to partial sterility, since homologous
chromosomes derived from male and female parents of different species in
aneuploids fail to form normal chromosome pairs. When chromosome number
of such a hybrid double s, the hybrid will have duplicate of each chromosome
and the chromosome s will pair making the polyploid hybrid fertile. Such a
sudden change in chromosome number leads to sympatric speciation by
establishing instant reproductive isolation from parental species in plants .
Among plants, fertile individuals from sterile ones through polyploidy have
arisen in nature forming new specie s.
8 .10 ORIGIN OF REPRODUCTIVE ISOLATION AND
ORIGIN OF SPECIES
Two major theories have been propo sed to explain origin of reproductive
isolation:
8.10.1 Muller's View
A ccording to Hermann Muller ( 1940), reproducti ve isolation is due to the
differenc es in gene s that arise during origin of species and subspecies in the
gene pool s of allop atr ic populations. When a large population of a species
occupies different environments or becomes isolated into subgroups by
geographic isolation, each isolated subgroup behaves as an independent
population . The organisms of such allopatric populations do not interbreed and
no exchange of genetic material take s place. The evolutionary force s, like
mutations. recombinations, natural selection and genetic drift operate
independently in each population and each is subj ected to independent selection
pressure . In othe r word s, the gene pool of each population change s in respon
se to its particular environment and its organisms get adapted to the new
environment. Over a long period, the se changes result in entire reshuffling and
reconstitution of genes, chromosomes and the entire genotype. The se popu

514. lations with changed genot ypes if get an opportunity to interbreed or happen to
be sympatric, their changed genotypes do not permit them to interbreed or the
hybrids produced are steri le or inviable. In brief, reproductive isolation is the
byproduct of genetic divergence which occurs in allopatric popu lations during
the origin of species and subspecie s.
8.10.2 Dobzhansky's View
Ac cording to Dobzhansky, the reproductive isolation betwe en sexuall y
reproducing and cross fertilising species is cau sed by changes in the
complementary gene comple xes, favoured by natural selection. He noticed that
hybrid s of the species are often less adapted or may be even sterile or inviable
due to incompatibility of their gene combinations. Such hybrid s tend to be
eliminated by natural selection due to hybrid inviability, hybrid inferiority or
hybrid sterility. Along with the elimination of hybrid s, those genes of parent are
also elim inated which favour hybridi sation .
Therefore, natural selection acts against hybridi sation and brings about
reproductive isolation between closely related populations.
8.10.3 Dobzhansky-Mu ller Theory
Dob zhansky and Muller propo sed theory of allele substitution for reproducti
ve isolation between two populations. According to this theory fitness in
conspecific hybrid s is due to the combined action of the ' right' alleles at both
the loci. The 'wrong' combinations of alleles in interspecific hybrids result in
'Dobzhansky-Muller incompatibility'.
EXAMPLES:
Two populations of a species had initial genotype A
2
A
2
8
2
8
2
. Each population
undergoes an allele substitution at a different locus substituting either AI or 8,
yielding populations A,A,8
282
and A
2A2B
IB, genotypes (Fig. 8.9). As long as the
genetic background is 8
2B 2
both A
1A2
and AlA , have fitness equal to A
2A2
or
greater in population I. Likewise 8,B
2
and BIBI are equal or superior to 82B2, as
long as the genetic background is A
2A2
in population II. However, an epistatic
interaction between AI and 8
1
causes incompatibility and hybrid A,A
2B
IB
2
has
lowered viability or fertility. This theory is based on the incompatibility of
epistatic interactions among several loci. The degree of hybrid sterility or
inviability may increase exponentially with the passage of time.

515. 8.11 EVOLUTION OF REPRODUCTIVE BARRIERS
Ho w long does it take reproductive isolating barriers to evolve? Does premat
ing and postmating isolation mechani sms evolve at approximately equal rates
between allopatric species? Is there a speciation clock or does reproductive
isolation evolve in a more or less clockwise fashion?
C oyne and Orr have answered these questions based on large scale studies on
the patterns of speciation conducted in Lepidoptera, Frogs, 8irds and a Freshwat
er fish. The general pattern of evolution of reproductive barriers is:
• Reproducti ve isolation increases with time .
• Reproductive isolation increases in a linear fashion till reprodu ctive isolation
is fully established.
• Rough 's peciation clocks' exist for large taxa.
• Hybrid sterility evolves well before hybrid inviability.
• In sympatric speciation reproductive isolation is almost instantaneou s, if it is
due to polyploidy .
• In allopatri c species, the pre-mating and post-mating reprodu ctive isolation
evolve at appro ximatel y equal rates.
212 ~ Evolutionary Biology
• Allopatric speciation
• Behavioural isolation
• Conspecific
• Ecological barriers
• F
2
breakdown
• Geographic isolation
• Hybrid inferiority
• Hybrid inviability
• Isolating mechanisms
• Premating isolating barriers
• Speciation
• Temporal isolating barriers
KEY TERMS
• Aneuploidy

516. • Biological species concept (BSC)
• Dobzhansky-Muller incompatibility
• Ethological isolation
• Gametic incompatibility
• Haldane's rule
• Hybrid inferiority
• Hybrid sterility
• Postmating isolating barriers
• Prezygotic isolating barriers
• Sympatric speciation
• Tiglon or Tigon
• Liger
REVIEW QUESTIONS
I . What is isolation? Describe various isolating barriers and the importance of
isolation.
2. Discuss the role of isolating mechanisms in the process of evolution of new
species.
3. What are different biological isolating mechanisms in evolution? Briefly
comment on each of them.
4. Describe genetic basis of reproductive isolation.
5. Discuss isolation as factor of evolution.
6. Write a short essay on isolation.
7. Explain how are geographic barriers and reproductive barriers associated?
8. Describe role of geographical barriers in evolution.
9. Without isolation there is no speciation. Justify this statement.
10. Differentiate between allopatric and sympatric speciation.
II. Write short notes on:
(a) Ethological isolation (b) Physiological isolation
(c) Geographic isolation
12. Discuss Dobzhansky-Muller's view regarding the establishment of
reproductive isolation leading to the formation of new species.
13. How is reproductive isolation between species maintained?
14. Postzygotic isolation between populations or species is based on interactions
between nuclear genes and cytoplasmic genetic elements such as mitochondria.
How this incompatibility leads to hybrid sterility or hybrid inviability?
15. Explain which isolating barriers can reduce gene exchange between
geographically isolated sister populations of a species.

517. FURTHER READINGS
I. Brooks, R., 2002. Variation in Female, Mate choice within Guppy Populations:
Population Divergence, Multip le Ornaments and Maintenance of Polymorphism
. Genetica, 116: 343-358.
2 . Cabot , E.L. Davis, A.W., Johanson, N.A. and CI Wu, 1994. Genetics of
Reproductive Isolation in the Drosophila simulans Clade: Complex Epistasis
underlying Hybrid Male Sterility. Genetics, 137: 175-189.
3 . Coyne , J.A. and H.A. Orr, 2004. Speciation , Sinaur Associ, Inc.
4. Howard, DJ. and S.H. Berlocher, 1998. Endless Forms: Species and
Speciation, Oxford University Press.
5. Mayr, E., 1970. Populations, Species, and Evolution, Harvard University
Press [Species concepts].
6. Michalak , P., Mohamed, A.F. Noor, 2006 . Genetics of Reproductive
Isolation and Species Differences in Model Organisms. Evolutionary Genetics,
Oxford University Press, pp. 387-398.
7. Noor, M.A.F., 1999. Reinforcement and Other Consequences of Sympatry.
Heredity, 83: 503-508.
8. Orr, H.A., 1997. Haldane's Rule. Annu. Rev. Eco/. Syst., 28 : 195- 218.
9. Orr, H.A. and Presgraves, D.C., 2000 . Speciation by Postzygotic Isolation:
Forces Genes and Gene Molecules. Bio Essays, 22: 1085- 1094.
10. Presgraves, D.C., 2002. Patterns of Postzygotic Isolation in Lepidoptera.
Evolution, 56: 1168-1183 .
II . Wu, c.I. and M.F. Palopoli, 1994. Genetics of Postmating Reproductive
Isolation in
Animals. Annu. Rev. Genet.. 28 : 283-308.
DOD

520. UNIT III
Speciation

522. Chapter 9. Population Genetics, Gene
Frequencies and
Hardy-Weinberg Equilibrium
Chapter 10. Persistence of Variability within Populations: Polymorphism
Chapter 11. From Population to Species
(Speciation)
Chapter 12. Genetic Drift and Gene Flow Chapter 13. Natural Selection in
Action
Chapter 14. Evolution of Genes and Genomes

524. 9
Population Genetics, Gene Frequencies and Hardy-Weinberg
Equilibrium
9.1 POPULATION GENETICS
According to neo-Darwinism or modem synthetic theory, evolution is regarded
as a change in the gene frequencies or more accurately the allele frequencies in
the gene pool of a population or species by the action of natural forces i.e.,
natural selection, genetic drift and gene flow. The study of variation in the allele
frequencies in a population and the causes for such variation is referred as
population genetics. Although, the individuals in a population must differ
genetically for evolution to occur, it is the population that evolves and by
accumulating variation in its gene pool diversifies from its sister population and
change s into a new species.
9
.2
POPULATION
-------
A population is a group of organisms of the same species (conspecific
organisms) that live in the same area and interbreed producing fertile offspring.
Different populations of a single species are usually isolated geographically so
that the exchange of genetic material between them occurs less frequently. Such
isolation is common because of gregarious nature and uneven distribution of
organisms. It means, the members of a population breed freely among
themselves and are more closely related to each other. Because of less frequent
interbreeding, the gene pools of different populations show more differences.
Such an isolated group of individual s living together in an area and showing
free interbreeding has been variously described as local breeding population or
Mendelian population by Dobzhansky, panmictic population by Sewall
Wright and deme by Gilmour and Gregor.
9.2.1 Mendelian Population
Definition

526. The evolutionists and geneticists have defined Mendelian population as a local
population or community of sexually interbreeding or potentially interbreeding
individ uals living within a given geographical area at a given time. Each
member has an equal opportunity of mating with any other member of the
opposite sex.
9.2.2 Attributes of a Mendelian Population
• Individuals of a Mendelian populati on have somewhat similar genetic
constitution and gene alignment except for some uniqueness.
• Individual variation often reflect gen etic variation . These represent
differences in the composition of their genes or DNA segments.
• A population possesses a given gene pool and all the members of a population
share in the same gene pool and contribute to it.
• There is free gene flow among all the members of a population because of free
interbreeding.
• Each member of a popul ation has equal chances of mating with any other
member of opposite sex.
• The sister populations of a specie s are in occas ional reproductive contact, but
the chances of interbreeding are less than among the individuals of same
population i.e., interpopulation interbreeding is occasional and intrapopulation
interbreeding is frequent.
Interbreeding
Sister populations
I. Deme or population with free
interbreeding among its members
Spec ies
Race
II. Because of geographica l or other types of isolations , breeding with other populations becomes less
frequent and divergence of genotype begins III. Almost complete isolation; genetic Interchange very rare;
gene pools of populations show divergence and causes partial sterility of brids
1--"'1hows less frequent interbreeding
I~IShows interbreeding
Subspec ies FIG. 9.1: The interbreeding relationship between the individuals of a
population and

527. betwee n the gene pools of different populations, races and subspecies. Free
interbreeding is found among the members of a population but interbreeding
becomes less frequent

529. among sister populations, r aces or subspecies .
• Because of occasional breeding between individuals of sister populations, their
gene pools are interconnected and as a result, the gene pool of the entire specie s
gets reshuffled continuously.
9.3 GENE POOL
Gene pool is the sum total of all the copies of every type of allele at every locus
in all the members of a Mende lian population.
The allele s/gene s are embodied in the individuals and are passed on to the next
generation through reproductive gametes. Therefore, gene pool can also be
considered as gametic pool and can be defined as the sum total of alleles present
in the gametes of a Mendelian population.
If the gene pool of a population is described completely , it provides information
not only about the types of alleles present in the population but also the
proportions of different alleles and the way they are distributed in the
individuals. If only one allele exists for a particular locus in a population, that
allele is said to be fixed in the gene pool, and all the individuals are homozygous
for that allele. But if there are two or more alleles for a particular locus in a
population, its individuals are both homozygous and heterozygous.
Imagine a population of 500 wild flower plants with two alleles , Rand W for a
locus that codes for flower pigment. These show incomplete dominance. Here
each genotype has a distinct phenotype. Plants homozygous for R allele will
have genotype RR and bear red flowers ; plants homozygous for W allele
produce no pigment and bear white flowers. The plants heterozygous i.e. with
both Rand W alleles produce pink flowers. Suppose these alleles are present in
equal proportions (i.e., 50% Rand 50% W alleles) in the gene pool of the
population. In case these alleles are in equilibrium, they represent red : pink :
white flowered plants in the ratio of I : 2 : I , i.e., I Red (RR) : 2 Pink (RW) : I
White (WW). This ratio of alleles is maintained as long as random mating
occurs.
Locus X is
represented
~

-==~
by three alleles (X" X,and X,) in the population

530. In d iploid individuals only two of the three
alleles are present
FIG. 9.2: The gene pool of a population for gene locus X is represented by three
alleles X,. X
2
and X
3•
Each of the coloured circles represents an individual. The
allele proportions
here for locus X are 0.20 for X,. 0.50 for X
2•
and 0.30 for X
3•
9.3.1 Integrity of Gene Pool

532. The gene pool of a population maintains its integrity as long a s there is no
interbreeding between populations. Because of interbreeding between sister
populations, the genes from one sister population enter the other populations and
vice versa. This transfer
of genes is called gene flow. Gene flow leads to mixing and reshuffling of gene
pools. (Fig. 9.3)
9.3.2 Equilibrium
Th e gene pool maintains or tries to maintain a dynamic equilibrium in the gene
and genotype frequencies but these change due to mutations, preferential
interbreeding and natural selection.
9.3.3 Fluctuation in Size of Gene Pool
The size of the gene pool depend s on the number of genes and the individuals
carrying these genes. The gene pool becomes large by the addition of genes to
the gene pool. This is brought about by immigration and mutation. The gene
pool decreases in size by the removal of genes. This is brought about by
emigration, natural selection and genetic drift.
9.4 FUNDAMENTAL PRINCIPLES OF GENETIC VARIATION IN
POPULATIONS
Nearly all populations possess some le vel of genetic variation for many
characters. To measure genetic variation in a Mendelian population , we need to
count every allele at every locus in every individual in it. This will tell the
relative proportion or frequencies of all the alleles in the population. This is
called allele frequency (also referred as gene frequency).
Gene
•
flow •
Interbreeding

534. ••
•
• • • New gene pool• • •
••
FIG. 9.3: Gene flow from one population to another population by interbreeding.
0.50 0.50 i')'
c:
Ql
Ql
0.25 0.25 ~
u..
0.25
Aa aa A a Genotypes Alleles AA Aa aa Genotypes
Genotype frequencies , Allele frequencies (parental generation) among gametes
0.50
Genotype frequencies , (offspring generation)
FIG. 9.4 : Diagram showing frequency of three genotypes among females and
males for a locus with two alleles (A, a), in one generation, the allele frequencies
among their eggs and sperm, and the genotype frequencies among the resulting
offspring.
In sexually reproducing diploid populations, the alleles present in the eggs and
sperm, combine to form homozygous and heterozygous genotypes as shown in
Fig. 9.4. The proportion of alleles of one type at one locus in the gametes is also
called gametic frequency or allelic frequency and the total numb er of
individuals having same genotype for one pair of alleles repre sents genotype
frequency.
9.4.1 Gene Frequency or Allele Frequency
The gene frequency refers to the proportion of an allele in the gene pool as
compared with other alleles at the same locus, with no regards to their
distribution in organisms.
9.4.1.1 Explanation
Imagine a hypothetical population of hamsters with black and gray hair. These

535. are controlled by two allele s located on the same locus. These are A and a, of
which A is dominant and a is recessive. Three types of genotypes may exist in
the population when there are two allele s as follows :
AA Homozygou s dominant 25%
aa Homozygou s recessive 25%
Aa Heterozygou s 50%
222 ~ Evolutionary Biology
Th e ratio of A allele to a is
the frequency of A allele and
vice versa, the ratio of a to A is
the frequ ency of allel e a.
In c ase allel e frequ encies in parental generation are A = P
= 0.7 and of a = q = 0.3; in offspring s of second, third or
further generations, the genotypic
All
ratio and gene frequencies will sperm
remain the same as in the parental
in gene
generatio n in case population fol
pool
lows Hardy-Weinberg principle.
(Fig. 9.5).
Allele frequencies in parental generation: A =P=0.7 a O =q = 0.3
All eggs in gene pool r__---''''.....----,
\0
0.7 A 0.3 a
AA
0.7 A 0.49 Aa
0
0.21
aA
0 0
0.3 a 0.21 aa
00
0.09
Allele
frequencie s have not
changed

536. 9.4.1.2 Calculation of
Genotype frequenc ies Allele frequenc ies
Gene Frequency or
in
offsp ring generation in offspring generation
Allele Frequency
AA
=
p
2
=
0.49
A
=
P
=
1 0.49 + (0.42) =0.70 Suppo
se
a
diploid popu
lation
2
=
q
=
0
.
09 +
-
(0.42) =0.30has 1000 individuals, and that Aa = 2pq = 0.42 a
1 2
for a pa rticular
gene locus in aa = q2 = 0.09
tha t pop ulation , there are only
Fig. 9.5: A Punnett Square to illustratetwo allel es A and a or AI
and Hardy-Weinberg principl e.
A
2
. There will be thre e possible
genotypes for this locus AA and aa (or A lAI and A
2A2) ,
both homozygous and
Aa or A
I
A
2
heterozygous.
L et us say that out of 1000 individuals 400 have genotype AA (= A lAI) ' other
400 are heterozygous with Aa (= A
1A2
) genotype and 200 are aa or A
2A2
. Then
allele frequency of A and a in a popu lation can be calculated by the following

537. formu la:
(i) Allele fr equ enc y of A or AI
Number of allele A in the populationp =
Sum tota l of alleles on the same locus in the
population (2N) 2N
AA
+ N
Aa
2N
(ii) Allele frequency of a or A
2
Num ber of allele a in the population
q =
Sum tota
l of alleles on the same locus in the population (2N) 2N
aa
+ N
Aa
2N
IM:I•••' Calcu lation of allele frequency in a population of 1000 individuals
1 . Total number of individuals in the population = 1000 Frequency Percentage
frequency
2. Total number of alleles on one locus contributed to the gene pool of
population by 1000 individuals will be
= 2000
800 = 0.4 3. Number of dominant alleles (A) 400 +400 = 800
2000contributed
to the gene pool by 400
homozygous dominant individuals will be
4. Number of dominant alleles (A) contributed
400
by 400 heterozygous (Aa) individuals will be = 400 - = 0.2
800 +400
2000 5. Frequency of allele A
2000
= 0.6 0.4 + 0.2 = 0.6 60%
6. Number of recessive alleles (a) contributed to
400
the gene pool by 200
homozygous 200 + 200 = 400 - = 0.2 individuals (aa) will be
2000
7 . Number of recessive alleles (a) contributed
400
to the gene pool by 400
heterozygous = 400 - = 0.2 individuals (Aa) will be
2000
8. Frequency of allele a will be
400 +400
= 0.4 0.2 + 0.2 = 0.4 40%
2000
The above numera ls can be represen ted as follows:
1. Total no. of individuals = N

538. 2 . Total no. of alleles A and a (since the organism is diploid) = 2N
3. No. of homozygous dominants = D
4. No. of homozygous recessives = R
5. No. of heterozygotes = H
6. Total number of dominant allele (A) = 2D + H
(a) The number of dominant allele (A)
contributed to the gene pool by homozygous
dominants = 2D
(b) The number of dominant allele (A) contributed
by heterozygotes H 2D+H
7. The frequency of allele (A) : = -
2N
8. The number of recessive allele (a) contributed to
the gene pool = 2R + H
( a) Contributed by homozygous
recessives (aa) = 2R
(b) Contributed by heterozygotes (Aa) = H
2R+H
9. Frequency of allele a =
2N
Therefore , th e allele frequency can be calc ula ted by dividing the number of a
particular allele in qu estion with the total number of all eles present on that
locus in the pop ulation.
If frequency of allele A is represented by p and that of a by q and at gene
equilibrium their total frequency is represented by I, then at equilibrium: p +q
=1 H
or p = I q = 20 + 2
H
or q = I P = 2R + 2
9.4.2 Genotype Frequency
The genotype frequency is the total number of one kind of individuals from a
population all of which exhibit similar character with respect to the locus under
consideration.

539. In a population , there are two allele s at one gene locus (AI and A
2)
and they are
related as dominant and recessive. Naturally, individuals with three genotyp es,
i.e., homozygou s domin ant, heterozygous and homozygous recessive will occur
in the population.
If N = Total numb er of individuals in the population
D = Number of homozygous domin ants
H = Number of heterozygous individuals
R = Number of homozygous recessives
Th en, genotype frequency of AlAI individuals =
genotype frequency of AIA
2
individuals =
genotype frequency of A
2A2
individuals =
It means genotype frequency for a particular type of gene combination on the
same locus can be determined by dividing the number of individuals with that
genotype by the total number of individuals in the population .
9.4.2.1 Calculation of Genotype Frequency
(A ) The frequency of homozygotes is calculated by squaring the frequency of
the allele A and allele a.
(B) The frequency of heterozygotes is calculated as follows:
(a) Gene frequen cy of A allele = p
(b) Gene frequency of allele a = q or I P
(c) Genotype frequency of AA = p 2
(d) Genotyp e frequency of aa = q2 = (1 p )2
(e) Genotype frequency of Aa = 2pq
EXAMPLES:
(a) Genotype frequency of recessive allele (aa) in human population, i.e., q2 is 1
= 20 000 = 0.00005
(b) Gene ' frequency of recessive allele aa, i.e., q = ~0.00005 = 0.007 (c) Gene
frequency of dominant allele AI or (A) i.e., p = 1 q = 1 - 0.007 =
0.993
(d) Genotype frequency of heterozygotes Aa, i.e. , 2pq = 2 x (0.993 x 0.007)
= 0.014

540. (e) Genotype frequency of homozygous (AA) = P x P = 0.993 x 0.993 = 0.986
(A) Parental genotype frequencies (B) Parental allele frequencies (not In equilibrium)
1.0 1.0
0.8 0.8 ~
c:
0 .6
~
0.6 C" Q)
0.4
0.4
.l::
0.4
"* 0.4 0
.2
~ 0.2 0.2
0.0
AA Aa aa
0.0
A a (0) (H) (R) (p) (q)
(e) Offspring probability of a given mating producing the genotype AA Egg A x
Sperm A = P
x
P = p2 = 0.6
2
= 0.36
Aa { Egg A x Sperm a = p x q = pq = 0.6 x 0.4 = 0.24 } = 0.48
Egg a x Sperm A = q x p = pq = 0.4
x
0.6 = 0.24 aa Egga xSperma=qxq=q2
=0.4
2
= 0.16
(0) Offspring genotype frequencies (E) Offspring allele frequencies
1.0 1.0
0
.8
0
.8
Allele frequencies have not changed from one ~
generation to the next
0 .6
0 .6

c:
~
0.6
0.48 I
0.40.4 "* 0.4
~
0.2 0.2
0.0 AA Aa aa 0.0 A a
FIG. 9.6: Genotype frequencies and allele frequencies in paren tal populat ion
and in offspring population after one generation. The allele frequencies do not
change from one generation to the next when evolut ionary forces are not acting
on population's gene pool.

541. The frequencies of different all eles at each locus and the frequencies of different
genotypes in a Mendelian population descr ibe its genetic structure. Allele
frequencies measure the amount of genetic variation in a population; genotype
frequencies show how a population's genetic variation is distributed among its
members. These measurements help in assessing whether genetic structure of a
population is undergoing any change or not, over generations and whether
natural seletion is operatng or not on the gene pool of the population.
9.5 HARDY-WEINBERG EQUILIBRIUM
In 1908 , British mathematician, Godfrey Hardy and the German physician ,
Wilhelm Weinberg independently derived a mathematical equation to represent
the distribution of alleles and genotypes pertaining to a specific gene locus in a
panmictic Mendelian population. Later, in 1929-30, R.A. Fisher and J.B.
Haldane in England and Sewall Wright in United States worked out that under
certain conditions, the genetic structure of a population may not change over
time. They established the concept of 'Hardy-Weinberg principle of equilibrium.'
9.5.1 Definition
T he principle of genetic equilibrium states that relative frequencies of various
kinds of genes in the gene pool of a large and randomly mating sexual, panm
ictic population tend to remain constant from generation to generation in the
absence of evolutionary forces, such as mutation, selection and gene flow.
This is called Hardy Weinberg Principle or Hardy Weinberg Law of Equilibrium.
9.5.2 Explanation
Hardy Weinberg law describes a theoretical situation in which a population is
undergoing no evolutionary change. It explains that if the evolutionary forces are
absent, the population is large, its individuals have random mating, each parent
produces roughly equal number of gametes, the allele frequencies are identical in
males and females , and the gametes produced by the mates combine at random
and the gene frequency remains constant, then the genet ic equilibrium of the
genes in question is maintained and the variability present in the populations is
preserved. Suppose, there is a panmictic population with allele A and a. The
frequency of gametes with allele A will be the same as the frequency of allele A
and similarly the frequency of gametes with allele a will be equal to the
frequency of the allele a. Let us presume that the numerical proportion of

542. different genotypes in this population is as follows:
AA Aa aa
• Genotype frequencies 36% 48% 16%
0.36 0.48 0.16
• Allele frequency A = 60% or 0.6 and
a = 40% or 0.4
Since AA individuals make up 36% of the total population, they will contribute
approximately 36% of all the gametes formed in the population. These gametes
will
p ossess gene A. Similarly, aa individuals will produce 16% of all the gametes.
But gametes from individuals Aa will be of two types, i.e., with gene A and with
gene a roughly in equal proportion. Since these constitute 48% of the total
population, they will contribute 48% gametes, but out of them 24% will possess
gene A and the other 24% will have gene a. Hence, the overall output of the
gametes will be:
P arents Gametes Parents Gametes
36%AA
~ 36% A
16% aa
~
16% a
48%Aa ~ 24% A 48%Aa~ 24% a Total 60%A 40% a Generation I
Genotypes AA Aa aa
Frequency of
0.36 0.48 0.16
genotypes in

543. population 1
+ 0.24
/ \
0.24 + 0
1
Frequency of0.36 .16
allelesin '--y-J '--y-J
population p = 0.60 q = 0.40
® Gametes 0
Generation II
~

545. The addition of three types of genotype frequencies give Hardy-Weinberg
equation: p2 + 2pq + q2=1
FIG. 9.7: Calculating allele and genotype frequencies with the help of Hardy-
Weinberg equation.
If the gametes unite at random, the total number of different genotypes will be:
Gene S.No. Parents Sperm Ova Frequency in
Offspring
1. M 60 60 60 x 60 =36%
2. Aa 60 40 60 x 40 =24%
3. aA 40 60 40 x 60 =24%
4. aa 40 40 40 x 40 =16%
Offspring Genotype
M
Aa Aa aa
The abo ve concept can be translated into a simple mathematical expression:
• Frequency of gene A is represented by = p
• Frequency of gene a is represented by = q
and there i s random union of the gametes with gene A and a. At the equilibrium
state, the popu lation will contain following frequencies of the genotypes and
genes A and a generation after generation:
AA + 2Aa + aa genotype frequencies
or p2 + 2pq + q2
The above results are explain ed by relying on the theory of probability. In a
population of large size :
• Probability of formation of AA individuals = p x p = p2
• Probability of formation of aa individuals = q x q = q2
• Probability of formation of Aa individuals = 2 x p x q = 2pq
In the next generation , the probabili ty of frequency AA, aa and Aa individuals
will again be p2, q2 and 2pq.
The relationship between gene frequenc y and genotype frequency can be ex-
pressed as:
(a) if gene freq uencies of two alleles A and a are p and

546. q: p + q = I
(b) the genotype frequencies in the population will be:
(p + q)2= (1)2= p2+ 2pq + q2= I
It is known as Hardy-Weinber g formula or binomial expression of
HardyWeinberg principle.
From this binomial expression proposed by Hardy and Weinbe r g, it is clear that
in a large and randomly mating population not only gene frequencies but also the
genotype frequencies remain constant or tend to remain constant.
9.5.3 Conditions for Hardy-Weinberg Equilibrium
According to Hardy-Weinberg principle, the gene and genotype frequencies of
each allele in a population remain at an equilibrium (static) generation after
generation, if that population exhibits following attributes:
1. Extremely Large Pop ulation Size: The equilibrium in gene and genotype
frequencies occurs only in large sized populations . In small populations, there
will be significant sampling errors and random fluctuations in the gene
frequency by chance, the so called genetic drift or random drift.
2. Random Mating: The population shall be panmictic where every gamete has
an equal opportunity of fusing with any other gamete of the opposite sex. This
nonpreferential fusion of gametes is called random mating. The natural
populations may not fulfill this equality, because individuals show preferential
mating within a subset of the population and random mixing of gametes fails to
occur.
3. Biparental Mode of Reproduction: Hardy-Weinberg principle is applicable
only for biparent sexually reproducing species. Unisexual or asexually
reproducing populations do not follow Hardy-Weinberg law.
4. Homogeneous Age Structure: A population normally consists of individuals at
different stages of reproductive maturity. According to Hardy and Weinberg, the
population must be homogeneous in this regard.
Summary of Assumptions of Hardy-Weinberg Equilibrium
• Parents represent a random sample of genes and gene frequencies from the
gene pool of a population.
• Heterozygotes for any gene pair produce two kinds of gametes in equal

547. frequency showing complete segregation of alleles of a gene pair.
• Both the parents are equally fertile.
• All the gametes are equally fertile and have equal chances of survival and
forming a zygote.
• All the zygotes formed have equal chances of survival and developing into an
adult.
• Gene frequencies are the same in both male and female parents.
• Mating between the two sexes is nonpreferential and random.
• All genotypes have equal reproductive ability and equal chances of
contributing the genes/alleles to the gene pool of the population.
5. Absence of Evolutionary Forces: The gene frequency will remain static only
in the absence of evolutionary forces like mutations, selection, genetic drift and
migration, as follows:
• There shall be no gene mutations, because mutations affect allele frequencies.
• There shall be no gene flow either by immigration or emigration, because
migrants alter number of alleles by deleting from the gene pool or adding to it.
This shows absence of gene flow in or out of the population.
• All genotypes in a population shall reproduce equally successfully. There shall
be no selection and no preference for specific genotype or phenotype for

549. mating. The absence of selection means every gamete is viable, all gametes have
equal chances of becoming a zygote , every gametic union (zygote) also survives
and gametes or zygotes are not segregated into classes of varying survival value .
Under such conditions, the genetic composition of population will remain
unchanged.
• Gene frequencies are same in both male and female parents .
From the above discussion it becomes clear that populations exhibiting genetic
equilibrium or following Hardy-Weinberg's principle are static with zero
evolutionary rate. This means such popu lations do not evolve and any
population evolving is not static and does not exhibit genetic equilibrium.
9.5.4 Examples
1. PTe Taste in Human Population: In human population persons with gene T
find weak solution of PTC (phenyl-thio-carbamide) to be bitter in taste, whereas
to homozygous tt persons such solutions are tasteless . Moreover, persons are
unaware of their reaction to PTC and nobody selects his mate according to
whether he or she can or can not taste this substance. Therefore, the marriages
take place at random, and population is panmictic with respect to this trait.
Suppose, in a particular island or in a town, the number of homozygous tasters
(TT) and of homozygous nontasters is equal, the possible marriages could occur
between:
TT x TT or TT x tt or tt x tt
The marriages between tasters and nontasters and their progenies can be
represented as follows:
Therefore, the genotype frequency in first
generation will be TT = 25%, tt = 25% ~0.5TT 0.5 Itand Tt = 50%, but since the homozygous TT
Tt
tasters (TT) and heterozygous tasters (Tt) 0.5 TT
0.25 0.25are phenotypically alike, the
population will
be 75
%
tasters
and 25% nontasters.
The
0

550. .
5
It
Tt It
0.25 0.25
same results are obtained if we consider
the union of their gametes at the time of =0.25 TT
+
0.50 Tt
+
0.25 It fertilisation.
The proportion or frequency of genes T and t will remain the same. This could
be explained as under:
Let us once again presume that every
~
0.5T 0.5 t
individual produces equal number of func

552. TT Tttional gametes. The homozygous tasters 0.5T 0.25 0.25
(TT) and homozygous nontasters
(tt) will
produce all the gametes of only one type
0
.5
t
Tt It
0.25 0.25
i.e., T and t respectively; the heterozygotes
Tt will produce gametes with T and t in =0.25 TT
+
0.50 Tt
+
0.25 It
equal numbers. Therefore, the frequencies of the gene s T and t in the gene pool
will be:
Gene T 0.25 from homozygotes TT
{
0.25 from heterozygotes Tt
Total frequency of gene T = 0.25 + 0.25 = 0.5
Gene t {0.25 from homozygotes tt
0.25 from the heterozygotes Tt
Total frequency of gene t = 0.25 + 0.25 = 0.5
We see that the frequencies of gene T and t among the gametes giving rise to
second generation is the same as in the first generation and will remain the same
generation after generation. Similarly, the genotype frequencies , according to
Hardy-Weinberg's equation, will be:
0.25 TT + 0.50 Tt + 0.25 tt = I
or p2 + 2pq + q2 = I
2. MN Blood Groups: In human beings, MN blood graups depend on M and N
types of antigens. The formation of these antigens is under the control of
codominant alleles LM and L N. Suppose, in a human population, M, Nand MN
type of blood groups are in the proportion of 0.35, 0.15 and 0.50 respectively.
Then
• Genotype frequencies of MM , MN and NN blood types:
MM = 0.35, MN = 0.50 and NN = 0.15.
• Allele frequency of LM gene will be
2 x 0.35 + 0.50
p =

2
0.70 + 0.50 = 0.120 = 0.60
2 2
• Allele frequency of LN gene will be

553. 2 x 0.15 + 0.50
q =

2
0.30 + 0.50 = 0.80 = 0.40
Since p + q = 1
2 2
0.60 + 0.40 = 1
According to Hardy-Weinberg Principle L M LM / L N L N/ and L MLN should
appear in the frequencies represented as p2, q2 and 2pq:
p2 = (0.60)2 = 0.36
q2 = (0.40) 2 = 0.16
2pq = 2(0.60
x
0.40) = 0.48

555. 23 2 ~ Evolutionary Biology
9.5.5 Significance of Hardy-Weinberg Principle
Populations in nature rarely meet the stringent conditions necessary to maintain
them at Hardy-Weinberg equilibrium. It means Hardy-Weinberg Principle is
essential to determine whether the popu lation is evolving or not? In order to
ascertain that evolutionary agents are operating or not, the allele and genotype
frequencies of the population are determined generation after generation and
then compared. The pattern of deviation from Hardy-Wei nberg ratio will tell the
agent/agents responsible for evolutionary change. Thus, Hardy-We inberg
princip le:
• provides a theoretical baseline for measuring evolutionary change
• tends to conserve gains which have been made in the past and also to avoid too
rapid changes
• maintains hetero zygosity in the popu lation
• prevents evolutionary progress
9.6 HARDY-WEINBERG PRINCIPLE AND EVOLUTION
(FACTORS THAT CHANGE GENE FREQUENCY)
Unlike the theoretical static Hardy-Weinberg model of a genetic population, a
natural biologica l population is dynam ic. Its gene equi librium changes from
one to next generation by the disruptive action of evolutionary forces. R.A.
Fischer, Sewall Wright and J.B.S. Halda ne emphasised and studied dynamism
in populations.
The e volutionary forces change the gene poo l of the population. These include
genetic polymorphism or genetic variability (caused by gene mutations,
chromosomal aberrations, hybridisation, chang e in chromosome number, and
immigration or gene flow), genetic drift and selection.

557. Equ ilibrium in gene frequencies and genotype
4
.
Po
lyploidy
frequencies in the gene pool changes
Fig. 9.8: Facto rs that upset Hardy-Weinberg equilibrium in a popu lation.
9.6.1 Genetic Varia bility in Natural Populat ions
Natural populations are all polymorphic. It means in the gene pool of
populations, each gene is present in the form of two or more variants or alleles .
A population with only one variant of a character is monomorphic for that
character. The gene pool of a polymorphic population exhibits varied degree of
genetic variability. The genetic variability is mostly noticed as polymorphism in
the genes due to gene mutations and differences in arrangement of genes in the
chromosomes.
Even the populations exhibiting phenotypic similarity in characters of survival
value may be genetically polymorphic having heterozygous genotype. This
phenotypic similarity of traits in populations with genotypic variability for a
particular character or characters is called canalisation. For example, in insects ,
wing shape and body size are important for survival in nature . In Drosophila,
cross-veinless wings represent a sex-linked character. Waddington exposed
normal flies to environmental shock and obtained cross-veinless flies. After
repeated experiments, he established a new stock. This shows that high
production of optimum phenotypes in a population does not necessarily be
homeostatic in genetic composition for that particular character. The effect of
selection is found to change the expression of genotype.
Essentialit y of Genetic Variability: Genetic variability in populations is essential
for evolution, because genetic variability leads to polymorphism in populations
on which natural selection and genetic drifts operate. Evolution will be very
slow if populations are genetically uniform and mutations arise only
occasionally to replace the pre-existing genotypes. There will be no evolution
and no genetic variability without mutations.
The rate at which mutations arise at a single locus is usually so low that their

558. occurrence results in very small deviations from Hardy-Weinberg expectations.
It means for large deviations other forces also introduce genetic variability.
Factors Responsible for Genetic Variability in Populations: Causes for genetic
variability or variations have already been discussed in Chapter 8. These include:
• Gene mutations that introduce multiple loci.
• Gene recombinations produced during sexual reproduction, i.e., (i) at the time
of gamete formation, i.e., meiosis, and (ii) due to nonrandom fusion of
genetically different gametes at fertilisation.
• Changes in the arrangement of genes in individual chromosomes.
• Changes in the number of genes in individual chromosomes.
• Changes in the number of chromosomes.
• Gene flow due to introgressive hybridisation.

560. Genetic Load and Genetic Death: Genetic var iability is essential for evolution.
But natural selection tends to favour superior genes and superior gene
combinations , and eliminates the inferior or disadvantageous ones and thus
tends to make a population homozygous. However, populations retain certain
disadvantageous or harmful genes. The existence of disadvantageous genes in
gene pool of a
234 lil Evolutionary Biology
population is called genetic load. These are carried in heterozygous genotype
and confer selective advantage on the phenotype in certain environmental
conditions , e.g., sickle cell trait in malarial infested regions . The presence of
harmful alleles makes a population less fit. Thus , no population in nature has
genetic perfection.
In a genetically imperfect population , the individuals homozygous for harmful
or disadvantageous genes are unable to contribute to the gene pool of the
population either because of actual death before reproductive age or because of
sterility or inferiority or inability to find a mate or because of reduced
reproductive ability. This failure of individuals to produce offspring or death
without reproduction is called genetic death.
The values of genetic load and genetic death can be expressed in terms of
frequency and number of individuals eliminated because of selection. This is
called selection pressure.
For example , if a gene is deleterious in homozygous condition , the frequency of
homozygotes before and after selection will be as follows:
AA Aa aa
(a) Frequency at fertilisation p2 2pq q2
(b) Relative adaptive value 1 1 1 s* (c) Frequency after selection p2 2pq q2 _
sq2
*s is selection pressure
The loss of frequency of incurred genetic load is equal to sq 2. This value of sq2
is equal to the mutation rate at equilibrium, i.e., genetic load caused by a
deleterious homozygous recessive is equal to its mutation rate. It means if in a

561. population, the mutation rate is constant , it will not make much difference to the
genetic load. But selection pressure higher than the rate of mutation will tend to
eliminate the gene from population and low selection coefficient will permit the
gene to be retained in the gene pool of the population. Any increase in mutation
rate will cause a corresponding increase in genetic load and genetic death.
Mutational and Segregational Loads: The mutational load is the extent to which
the population is impaired by recurrent mutations. It is a part of genetic load of
every species . Mutational load has two components:
(a) produced by the deleterious mutations, and
(b) produced indirectly by beneficial mutations .
According to Muller and others , both these components of mutational load arise
because an ideal population consists only of individuals that are homozygous for
all beneficial genes.
The segregational load or balanced load is seen in populations where het-
erozygous genotype is superior to both the homozygotes. For a gene with two
alleles, the segregational load will be:
(a) for dominent homozygotes AA =p2s and
(b) for recessive homozygotes aa =q2t.

563. 9.6.2 Gene Flow in Populations
Populations of any species are geographically isolated, but migration of
individuals does occur leading to the movements of gametes between closely
related populations. This immigration of individuals adds new alleles to the gene
pool of the population and is called gene flow. If gene flow continues
unopposed, it causes all such populations to converge to have the same allele
frequencies or the average allele frequencies. The rate of gene flow among
natural populations can be estimated by following the dispersal of marked
individuals or their gametes.
9.6.3 Genetic Drift
I n small populations, the changes in allele frequencies are not adaptive or
directional. They show nondirection al fluctuations generation after generation
purely by chance. The random loss of individ uals and their alleles may decrease
the allele frequencies to great extent in a single genera tion. Sim ilarly, the allele
frequencies of certain genes may increase far beyond the normal range in one
generation just by chance. These deviations are nondirectional and nonadaptive.
The harmful alleles may increase and rare advantageous alleles may be lost
because of genetic drift. Even in large populations, genetic drift can influence
frequencies of alleles that do not influence the survival and reproductive rates in
the populations.
The reasons for such non-directional deviations in allele frequencies in small
populations are:
• bottleneck effect, and
• founder 's effect.
1. Bottleneck Effect: Large populations occasionally pass through seasonal or
yearly phenomenon of cyclic fluctuation in their population density leaving only
a few individuals to survive. These form the progenitors for the future
generation. This few surviving individuals may cause random loss of individuals
and their alleles leading to large changes in their allele frequencies from one
generation to the next. The period of low population dens ity in the seasonal
cycles represents a bottleneck period and changes in allele frequencies occurring
during this bottleneck period constitute bottleneck effect.
The bottleneck effect may result in the loss of some advan tageous alleles or in
the increased frequencies of even harmfu l alleles. The lost alleles in different

564. subpopulations or demes of the same large popu lation may be different
depending on chance.
2. Founder Effect: When a few individuals become isolated from a large
population and invade a new isolated geographical region and establish a new
population, they become founder members. They will carry only a limited
portion of parental gene pool. The population arising from these founder
members is unlikely to have all the alleles from the gene pool of source popu
lation. The resulting diversion in the gene pool of new population is called
founder effect.
236 ~ Evolutionary Biology 9.6.4 Natural Selection
Natural selection is based on differential success in survival and reproduction.
Individuals in a population that are better suited to their environment tend to
produce more offspring than those with traits that are not suited so well. The
relative success of different phenotypes within a population leads to changes in
allele frequencies and thus disturbs Hardy-Weinberg equilibrium. It is
determined by the average number of offspring they produce over their lifetimes.
Therefore, natural selection is described as the differential reproduction of
genotypes and is linked to increased adaptability to the environment.
9.7 GENETIC LANDSCAPE OF A POPULATION AND EVOLUTION
• Evolution is basically the changes in allele frequencies in the gene pool of
populations and population is considered to be the primary unit in evolution.
During evolution, it is the total gene pool of the population that changes or
evolves and not the individual. Diversity in the population is maintained by
mutations, recombination and assortment of genes or chromosomes.
• Each population or its gene pool contains a collection of genotypes. These
genotypes can be plotted on a landscape based on their adaptive values. The
relative fitness of all genotypes are represented by relative heights of the points.
These are called adaptive heights.
• The genetic landscape of a population has many adaptive peaks separated by
large stretches of adaptive valleys.
• Each adaptive peak represents a genotype most adapted or most beneficial for a
particular environment.
• Adaptive valleys represent genotypes that are much less fit.

565. • The valleys represent gene combinations that:
- could have a high adaptive fitness under some different conditions, or
- have monstrous combinations that could not survive under any circumstances.
• Populations sitting at the peaks of adaptive heights are most adapted for that
habitat. Populations that occupy peaks close to one another on genetic landscape
will be adapted to similar ecological niches and are closely related
phylogenetically.
• In a population, when the adaptive peak is moving back and forth over the
seasons, it indicates that individuals with one genotype have a higher fitness
Valley
.cr-- Adaptive peak
Frequency of Gene 1
FIG . 9.9: Representation of genotypes as points on a map. The heights of peak
represent adaptive values of each genotype .
Population Genetics, Gene Frequencies and... [i] 237 at one time , but in altered
seasonal variations, the greater fitness shifts to another genotype.
•Once a population has reached an
adaptive peak, further evolution will
depend upon the origin of new selec
tive environment which will lead to
the formation of new adaptive peaks .
•A population on one adaptive peak
cannot reach another higher peak
without going through a nonadaptive
vall ey. It means as environment
changes, the population becomes less
adapted to the changed environment.
If it has to reach adaptive peak again
FIG. 9.10: Diagram showing that population that resides an adaptive peak has
most suitable genotypes with the fit
individuals occurring near its edge. it needs change in its genetic structure

566. so as to be be st adapted to the changed environment. The volume a population
occupies in a niche space also changes. Thus, the landscape changes with
environmental changes. Some valley s may have genotypes of high adaptive
value in changed environment and may form the peak .
Adaptive peak
Route of
migration
of a population from one
adaptive peak to the other through nonadaptive
valley
FIG. 9.11: A small population coming from a large population to occupy a new
adaptive peak passes through nonadaptive valleys.

568. Genotype Dimension B
FIG. 9.12: Diagram showing the gradual change in the genotype of a population
and changes in volume it occupies in a niche in the changing environment.
238 iii Evolutionary Biology
• Each adaptive peak represents a genotype most adapted or beneficial for
particular environment, while the adaptive valleys represent genotypes that are
much less fit.
• Majority of popu lations are broken into sma ll groups or subpopulations or
demes or local populations. These local populat ions are small enough for the
nonselective fixation of genetic differences due to genetic drift. But these are not
so widely separated as to completely prevent gene exchange and introduction of
new genetic variability. Therefore, the adaptive landscape is occupied by a
network of demes, occupying different adaptive peaks.
• The selection takes place (a) among genotypes competing within demes, and
(b) between demes comp eting within a general environment.
SHI FTING BALAN CE PROCESS
The distribution pattern of adaptive peaks of demes on the genetic landscape and
the changes introduced by the evolutionary forces have been described by
Wright as the shifting balance process. It can be summarised as under:
(a) Random genetic drift acts upon heterozygosity and polymorphism. It changes
gene frequencies in the demes and allows the demes to move across nonadaptive
valleys to different parts of the adaptive landscape.
(b) Selection pushes some of these demes up the nearest adaptive peak by
changing gene frequencies, i.e., by making some gene loci homozygous or
nearly homozygous.
(c) Polymorphism in demes is retained or variability is introduced through
mutation and migration .
(d) Genetic changes introduced this way provide further opportunity for genetic
drift to trigger changes in the adaptive landscape and leads to the occupancy of
new higher adaptive peaks.
Thus, populations are highly dynamic. They keep evolving in the direction of
evolutionary force.
• Adaptive heights

569. • Allele frequency
• Founder effect
• Genetic death
• Genetic landscape
• Genotype frequency
• Mendelian population
KEY TERMS
• Adaptive peaks
• Bottleneck effect
• Gene flow
• Genetic drift
• Genetic load
• Genetic variability
• Nonrandom mating
• Adaptive valleys
• Deme
• Gene frequency
• Gene pool
• Genetic equilibrium
• Hardy-WeinbergPrinciple
REVIEW QUESTIONS
I . Define a population or a Mendelian population. With the help of a diagram
depict the inter-relationship between the gene pools of individuals of a
population, populations, races and species .
Population Genetics , Gene Frequencies and... ~ 239
2 . Discuss in brief the attributes of a population or a Mendelian population.
3. State Hardy-Weinberg's law of equilibrium. Discuss its salient features.
4. What do you understand by the following terms :
(a) Gene pool
(b) Alle le frequency
(c) Genotype frequency
(d) Genetic equi librium
(e) Deme

570. (f) Genetic landscape
5. Under what conditions the gene frequency in the individuals of a population
remains constant.
6. Explain the terms gene flow, random mating and their association with genetic
variability in populations.
7. Enumerate various evo lutionary forces that tend to disturb genetic
equilibrium and introduce changes in the gene pool of a population.
8. Mention significance of Hardy-Weinberg 's law.
9. How you can calculate the genotype frequency of heterozygous genotype of
heterozygotes?
10. Comment upon genetic variability of natural populations.
I I. Explain in brief genetic load, mutational load and segregational load. 12.
What do you understand by shifting balance process during population
evolution? 13. Justify the statement, a population on one adaptive peak can not
reach another higher peak without going through nonadaptive valley of genetic
landscape. 14. What do you mean by the statement 'adaptive peaks generally
shift geographically'? 15. Give reasons for the following :
(a) Genetic drift operates only in small popu lations.
(b) Genetic drift are the result of sampling error.
(c) Even harmful alleles or gene combinations are fixed in the populations.
FURTHER READINGS
I. Crow, IF., 1988a. Righty years ago: The beginnings of population genetics.
Genetics, 119. 473-476.
2. Crow, J.F.; Hardy, 1999. Weinberg and language impediments. Genetics. 152,,
825.
3. Darwin , 1859. On the Origin of Species by Means of Natural Sel ection. or
the Preservation of Favoured Races in the Struggle for Life. London, UK, John,
Murray. (link) .
4. Dobzhansky, Th., 1937, 1941, 1951. Genetics and the Origin of Species (three
cds.). Columbia University Press, New York.
5. Dobzhansky, Th., 2003 . Dobzhansky's Genetics of Natural Populations I-
XLIII, R.C. Lewontin, J. A. Morre , W. B. Provine, and B. Wallace (eds.).
Columbia University Press, New York. [This volume is a coolection of the 43
papers in Dobzhansky's
240 [j] Evolutionary Biology
influential series on Drosophila population genetics, with various coworkers,

571. along with introductory articles by Provine and Lewon tin.
6 . Edwards, A.W.F. and Hard y, G.H. 1908 and Hardy-Weinberg Equilibrium
Genetics, 179, 1143-1150 (2008).
8. Falconer, D.S., and T.F.C. Macka y, 1996. Introduction to Quantitative
Genetics, 4th ed. Longman, London, UK.
9. Futuyma , D.J., 1998. Evolutionary Biology, 3rd ed., Sinauer and Associates,
Sunderl and, MA.
10. Gille sp ie, J.H ., 2004. Population Genetics: A Concise Guide, 2nd ed.,
Baltimore , MD : John s Hopkins University Press.
I I. Hardy, H.H ., 1808. Mend elian Prop ortions in a Mixed Population. Science,
28,49-50 . 12. Hartl, D.L. , and A.G. Clark, 1997. Principles ojPopulation
Genetics, 3rd ed. Sinauer Asso ciates, Sund erland, MA .
13. Hedrick , P.w., 2000 . Genetics oj Populations, 2nd ed., Jone s and Bartlett,
Sudbury, MA.
14. Mendel , G. Versu ches tiber Plflanzen-hybrid en. Verhandlunden des
naturforschend en Ver-eine s in Brunn , Bd. IV fur das Jahr 1865, Abhand-
lungen (188 6): 3-47 (link ). 15. Monagh an, F. and Corcos, A. 1984. On the
Origins of the Mendelian laws. Journal oj Heredity, 75, 67-69.
16. Pagel, M. (ed. in chief), 2002. Encyclopedia ofEvoution, 2 Volumes, Oxford
University Press, New York.
17. Weinberg, W., 1908. Uber den Nachweis der Vererbung beim Mencshen.
Jahr eshefte des Vereins Verterlandische Naturkdunde in Wiirttemberg 64, 369-
382 . 18. Wiggington, J.E., Cutler, D. 1. et al. 2005. A Note on Exact Tests of
Hardy-Weinberg Equilibrium. The American Journal oj Human Genetics, 76,
887-893 .
DOD

572. 10
Persistence of Variability within Populations: Polymorphism
10.1 VARIABILITY WITHIN POPULATIONS
Genetically uniform natural populations of any significant size do not occur in
nature. Natural populations are highly variable genetically and morphologically.
The genetic variability among populations arises due to gene mutations, genetic
recombinations, chromosomal changes and changes in gene expression due to
environmental influence. However, both natural selection and genetic drift tend
to reduce variability in populations because of the following reasons:
1. Adaptability: Variability i s key to adaptability. According to Fisher (1930) the
rate of increase in fitness within any population is proportional to the variance in
fitness at that time and in that environment. This is called ' fund a mental
theorem of natural selection'.
Ayala (1965 , 1968) showed that Drosophila populations derived from a mixture
of several natural populations adapted twice as fast to the experimental
temperatures as the similar populations derived from single source. This shows
that evolutionary changes occur more rapidly in variable (heterozygous)
populations than in uniform (homozygous) populations.
2 . Evolution: A variable population has an increased chance of changing or
evolving under changed environmental conditions. It is expected that a variable
population would have some genotypes better suited to the changed
environment.
3 . Polymorphism: Variability leads to polymorphism in populations. It includes
genetic polymorphism, biochemical/physiological polymorphism and
morphological polymorphism.
10.2 POLYMORPHISM
10.2.1 Definition
Polymorphism is defined as the existence of two or more forms (= morphs) of a
phenotype in the same species within the same population at the same time and

573. place.
242
~
Evolutionary Biology
The differences in such forms may be morphological, physiological or
biochemical and are genetically determined. E.B. Fo r d (1965) defined
polymorphism as th e occurrence of tw o or mo re distinct forms of individuals
in th e sa me population or species and in the sa me locali ty.'
F or evolutionary biologists genetic polymorphism in populations is the basic
requirement on which evolutionary forces can operate. The gene tic polymo
rphism is th e presence in th e gene poo l of a na tural population of two or more
alle les for an y gene locus in frequenci es too large to be explained by mutations
alone.
10.2.2 Salient Features of Polymorphism
1. Polymorphism is due to the existe nce of more than two types of genotypes in
a population.
2. Different forms are adapted to differen t types of environments.
3. All Mendelian populations are polymorphic.
4. Polymorphism increases the efficiency in the exploitation of resources of the
environment.
5. Polymorphism helps the species to survive efficient ly in a variety of
environments.
6. Polymorphism is essential for evo lution .
7. Polymorphism results from evolut ionary proce sses.
8. Polymorphism is inherited and is modified by natural selection.
9. Polymorphism is due to the presen ce of different forms (alleles) of the same
gene in the population's gene pool and the switch mechanism that determines
which morph is needed to appear in the given environment.
10. Pleiotropic genes and epistatic genes interact and produce polymorphism in
the population.
Most genes have pleiotropic effect i.e.. produce more than one phenotypes. Due
to physiological or cryptic effect the pleiotropic genes produce polymorphism in
the populatio ns.
I I. Epistatic genes prese nt on different loci modify genes either increasing or
decreasing effect. This the expression of other way, two or more genes produce
coordinated changes in more than one characteristics as in the case of mimicry.

574. 10.2.3 Types of Polymorphism
Following types of polymorphism have been identified in the populations:
• Balanced polymorphism
• Transient polymorphism
10.3 BALANCED POLYMORPHISM
Definition
In bal anced polymorphism, individuals with two or more forms (morphs) or
genotypes coex ist in the same popu lation of a species in stable environment and
show almost constant ratio.
It means that in a popul ation showi ng balanced polymorphism, the genotype
frequencies of various forms occur at equilibrium. It can be defined as persistent
polymorphism, perpetuated by stabilising selection by favour ing heterozygotes.
10.3.1 Salient Features of Balanced Polymorphism
I . There is a balan ce of selective forces so that none of the forms tend to be
eliminated or both the alleles are maintained at appreciable frequencies in the
population.
2 . Each form (morph) has a selective advanta ge of equal intensity.
3. None of the forms has selective advantage over other.
4. Heterozygotes are at a selective advantage. This is called heterozygote
superiority against homo -zygotes.
5. The rare or the rarest allelom orph is advantageous. As it becomes common,
its advantage decreases or it becomes disadvantageous in changed environment.
6. In balanced polymorphism different alleles of a gene are maintained in the
population because each is favoured by a separate environmental force.
7. Balanced polymorphism is genetic polymorphism. It is steadily maintained by
natural selection.
10.3.2 Examples
1. Sickle Cell Anaemia
In human beings, the persons exhibiting sickle cell anaemia have RBCs which
becom e distort ed and sickle-shaped in oxygen deficiency. The sickle-shaped
RBCs are fragile and clog the blood vesse ls. This leads to necrosis of various
tissues and damage of the orga ns. This causes seve re anaemia and heart failure.
This conditi on is fatal and about four out of five sickle cell anaemic children die

575. before they reach the reprodu ctive age. Sickle cell anaemia occurs pre-
dominantly in native Africans, where it is six times more common. This disease
was discovered by the American physician, James B. Herrick ( 1904), who
examin ed an anaemic West Indian, black student in Chicago . James V. Neel (
1940) established that sickle cell anaemia is inherited as a simple Mendelian
character:
• Normal haemoglobin is produ ced by two dominant genes Hb
A/Hb A.
• Sickle cell anaemic patients have both the genes recessive Hbs/Hbs and are
strongly selected.
244 ~ Evolutionary Biology
• Persons heterozygous for these alleles (HbAlHb
S
) exhibit sickle cell trait.
These have low capacity of carryin g oxygen, possess both kind s of
haemoglobin in nearly equ al quantit ies and suffer fro m mild anaemia.
I n a norm al hum an population natu ral selection tend s to eliminate gene Hb f
from the population because hom ozygou s Hbs/Hbs per son s do not surv ive.
A.C. Allis on ( 195519 6 1) showed connection between sickle-ce ll anae mia
and malaria. He found that the frequ ency of gene Hb" is unusually high in areas
prone to malaria in East Africa, where Hbf gen e frequency reaches as high as
20% or more. In some African tribes, its frequency is as high as 40%. Alli son (
1959) explained that high frequ ency of Hb f gene is due to selective adva ntage
of heterozygotes (HbAlHb
S
) .
In m alaria infested areas, sic kle-ce ll heterozygotes (HbA/Hb
S
) or carriers for
sickle cell trait are resistant to ma larial infection. They are less infected with
malaria parasite than norm al homozygou s (HbAlHbA) non sickl ers. The
heterozygou s persons enjoy highest survival because malarial parasites use up
oxygen and cause sickle cell haemoglobin to clump. Th is mak es RBC s of
heterozygotes sticky and sic kleshaped. The y adhere to capillary walls and are
engulfed by macrophages along with the parasites which are also killed along
with RBCs.
Th ese hetero zygotes are resistant to ma laria and enjoy high est surviva l rate in
malaria infested regions. The normal homozygous dominants HbA/HbAare
susceptible to malaria and have low survival rate. Th is advantage of hetero
zygotes resul ts in stable polymorphism of both the alleles HbA and Hbf of the

576. gen e Hb.
I n West Africa, ther e is a third allelomo rph Hb" which produces another abn
orm al haemoglobin-C. The homozyg ote HbCfHbc suffers from severe anae mia
simi lar to Hbs/Hbs. The heterozygote s HbslHbc also suffer from anaemia, but
hetero zygotes Hb A/Hb
c
are at an advantage ove r hom ozygotes Hbc/Hbc.
The above two cases strongly support the view that the polymorphism for Hb
gene is maintained in malaria-infested areas by stabilising natural selection.
ASIA
AFRICA
Equator
HbS allele Equator frequency (%)
I
ATLAN TIC o 0.0-2.5
ATLANTIC
I _ Malaria
OCEAN 2.5-5.0
OCEAN 5.0-7.5
INDIAN
o
7.5-10.0
700 1400 ml
10.0-12.5OCEAN
o 1000 2000 km >12.5
FIG. 10.1: Relationship in the geographical distribution of malaria and sickle-
cell anaemia . A. Distribution of malaria in Eurasia and Africa; B. Distribution
of sickle cell anaemia.
Dies of malaria Lives and reproduces

577. ~
~
Dies of
s
ickle-cell anemia Dies of malaria
~
Lives and reproduces
~
Dies of
s
ickle-cell anemia
FIG . 10.2: Stabilising selection can produce balanced polymorphism.Two or
more alleles, each producing a different phenotype, may be maintained in a
population by opposing environmental pressures . The alleles for normal (Hb
A
)
and sickle-cell hemoglobin (Hb
S
)
are maintained by selection against both homozygotes . Heterozygotes (HbA
HbS) reproduce the most, thereby keeping both alleles in the population .
2. ABO Blood Groups
In human population, the existence of A, B, AB and 0 blood group s is also an
example of balanced polymorphism. Though, the genotypic frequencies within
different populations may vary, they remain constant from generation to
generation within each population. This is because none of the alleles for these
blood groups has a selective advantage over the other. Persons of blood group 0
have a greater life expectancy than those of other blood groups, but are prone to
duodenal ulcer.
3. Tay-Sachs Disease
Tay-Sachs disease is recessive and untreatable. It is common in children of

578. Jewish heritage , because lout of 40 Jewish persons is heterozygous wherea s
lout of 380 non-Jewish persons is a heterozygous carrier. This is because the
grandchildren of heterozygous persons suffering from Tay-Sachs disease were
resistant to pulmonary tuberculo sis (TB). The incidence of Jewish tuberculo sis
patients from Eastern Europe is relatively high . This finding indicate s that the
heterozygous carriers of Tay-Sachs disease are protected from pulmonary
tuberculosis.
4. Cystic Fibrosis
Cystic fibrosis of pancreas is another recessive disorder found in children. One
out of 2,500 children is born with this disease . Cystic fibrosis provides
protection

580. 246 Ii] Evolutionary Biology
agai ns t chole ra, beca use th e bacteri al toxin sma ll int estine cannot do so .
This prevents pro tect ion against cho lera and diarrhoea. Cys tic fibro sis ca
rriers have selective adva ntage aga inst diarrhoea and a greater reproductive
fitness of the hetero zygou s carri er sugg ests that the detrimental ge ne for cyst
ic fibro sis is bein g maintained at exceptio nally high frequencies by balan ced
polymorphism .
5. G6PD Deficiency
RB Cs of certain persons are deficient in enzyme glucose-6-phosphate
dehydrogenase. Such persons are not affected by malaria because malari al
parasite is unable to util ise glucose and fails to multiply. Variant A of G6 PO
with 85% activity rea ches 40 % survival in sub-saharan Africa having endemic
malari a, but in general has less than 1% survival outside Africa and Middle
East.
Si milarly, there is strong correlation between malaria and thalassem ia.
Thalassemia allele persists in hum an population and is more common in Italian
Penin sula and Sardin ia. Thalasse mia patients do not suppo rt surviv al of
malarial parasite.
6. Dro sophila polymorpha
The population of D. po lymorpha presents three types of colouration of their
abdo
m en. The light co lour, dark colo ur and intermediate colour. The light colour is
due to recessive genes (aa) in hom ozygou s state and dark colour due to hom
ozygou s dom inant ge nes (AA) . The hete rozygotes (Aa) with intermediate
colour are most abunda nt. It means heterozygous flies have some adaptive
advantage. th at op en s chloride cha nne ls in loss of water from int estine and
10.3.3 Experimental Verification of Balanced Polymorphism
Th e geneticist P.M. Sheppard of University of Liverpool introduced a
population of Drosophila melanogaster in a breeding cage. The population con
sisted of 86% norm al and 14% with stu b ble gene, which affects the bristle s of
the fly. The stubble

581. ii.'1-'I:::Ibll List of Balanced Polymorphism Heterozygous for
disease
1. Sickle cell disease
Provides protect ion f rom Malaria
2. Tay-5achs d isease 3. Cystic fi brosis
Tubercu losis
Diarrheal disease or Cholera
4. G6PD deficie ncy Malaria
5. Phenylketonu ri a (PKU)
Spontaneous abortion Reason for protection
Red blood cells are inhospitable for malarial parasites .
Not known
In heterozygotes . less number of chloride channels prevents loss of water from
intestine
Red blood cells are inhosp itable to malarial parasite
Excess of amino acid pheny lalani ne in carriers inactivates fungal toxin
Ochratoxin that causes miscarriage.

583. ge ne in homozygous state is lethal. All stubble flies would be heterozygous.
Under normal circumsta nces, the heterozygous stubble flies have a low survival
rate in comparison to normal homozygous flies. Sheppard removed 60 per cent
norma l flies from the cage population from each generation . Thus the
heterozygotes were rendered superior in fitness. As a result, the frequency of
stubble gene increased in the early generations but became established in about
0.365 flies out of one, thoug h the stubble gene is lethal.
10.3.4 Explanation of Balanced Polymorphism
According to the theory of natural selection, the selection operates at all time to
reduce the frequency of an abnormal gene and to eliminate it from the
population . The occurrence of deleterious genes at unexpectedly high
frequencies in popu lations can be exp lained only by balanced polymorphism.
Fig. 10.3 shows equilibrium frequencies of two alleles (AI) and (Az) under
different conditions of selection.
1 . In case 1, when AlA. and A
IA2
individuals have equa l reproductive fitness
and A
2A2
genotype is completely selected out, the recess ive gene, 'A
2'
will be
eliminated and only A 1 allele will be present in the population in due course of
time . This represents natu r al selectio n.
2 . In case II , the heterozygotes A.A
2
exhi bit maximum reproductive fitness,
both the homozygotic genotypes are selected out. Both the genes AI and A
2
are
retained at appreciable frequencies with maximum of 50 per cent each. This
represents balanced
11
0
A, A, (Fit) A,A, (Fit) 1
00
A,A, (Lethal) r-
90
80
7 0
II.A , A, - Lethal

584. 60 A,A, Fit A,A, Lethal
50 r-
r-
40
30
20
10

586. D
II
Frequency of allele A,
III.
IV A, A,- Lethal A, A,- Semilethal A,A, Fit
A,A,- Fit A,A, Semilethal
A.A. Lethal
- r-
r-r-
III
D
IV
Frequency of allele A, FIG . 10.3: Equ ilibrium of alle le frequencies in same
population under different selection pressures.
248
~
Evolutionary Biology
po lymorphism or heterozygou s supe r iority. As in the case of sickle cell
anaemia, the heterozygotes are maximally fit and both alleles Hb
A
and Hb " are
maintained in the population even though Hb " is lethal.
3 . Case III presents selection fitness of heterozygotes and different relative
fitness of both homozygotes. The genotype AlAI leaves only half as many
progeny as the heterozygotes. The recessive homozygotes are inviable and leave
no offspring. More AI alleles are transmitted than allele A
z
to next generation. It
means allele AI has higher frequency (0.67) than allele A
z
(0.33).
4. In case IV, the recessive homozygotes A
2A2
are not as disadvantageous as
dominant homozygote AlAI' The homozygotes AlA. are nonviable and do not
contribute alleles to the next generation. Recessive homozygotes are less adapted
than the heterozygotes. Therefore, both alleles are retained in the gene pool in
relative ly high frequencies, recessive allele 0.67 and dominant allele 0.33.
10.3.5 Significance of Balanced Polymorphism
Balanced po lymorphism is important in evolution because it maintains
variability in the popu lation. This variability helps the population to react
rapidly to an environmental change and to avoid extinction. Also it paves way to
the origin of new species. It means balanced polymorphism represents stabilizi
ng selection.
10.3.6 Balanced Polymorphism and Superiority of Heterozygotes
If the heterozygote has higher fitness and enjoys a selective advantage over both

587. types of homozygotes, both recessive and dominant alleles are propagated in
successive generations and persist in the population indefinitely, even if one
allele is lethal when homozygous.
Such a heterozygote a dvantage is called heterozygot e supe r ior ity or over-
domina nce or single locus heterosis. It produces balanced polymorphism for the
gene locus and keeps an equi librium between allele frequencies.
Genotypes that are heterozygous at many gene loci are more fit than more
homozygous genotypes. As discussed earlier that being a sickle cell carrier in a
malaria infested environment, Hbf allele confers a selective advantage. Frank B.
Livingstone revea led the relationship between the spread of agriculture, malaria
and sickle cell anaemia.
• The clearing of forest for agriculture provided new breeding areas for
mosquitoes that are responsible for spread of malaria.
• Spread of malaria generated selecti ve pressure in favour of sickle cell allele
which in heterozygous state imparts resistance to malaria.
Communities in Africa depend ing mainly on agriculture have the highest
frequencies of sickle cell trait, while hunting comm unities in Africa show a very
low incidence of malaria and an equally low frequency of sick le cell allele. In
case malaria disappears, it will disrupt the balanced polymorphism and
frequency of allele for sickle cell anaemia will begin to decline.

589. 10.4 TRANSIENT POLYMORPHISM
Transient polymorphism is found in populations undergoing a strong selection
pressure and one form or morph is being strongly favoured and the other is
getting eliminated. The transient polymorphism represents a temporary
condition. It lasts for a short period till the disadvantageous allele is either
completely eliminated or is reduced to such a low frequency that it is only
retained due to recurrent mutation.
10.4.1 Salient Features of Transient Polymorphism
Transient polymorphism is seen under following situations:
I . A newly arisen mutation is found to be advantageous and is favoured by
selection. It spreads through the population and becomes more abundant, while
the wild type becomes rare due to negative selection.
2. In a changed environment, a previously rare allele may become advantageous
and is favoured by selection. It spreads throughout the population.
EXAMPLES:
I . Industrial Melanism : Indus tria l melanism in peppered moth, Biston
betularia, is an example of transie nt polymorphism. Up to 1848, peppered
moths found in and arou nd Manchester in the nonsooty atmosphere were
creamy white with black dots and darkly shade d areas. They were called non
melanic form s. In 1848, a single black coloured moth appea red among
nonmelanic forms. This melanic form is called carbonaria. By 1895, about 98%
population of peppered moth in Manchester was melanic.
The black or me lanic form arose by a recurring random mutation. Up to 1848,
in the nonsooty atmosp here of Manchester, the tree trunks were covere d with
light coloured lichens. The nonme lanic forms were able to successfully
camouflage with the lichens while resting and were able to survive, while the
melanic forms were easily spotted out and preyed upon and were eliminated. As
a result of rapid industrialization in Great Britain, large amount of soot and other'
gases were poured into the atmosphere. These made the environment sooty and
dark and destroyed the lichens. In the dark environment light coloured
nonmelanic moths were easily spotted by the predators and were being removed.
The transient po lymorp hism in peppered moth was only for a short period and
was favoured by strong selection. The transient polymorphism is seen during

590. directional selection, where one form or character is gradually replaced by
another one.

592. 2. An intermediate variety of peppered moth is called insularia. The insularia
pattern is dominant and is controlled by a different allelomorph at the same
locus. Carbonaria is dominant to insularia which in tum is dominant to
nonmelanic form.
The distribution of insularia also indicates transitional polymorphism. The
insularia is also a melanic form but is found in the marginal regions where
industrial pollution is less and carbonaria form is still rare. It means during early
stages of pollution, insularia form became advantageous.
250 !il Evolutionary Biology 10.5 ORIGIN OF POLYMORPHISM
Polymorphism in a population may develop and maintained due to the following
reasons:
1. Changes in the environm ent
2. Recurrent mutations may introduce a variety of new alleles
3. Gene flow from other populations
4. Superiority of heterozygotes
5. Frequency dependent selection
6. Selection of neutral alleles by genetic drift
Natural selection may produc e selection pressure against some features and may
support the distribution of other characters. This may be described as frequency
dependent selection.
Dolinger and his coworkers presented an examp le of frequency dependent
selection and polymorphism. Some varietie s of lupine plants produce alkaloids
which are poisonous . These are secreted by the plants for protection against
herbivorous animals including larvae of butterfly. These larvae develop
resistance to the most common alkaloid poisons . The lupine plants with
common alkaloid genotype are attacked by herbivores, whereas plants with
polymorphic alkaloids have better chances of survival. For survival even
caterpillars of butterfly develop resistance to all kinds of alkaloids. The
frequency dependent select ion tends to maintain polymorphism both in plants as
well as in caterpillars and other herbivores.
Centre of Origin: Vavilon ( 1926) proposed that genetic variability in a
population is maximum in the territory where species arose and subsequently
spread elsewhere . The polymorphism enables a species to spread and occupy
different habitats.

593. 10.6 MECHANISMS TO MAINTAIN POLYMORPHISM WITHIN
POPULATIONS
A population needs to maintain polymorphism for its survival. In the long run,
genetically variable populations survive at the cost of genetically uniform ones,
especially in times of environmental change . The various mechanisms
associated with maintaining the polymorphism are:
10.6.1 Internal Mechanisms
The internal mechani sms that preserve variability in population are :
• Transmission of genetic material (DNA) by chromosomes
• Gene expression
• Modification of gene expression by other genes
These mechanisms can be discussed under followi ng three heads: 1.
Suppression of Phenotypic expression

595. 2. Cytogenetic mechanism 3. Heterozygote superiority
10.6.1.1 Suppression of Phenotypic Express ion
Natural selection operates only on those alleles that are expressed as phenotypes.
The mechanisms that prevent phenotypic expression of alleles help in
maintaining the alleles in the gene pool. This phenotypic suppression is seen in
following situations:
I . Recessiveness: A recessive allele is expressed only in homozygous condition.
In heterozygotes, the recessive allele fails to produce its phenotypic effect and is
not exposed to natural selection.
2. Interaction between Genes: Genes present on different loci often interact and
control, modify, suppress or even add to the expression of other genes . In these
cases , the gene whose expression is altered (i.e., hypostatic gene) is exposed to
natural selection in an altered form.
3 . Incomplete Penetrance and Limited Expressivity: Certain genes produce their
phenotypic effect under specific environmental conditions or they are not
expressed fully. In these cases also, only those genes are exposed to natural
selection which are expressed.
4. Presence of Supergenes: Supergenes are several tightly linked genes present
on a single chromosome and interact to produce morphological differences in the
members. These may arise due to
• Supression of crossing over
• Translocation of chromosome fragments
• Cistron duplication.
According to E .B. Ford chromosome arragnement produces supergenes. It
means component genes in a supergene are initially present on separate
chromosomes and subsequently got reorganised on one chromosome. But
according to Turner's sieve hypothesis supergenes arise in situ on the same
chromosome only.
10.6.1.2 Cytogenetic Mechanism
The cytogenetic mechanisms, like linkage and unequal segregation, tend to
retard the elimination of unfavourable alleles and maintain variability within the

596. population.
10.6.1.3 Heterozygote Superiority (Heterosis)
In some cases heterozygotes have a selective advantage over homozygotes . As a
result both the alleles persist in the population indefinitely. Heterozygote
superiority may be due to following mechanisms:

598. I. Homozygous Inferiority: When each homozygous condition is inferior or less
adapted than the heterozygous condition, the better survival of heterozygotes
maintains balanced polymorphism of alleles within the population.
252 ~ Evolutionary Biology
2 . Heterologous Advantage: In some cases heterologous enzyme s are found to
have better activity. These heterologous or heteropol ymer enzymes are produced
by heterozygous genotypes. It means heterozygous genotypes are selected by
natural selection.
10.6.2 Ecological Mechanism
To maintain genetic variability in populati ons several ecological or external
mechanisms operate on the gene pool of popu lations. These include :
• Inefficiency of natural selection
• Compromise selection of a phenotype when a popu lation is subjected to two
opposite forces of selection
• Changes in selection pressure with time and season
• Changes in selection pressure in space (i.e., in different habitat s)
• Selection in favour of rare mutant alleles
• Frequency depend ent selection
• Density dependent selection
• Sexual selection
• Balanced polymorphism
• Genetic polymorphism
• Heterosis
• Transient polymorphism
KEY TERMS
• Cystic fibrosis
• G6PD deficiency
• Over domin ance
• Frequency depend ent
• Hetero zygote superiority
• Sickle-cell anaemia

599. REVIEW QUESTIONS
I . Differentiate between balanced polymorphism and transient polymorphism.
2. With suitable examples explain
th~
phenomenon of polymorphism .
3. How do you explain the high frequency of gene for sickle cell anaemia in
human populations living in malaria infested areas?
4. Write short notes on:
(a) Balanced polymorphism
(b) Transient polymorphism
(c) Polymorphism in land snail
(d) Tay-Sachs disease
(e) Heterozygote superiority
5. Discuss significance of balanced polymorphism taking examp le of sickle-cell
anaemia .
6. Explain with reason :
(a) High frequency of sickle-cell anaemia allele in persons associated with
agriculture. (b) Resistance against cholera/diarrhoe a in persons heterozygous for
cystic fibrosis.
FURTHER READINGS
1. Allendorf , F.w. , and G. Luikart, 2007. Conservation and the Genetics
ofPopulations. Blackwell, Oxford.
2. Allison, A.e., 1955. Aspects of Polymorphism in Man, Cold Spring Harbor
Symp. Quant, Bioi. 20 : 239-255.
3. Avise, J.C., 2000. Phylogeography, Harward University Press, Cambridge ,
MA.
4. Bakker, E.G., C. Toomajian, M. Kretiman, and J. Bergelson, 2006. A
Genome-wide Survey of R Gene Polymorph isms in Arabidopsis, Plant Cell, 18:
1803-1818.
5. Begon, Townsend, Harper, 2006. Ecology: f rom Individuals to Ecosystems,
(4th ed.), Blackwell, Oxford.
6. Clark, w.e., 1976. ' The Environment and Genotype in Polymorphism".
Zoological Journal of the Linnean Society, 58 (3) : 255-262 .
7. Dean, M., M. Carringtom , and S.1. O'Brien, 2002. Balanced Polymorphi sm
Selected by Genetic versus Infectious Human Disease . Annu. Rev. Genomics
Hum. Genet., 3 : 263-292.
8. Dobzhansky, Theodosius., 1970. Genetics of the Evolutionary Process. New
York: Columbia.

600. 9. Ford, E.B., 1975. Ecological Genetics, (4th ed.), Chapman and Hall, London.
10. Futuyma, 0.1 ., 2009. Evolution (2nd ed.). Sinauer Associates INC. II .
Hedrick Philip, 20 II . Genetics and Populations. Jones and Bartlett Learning, p.
104. 12. Huxley Julian S., 1955. "Morphism and Evolution". Heredity, 9 ( I) : I-
52. 13. Williams, G.c., 1957. Pleitropy, Natural Selection, and the Evolution of
Senescence. Evolution, II : 398-411.
DOD

601. 11
From Population to Species (Speciation)
11
.1 SPECIES AND SPECIATION
Accordin g to modem concept, evolution is nothing more than change in the
gene frequenci es in the organisms of a populati on through time. Accumulation
of these changes in gene frequencies in the gene pool of population makes it
reproductively isolated from other sister populations and leads to the origin of
new specie s.
This pro cess of genetic divergence of gene pools of populations to form new
species is called speciation. It means a spe cies is a group of organisms that can
interbreed and produce fertile offspring . All the organisms of a species share in
the same gene pool.
11.2 THE SPECIES CONCEPT
Though th e concept of species is very old and has wide usage, it is difficult to
define the word species in a way that can be applied to all living organisms and
that can satisfy all naturalists. For taxonomists, species is the lowest taxonomic
group of biological classification. It represents a group of individuals closely
resembling each other. In evolutionary studies, a species means a group of
organisms that can interbreed and produce fertile offspring. This is called
biological species concept (BSC).
Today, bi ologists define species as populations of organisms that have a high
level of geneti c similarity, share a common gene pool and are potentially
capable to interbreed and produce fertile offspring.
11.2.1 Different Criteria to Define a Species
I . Morphological Similarities: A species is a group of individuals that resemble
in most of their visible morphological characters, sex for sex and variety for
variety.

602. 2. Ecological Basis: A specie s is a group of organisms that share the same
ecological niche. No two species can share the same ecological niche.
From Population to Species (Speciation) iii 255
3. Genetic Basis: A species is a group of organisms that show close similarity in
their genetic karyotype.
4. Evolutionary Aspect: A species is a group of organisms that share a unique
similarity in structural and functional characteristics.
5. Biological Aspect: A species is a group of organisms capable of interbreeding
and producing fertile offspring.
11.2.2 Attributes of a Species
I . Members of a species exhibit distinctive features in common, in which these
are different from all other such groups .
2. Intermediate or transitional forms between the species are usually not found
i.e., species are reproductively isolated .
3. Members of separate species do not usually interbreed, so that natural hybrids
of the species are either totally absent or are very rare. Even if hybrids are
produced artificially, these are usually sterile.
4. Allied species usually have separate but adjoining territories.
5. Specie s occupies a specific ecological niche, unoccupied or unutilised by
another closely related species.
6. Species interacts with the environment and with other species present in a
given environment.
7. Species have fully effective reproductive isolating mechanisms.
8. A species is a genetic unit consisting of a large intercommunicating gene pool.
11.2.3 Evolving Species Concept
Upto 19th century, species were regarded as 'man made categories of thought'
without stability of characteristics. Carolus Linnaeus (1707-1778) for the first
time empha sised that species have constant characteristics and are sharply
delimited against each other. He defined species as 'the group of organisms that
are alike even in minute details of body structure.' Linnaeus presumed that each
species was created separatel y. Species could neither appear or disappear, nor
could change. These are totally unrelated entities.
Charles Darwin connected species diversely to evolution. In I940s , Mayr

603. developed biological species concept. According to him "a species is a
population or group of populations, whose members can interbreed and produce
fertile offspring. Today most species are assigned based upon the ability to
produce fertile offspring . new species arise when members of a population can
no longer interbreed.
Following three main evolving species concepts are being discussed here: 1.
Morphological or Typological Species Concept
According to taxonomists a morp hological or taxonomic species is 'a group of
individuals that resemble in most of their visible morphological characters with
adjacent local populations and differ only in variable characters that integrade
marginally.'
Exception: Morphospecies concept is not applicable in all the cases. Very often
greater morpho logical differences are seen among individuals of a single
population or between different populations of the same species. For example,
the males and females of river duck, mallard are so different that a they were
originally placed
yii·i-'.::&I_. Different species concepts t, heir Features and Scient
ists
Name of species concept
1. Morphological spec ies
concept
Reference Carolus Linnaeus
2. Biological species concept
3. Phylogenetic species concept
4. Evolutionary species concept
5. Cladistic species concept
6. Phenetic species concept
7. Recognition species concept
8. Ecological species concept
9. Polytypic species concept
Features of the concept

604. A group or a population of
morp hologically similar organisms which differ from other such local
populations.
A gro up of actually or potentially interb reedin g popul ati ons that prod uce
fertile offspring but are reproductively isolated from other such groups.
The smallest monophyletic group in a phylogenetic tree having common
ancestry and different from other similar groups or populations.
A single lineage of populations or organisms evolving by the accumulation of
genetic changes in the gene pool of populations.
Species are unbranched lineages in a phylogenetic tree
Clus ter of individuals having phenotypic similarities based on available data.
The most inclusive population of biparental organisms that share a common
fertilisation system. A group of organisms that
occupy the same adaptive zone or ecological niche and have minimal
morphological differences but are distinct from other
lineages
A species consisting of
geographically isolated
populations or subspecies that occupy different areas but can interbreed when
they meet. Mayr (1942)
Cracraft (1989)
Simpson (1951) and Wiley (1978)
Henning (1968); and Ridley (2004)
Sokal and Crovello (1970)
Paterson (1985)
Van Valen 1976
Mayr 1970
Mallet 1995, 2004
in separate species . The males were described as Anas boschas and females as
platyrhynchos. In birds of paradise, humming birds, tanagers , wood warblers,
etc., females differ more from males of their own species than from females of
other related species . In deep sea fishes, males are dwarf and attached to the
body of females.

605. • In sibling species, the morpho logical differences are almost absent, though
they otherwise are distinct species .
• Adaptive convergence offers examples of apparent morpho logical similarities
in unrelated groups of organisms which occupy same habitat.The logic behind
morphospecies concept is that the two populations can have distinguished
features only if they arise independently and do not have free gene exchange .
Advantages of Morphospecies Concept:
• Morphospecies concept is a useful criterion for classification when no data on
the extent of gene flow is available.
• This concept is equally applicable to sexual, asexua l or fossil species.
Drawbacks of Morphospecies Concept: The biggest drawback of morphospecies
concept is that the features used to distinguish species are subjective. Different
taxonomists working on the same populations disagree on the characters that
distinguish species . Some regard polymorphic variants of a population as
species , others . consider the geographically isolated populations to be the
species .
2. Genetic Species Concept
Lot sy (1918) and some other genetici sts defined species purely on the basis of
genetic similarity. According to this concept "A species is a group of genetically
identical individuals". But the definition is incorrect because except for identica l
twins no two individua ls are genetica lly similar.
3. Biological Species Concept
The biological species concept was introduced to replace morphological species
concept by Dobzhansky in 1937 and Mayr in 1970. According to Dobzhansky "S
pecies are Mendelian populations which share in the common gene pool ".
Definiton: Mayr (1970) gave the most acceptab le definition of species:
'A species is an array of actually or potentially interbreeding natural populations
that are reproductively isolated from other such groups under natural conditions.'
Essential Features of Biological Species Concept: These features are:
• A biolog ical species is a dynam ic unit, a stage in the process of evolution and

606. not a fixed entity as taxonomic species .
• Main feature to identify different species is the presence of reproductive
isolation between them.
• New species arise only when the gene pools of different populations of a
species become reproduc-tively isolated , i.e., the populations of a species evolve
into separate species when their members are unable to interbreed and to
produce viable fertile offspring .
• The isolated populations accumulate differences in their gene pools and persue
independent evolutionary paths.
According to biological species concept if two or more species occur in the same
region and even if their areas of distribution overlap there is generally no
interbreeding (complete reproductive isolation) between them. As a result, the
intermediate or transitional forms between the species are usually lacking.
Significance of Biological Species Concept: Prior to biological species concept
there was lot of confusion in defining a species. A group of taxonomists would
regard every polymorphic variant of a population as a species , others would call
every geographically isolated population a species.
• Biological species concept helps in solving the riddle of sibling species that
exhibit morphological similarities but are reproductively isolated , e.g., two
species of Drosophila , D. pseudoobscura and D. persimilis.
• Biological species concept could assign species to individuals which have
marked morphological difference s but do not have reproductive isolation.
Drawbacks of Biological Species Concept: Although biological definition of
species comes close to the biological realit ies, it has some limitations.
• The concept can not be applied to species living either in geographically
isolated areas or at different times, because it does not tell whether populations
isolated from each other in space or in time could actually interbreed or not.
• The definition is not workable to the museum taxonomists and palaeontologists
who work with dead specimens and fossils.
• This definition is limited to sexually reproducing species. Asexual and
obligatory parthenogenic organisms can never be tested by this criterion.
A B

607. /
~ D- ~E~1J""" lJ A lJ lJ AD DDD
n lJD A~D) is lJss A ts DD: D n Dn A
U ', lJUlJU lJ ts Ii Ail
D DDD -.-;~ lJlJQ _~/ lJil lJ D --------~:1<::-------lJ
.------
Population
Area where ----•••. populations of
Popul~tion
of species A species A and B overlap
of species B
bu t do not interbreed
FIG. 11.1: Biological species concept that two species are reproductively
isolated and the naturally occurring
hybrids are absent even in that area where populati ons of two species overlap.
Biological and Morphological Species Concepts Contrasted The biological
species concept has an objective approach, since the behaviour of organisms
themselves is the determining factor to their relationship and distinctness.
The morphological species concept on the other hand is subjective , because it
depends upon the judgement of taxonomists as to what should be the degree of
morphological similarities worthy of species status. However, there is a high
degree of coincidence between the two definitions of the species, because
individuals that look alike tend to interbreed , and individuals that interbreed
tend to look alike, since these have large number of genes in common. But
sibling species are an exception to this.
Sibling Species
The sibling species are morphologically similar but reproductively i solated
species . Despite their morphological similarities, they are true species because
of reproductive isolation.
Sibling species are identified by differences in ecology, behaviour, mating call,
breeding and in structure or number of chromosomes.
4. Phylogenetic Species Concept According to phylogenetic species concept , 'a
species is the smallest monophyletic group on a phylogenetic tree.

608. On phylogenetic tree the branches represent populations that are changing and
evolving through time (Fig. 11.2) and the tips on the branches are monophyletic
species. Monophyletic Monophyletic
A monophyletic group is group
group
called a clade or lineage. It
consists of the ancestral pop
ulation and its descendants.
Advantages of Phylo
genetic Species Concept:
This concept has two distinct
advantages:

609. i
Each bran ch tip represents a phylogenetic species
• It can be applied to
fossil, asexual or sexual FIG. 11.2: Phylogenetic species on a phylogenetic tree.
populations equally. Group 8 with species 8, and 8
2•
group C·with C, C
2
•
The
concept
is quite
and group°having 0,. 0 2 and 0 3 species are three overlapping monophyletic
lineages.
logical because popu

611. lations are distinct as monophyletic groups and are free from gene flow and have
evolved independently.
• Phyflogenetic approach can help in recognition of many more species than
either by morphospecies or biological species concept.
Disadvantages of Phylogenetic Species Concept: The only disadvantage of this
concept is that only a few well-estimated phylogenies are available.
In actual practice , taxonomists and evolutionists use all the species concepts
summarsied above .
11.3 SPECIES CATEGORIES
Organism s of any species rarely exist naturally as a sing le large population. It
comprises of several popu lations called dem es. Interbreeding is very frequent
among the individuals of a local population or deme but is occasional among
different populations of a specie s. Interbreeding is absent among the individuals
of different species.
The accumulation of genetic hybridisation, genetic drift and in populations of a
species that are geographically iso lated. In the absence of geogra phical isol
ation , in terbreeding among the members of different populatio ns of the species
swamps away any differences that might have arisen in natural way.
As a result of genet ic divergence, isolated populations gradually differentiate
into several subcategories. These are:
differenc es caused by mutations, recombination, natural selection increase
genetic divergence
11.3.1 Demes
A d eme is a local population of a polytypic species, present in a given locality.
All the members of a deme share a single gene pool because of free
interbreeding. Demes are open genetic system s that have gene flow from
adjacent populations. The individuals of a deme have genetic similarities.
11.3.2 Geographical Races
Population s which are distributed over a wide geographica l range or have
occupied well separated geographic habitats for a long time, usuall y exhibit
distinct morphological or phenotypic differences. These differenc es are acquired
in adaptation to climatic factors. These phenotypic difference s are controlled

612. genetically. It mean s climatic factors produce change s in gene frequenc ies
within the gene pool of populations. Such popu lations of a species showing both
phenotypic and genotypic difference s in response to climate are called
geographical r aces. Therefore, races can be defined as 'geograph ical agg reg
ates of po pula tions that differ in genetic traits.' They are regarded as units of
organsiation below species level. Races represent a transient stage in the
transformation of populations into spec ies.
EXAMPLE 1: J. Van der Vecht observed three geographical variants or races of
carpent er bee (Mylocopa nobilis) on the mainland of Celebes and other three on
the adjacent small islands . These geographical variants from different islands
differ conspicuously in the colouration of their body hair and are easily
distinguished. These are called races.
EXAMPLE 2: The gypsy moth, Lyman/ria dispar, is distributed throughout Eastern
Asia and Japanese islands. Over this distribution range, a variety of climatic
conditions exist and ten geographical races of gypsy moth occur. These races
differ from each other in hatchin g timing of their eggs. The northern races hatch
later than the southern. The hatching time is controlled genetically because these
races when placed under identical environmental conditions still hatch at
different times.
Sangihe
Island

614. FIG. 11.3: Geographical isolates or races of carpenter bee (Mylocopa nobilis) on
Celeb and neighbouring islands, identified by colour patterns in their females.
11.3.3 Semispecies
S emispecies are two or more parapatric populations that are geneticall y
differentiated but are not fully isolated reproductively. Since they have not
achieved full reproductive isolation, they are called semispecies. Mayr (1963)
called a collect ion of semispecies as superspecies.
11.3.4 Ecological Races (Ecotypes)
T hese are populations of a polytypic species, which occur in different ecological
habitats but in adjacent geographic al areas. They possess phenotypic differences
determ ined genetically, and are considered to be equivalent to subspecies.
Fo r exam ple, a species of plant, Cilia achilleafolia, has two ecologi cal races
along the coast of California. One race ' the sun race' is found on the exposed
southern facing grassy slopes, whilst the other 's ha de race' occurs in the shaded
oak woodlands and redwood groves . The two races differ in the size of petals,
determined genetically.
11.3.5 Clines
These ar e the groups of local populations of a widely distributed species, which
exhibit regular and gradual stepwise phenotypic and genotypic differences
throughout the geographical range. The regular or continuous variations are
related with the ecogeographical va riations. More than one clines may be
exhibited in a species and they may run in the opposite direction .
The clines are 'populations that show more or less continuous variation in a
particular character corres-ponding with the gradual change in the environmental
terrain.'
A ll the above categories are based on different degrees of genetic dissimilarity
which may interfere with the breeding potential of members of these populations
if brought together.
11.4 ORIGIN OF SPECIES (SPECIATION)

615. Formatio n of new species from the parental species is called speciation or origin
of species.
Thoug h Charles Darwin in his book, T he Origin of Species, did not propose
complete mechanism of speciation, Ernst Mayr recognised that speciation
depends on two factors :
• Isolation of gene pool of population s
• Genetic divergence of their gene pool
11.4.1 Isolation of Gene Pools of Related Populations
(Reproductive Isolation)
Th e central event in speciation is the separation of gene pool of ancestral
species into two or more isolated gene pools. When isolated gene pools become
so much different that interbreeding and exchange of genes between them is
almost impossib le, the populations with these gene pools become independent
species establishi ng reproductive isolation. If there is free gene flow in the gene
pools of these populations, the two gene pools exchange genetic changes also
and the two gene pools tend to be similar.
11 .4.2 Genetic Divergence
The isolated gene pools need to accumulate large number of genetic differences
so that their gene pools do not permit free gene flow. If isolated populations are
reunited they either fail to interbreed or do not produce fertile offspring. Their
gene pools diverge due to different environmental pressures and natural selection
or due to genetic drifts.
11.5 MODES OF SPECIATIONS
Several p ossible mechanisms have been proposed to account for the occurrence
of speciation. Based on different modes, following types of speciation have been
recognised:

617. Possible Modes of Speciation [Based on Mayr (1963) and Endler (1977)] 1.
Phyletic Transformation: Speciation in Time
The gradual replacement of old species by new one without branching in due
course of time.
2. Fusion of Species: Speciation in Time
Formation of one species from two or more due to complete breakdown of
reproductive isolation.
3. True Speciation: Speciation in Space
Splitting of species because of reproductive isolation.
(a) Sudden or Instantaneous Speciation or Quantum Speciation or Sympatric
Speciation
Sudden change in the gene pool due to chromosomal mutations, polyploidy or
hybridisation or macromutation.
(b) Gradual Speciation (through populations)
• Allopatric or Geographical Speciation: New species arise through isolated
allopatric populations
(i) Vicariance
(ii) Peripatric speciation
• Parapatric Speciation: New species arise through populations that maintain
genetic contact wherever they are contiguous.
• Alloparapatric Speciation: New species arise through populations that are
allopatric first but become parapatric later, before complete reproductive
isolation is established.
11.5.1 Phyletic Transformation or Autogenous Speciation Phyletic speciation is
gradual transformation of one species A into another species B over a long
period by accumulation of slow changes in the gene pool. Neither
t
the re is splitting of the parental gene pool nor evo lution of reproductive
isolation. The gene pool of parental spec ies modifies du e to changes in gene
frequenc ies, chromoso ma l inversions, other structural changes in the
chromosomes. These changes accumulate in response to:
• adaptations to a shifting environment
FIG. 11.4: Phyletic speciation : transformation of a species in time leading to the

618. origin of new species from the pre-existing one.
• increasing specialisation for a particular environment
• improved adaptations in a constant enviro nment
In phyletic speciation, evo lving species present a line of success ion in which
one
species is replaced by other.

620. 11.5.2 Speciation through Fusion of Species
A n ew species may arise by the fusion of two already existing specie s through
the breakdown of their reproductive isolation . Fusion of gene pools is possib le
through hybridisation. The new specie s formed has combination of characters
from both parent species but is reproductively isolated from either of them.
Several new plant species have arisen in nature by hybridis ation .
11.5.3 True Speciation
Tru e speciation is the origin of two or more species from one parental species .
It requires splitting of parental gene pool or origin of two or more new gene
pools from the gene pool of sing le specie s and establishmen t of reproductive
isolation between them. The true speciation may be of two types:
• Sudden speciation or Quantum speciation
• Gradual speciation
1. Sudden Speciation or Quantum Speciation
In this type of speciation , chromo somal rearrangements produce reproductive
isolation and result in speciation. These evolutionary events occur sudden ly and
intermittently. Quantum speciation may occur due to chromosomal aberrations
(inversion and transloca tion ) or change in chromosome number (polyploidy,
autopolyploidy and
amphidiploidy). Quan tum Model
Michael White studied evolution in
~~ pa ren ta l
population flightless grasshoppers in Austral ia. In one
~
of its sma ll population, a tran slocation
occurr ed by chance . Trans location hetero
zygote s were found to have somewhat
reduc ed fi tness , perhaps because of
abnormal meiosis . But translocation
homoka ryotyp e (with two copies of o
translocation chromosomes or two normal
chromosomes) were able to produce normal o~
offspring. The translocation homokaryotypes
with a chromosome number less or more
than norma l were found to establish them

621. as distinct semispecies , because they
hado
Secondary
@t
popu
lation @:9
ogJo
o
~r
Reduced reproduct ive isolation in secondary population
establi shed reproductive isolation from the
parental population. This is also describe d
as statispatr ic speciati on or Sympatric
spe ciation.
New population with
altered gene pool
@f
Bo th autoploidy and allopolyploidy have FIG. 11.5: Quantum mode l of
speciation . played major role in the origin of new
species from the pre-existing ones specially in plants.

623. Accord ing to Simpson, the quantum evolution represents accelerated pace of
phyletic gradualism because ofaccumulation of gene tic changes in relatively
rapid succession. In quantum speciation biological barri er to gene exc ha nge
arise within an initiall y randomly mating population w ithout an y spatia l seg
regation of the incipient spe cies. It means quantum or sudden speciation occur s
due to disruptive selection.
2. Gradual Speciation
FIG . 11.6: Gradual speciation resulting in the formation of a number of species
when different allopatric populations become reproductively isolated.
Gradual spec iation is a microevolutionary event. The new species originate
from daughter populations of the same species by gradual accumulation of many
minute gene differences over a long period of time und er the influence of
natural selection. In this proc ess one of the species gives rise to one or more
new species. The gradual speciation may be of following type s:
• Allopatric speciation
• Alloparapatric speciation
• Parapatric speciation
• Sympatric speciation.
11.6 ALLOPATRIC SPECIATION 11.6.1 Definition
Allopatric speciation is the divergence of physically separate populations of a
species, occupying separate geographical areas into new species. This is also
called Speciation by geographic isolation.
11.6.2 Methods of Physical Isolation of Populations
Physical isolation of an original large population of a species may occur in three
ways :
• by physical splitting of a large habitat with a wide spread population by som e
new physical barri er into two or more large isolat ed area s (Vicariance)
• by Divergenc e of a few members of a wid ely distributed population to some
new habitat (Founder effect)
• by the extinction of intermediate link s in a chain of interconnected populations

624. Th e physical splitting of habitat is called vicariance. Speci ation that begin s
with physical isolation of populations either by dispersal or vicariance is called
allopatric speciation.
11.6.3 Allopatric Speciation by Vicariance
Volc ano es, earthquakes, storms, tid al waves, glaciers, flood s and formation or
destruction of mountains and forests, and appearance /disappearance of water
bodies act as vicariance agents for spec iation in terrestrial populations.

626. 2&& ~ Evolutionary Biology (A) Allopatric speciation by vi cari ance
(B) Peripatric speci ati on (Founder eff ect)
(e) Parapatric speci ati on (0) Sympatric spe ciat ion or statlspatrlc speci ation Parental
~poPulationo (-------:J~Barrier I A small group.;.::;;;==- -..,O/ 0T
diverges =::s;;:::
~
O
from
o
parental
population •

627. o
Q
• 0
o
ndependent()
o
O T~

628. em specres
Barrier removed
or new species
disperse over it,
and become
sympatric species
LJiI:J
Range expan sion and two species become Sympatric species
Range expansion leads to sympatry Sudden genetic
differences result in reproductive isolation and produce sympatric species
D ivergent selection. even at a narrow environmental discontinuity. may oppose gene flow and result in
reproductive isolation
FIG. 11.7: Successive stages in the allopatric, peripatric, allopatric and sympatric
speciations.
The geographicall y separated populations are exposed to variable forces of
natural selection, genetic drift , mutations, random mating that cau se their gene
pools to diverge. Their gene poo ls become so different that the free gene flow
between isolated populations is totally prevented and reproductive isolation is
established. Finally, these reproductively isolated populations become
independent species. In case, the geographic barrier is lifted, the memb ers of
these spec ies meet but do not interbreed.
11.6.3.1 Stages in Allopatric Speciation by Vicariance
Allomen ( 1992) divided the process of allopatric speciation into following three
stages:
• Fragmentation of a widespread popu lation due to geog raphical barriers and
separation of its gene pool.
• Persistence of isolation of populations and their gene pool s, so that their gene
pools become different from that of parental population , i.e.. genetic divergence.
• Establishment of reproductive isolation between new population s and
formation of new species, i.e., speciation.
11.6.3.2 Examples of Allopatric Speciation by Vicariance

629. EXAMPLE 1: Pupfish of Nevada, California: The blue-gray pupfish which
inhabits a warm spring at the base of mountains near Death Valley, Nevada, has
evo lved by allopatric spec iation. This spring got isolated from other water
bodies about 50,000 years ago. The fish trapped in the spring, became
geographically isolated from main fish population. In due course of time its gene
poo l became so different that it became a distinct speci es.

631. EXAMPLE 2: Speciation in Warblers: Mengel (1964) presented an exam ple of
allopatric or geographic speciation in northern wood warblers in North America.
These warblers are represented by four species, one eastern and three western.
The eastern species is Black-throated green warbler, Dendroica virens and the
western species are-I. Black throated gray warbler, Dendroica nigrescens, 2.
Hermit warbler, Dendroica accidentalis and 3. Townsend's warbler, Dendroica
townsendi. (Fig. 11.9)
Mengel explained the formation of abo ve four species of warblers due to four
glaciations which occurred one after the other in northern part of America. The
parental species had a south-eastern distribution and adapted to coniferous fore
st during Pleistocene Nebraskan glaciation. Th e occurrence of thi s glaciation
compressed the conifers toward s south. But during interglaci al period , the
coniferous forest and also the warblers expanded through North and West. The
next glaciation (i.e.. the Kansan glaciation ) reached southern extrem ity in
Central North Americ a and created a geographical barrier separating the eastern
parental species from the western population. The western warbler population
evolved into a new species during interglacial period. Th e sec o nd a n d third
warbler species in West also evolved in the same fashion due to Illinoian and
Wisconsin glaciation which occurred in the west.
O ver the past several million years , glaciers in the Northern continents
advanced and retreated repeatedly. During glacial advances , growing ice fields
fragmented the forests and grassland habitats into small regions. The populations
of species occupying these isolated region s also got genetically isolated and
evolved into new species. Such speciation has also occurred in flowering plants,
insects, fishes and turtles.
1 . Geographic Barrier: A continuous population of lizards being isolated into
two populations due to changed course of the river.
2. Genetic Divergence: On being separated by river, the gene pools of two
populations begin to diverge due to mutations, selection and genetic drift.
3 . Speciation: Reproductive isolation established and two populations became
two distinct species.
FIG. 11.8: Allopatric speciation by Vicariance.

633. Black-throated
Gray Warbler
Dendroica nigrescens
Wilson's Warbler
Wilson ia pusilla
FIG . 11.9: Allopatric speciation of black throated green warblers into four
species in North America due to glaciation that acted as geographic barrier.
Another glaciation event is likely to cut off another species in the West.
EXAMPLE 3: Speciation in flightless birds (Ratitae): About 150 mya the flightless
birds enjoyed a continuous distribution on the supercontinent of 'Gondwa na'.
Continental drift led to the separation of supercontinent into present day
continents and islands about 20 mya. The flightless bird's population s diverged
by vicariance events and evolved into present day genera and species .
A. Gondwan a was the original home of B. Gondwana began to break up into
Ratitae separate continents
(
Equator
150 mya
Gondwana was a
The ancestral ratitae lived
"supercontinent" made throughout Gondwanaup of many plates
C. Ratitae speciated as the continents moved apart .

634. -
)'.
,~
t'~
\~

635. ~
Z
ealand
20 mya~
FIG. 11.10: Origin of different genera and species of flightless birds by
vicariance.
o
Marchena Genovesa
o
Secondary contact is
established and two
species become sympatric A few ground finches from South America
~San t iago ~ Baltra
o
Wide Ocean acted as strong geographic
barrier
One or two founders
establish a
Ipopulation
Reproduct ive isolation from
founding population is acquired
durin eo ra hic isolation
Floreana Dispersal and colonisation of
new island Espanola
FIG . 11.11: Allopatric speciation of Darwin's finc hes in Galapagos islan ds by
founde r effect, based on Grant (1986) explan ation .
11 .6.4 Allopatric Speciation by Founder Effect
Grant (1986) exp lained speciation in isolated popu lations of Darwin's finches

636. by founder effect on Galapagos Islands. He presumed that only a very few
individuals colonise a new locality and multiply to form an isolated founder
population. For examp le, a South American ground finch, Geospiza
magnirostris from the mainland of South America colonised Galapagos Islands.
These islands were separated from the mainland by vast stretch of ocean which
acted as a geographical barrier. The populations on different islands evolved
independently of the parent population and each became reproductively isolated
to form a new species.
Since each island was initia lly colon ised by a few birds, the number of
individuals remained small for several generations. Genetic drift altered allele
freq uency independently in founder finch popu lations on each island. This
allopatric speciation is called 's peciation by founder effect'.
Speciation by founder effect is more frequent and more rapid because of
following reasons:
• Founder members or the colonisers are exposed to new nove l enviro nment.
• In a novel environment, mutations are more likely to be adaptive and accumu
late as a result of natura l selection.
• Advan tageo us changes in each gene pool get fixed more rapidly in small
populations by genetic drift.
11.7 PERIPATRIC SPECIATION OF MARG INAL POPULATIONS 11.7.1
Definition
is origin of new species from peripatric populations that bud off from the It
periphery of a parental population. Such populations are also called marginal
isolates.
M ayr (1982) described founder effect speciation as peripatric speciation. His
hypothe sis was based on the observations that in many birds and mammal s, the
isolated marginal populations on the periphery of widely distribut ed parent
population become so distinct from parental population that these populations
are classified as distinct species.
11.7.2 Examples of Peripatric Speciation
EXAMPLE: Peripatrie Speciation in Paradise Kingfishers: In New Guinea ,
paradise kingfisher, Tanysiptera galatea is widely distributed throughout low-

637. land area (marked I, 2, 3 in Fig 11.12). But it has differentiated into several
distinct species on small islands (mark ed 4, 5, 6, 7 and 8 in the fig. 11.12).

639. Solomon Is
land 5
o
FIG. 11.12: Peripatric speciation among paradise kingfishers in New Guinea.
Tanysiptera galatea is distributed throughout New Guinea lowlands (islands
1,2,3); Its two other species T. riedelii occurs on Biak Island (No. 6) and T.
carolinae on Numfor Island (No.7).
Speciation is speedy in peripatric populations because of genetic drift and
fixation of random alleles.
11.8 PARAPATRIC SPECIATION
11.8.1 Definition
Parapatric (para , near) speciation is the development of reproductive isolation
among the members of a continuous population or spatially distinct populations,
between which there is some gene flow in the absenc e of geographical barrier.
It means parapatric popul ation s occupy adjacent regions with different selective
pressures. They coexist only in one or more overlappin g region s at the
peripheri es of their geographical distributions and are not completely isolated.
11.8.2 Mechanism of Parapatric Spe ciation
Parapatric speciation occur s in neighbouring populations of a widely distributed
species. Though separated, they share a border zone. Most individua ls mate
within their own popu lation s, but a few go and mate with individuals present in
the border zone . Such hybrid s formed in the border zone are genetically
different from the two original populations and form the initial step toward
formation of a parapatric species.
Rain forest Border zone Grassland (Larger birds, (Smaller birds) longer bills, legs and wings) FIG.
11.13: Parapatric speciation in little greenbul.
Th e hybrid population present in the border zone possesses new combination of
characters. Natural selection acts on this gene pool and further isolates it from
the two original popu lations. Ernst Mayr called it semigeographic or pa rapat r
ic speciation. Such parapatric popu lations are separate d by distance but are not
completely isolated.

640. According to Russel Landle (1982) prezygotic isolation arises in populations
isolated by long distance due to divergent sexual selection.
11.8.3 Examples of Parapatric Speciation
EXAMPLE 1: Parapatric Speciation in Greenbut: The bird, little greenbu l
(Andropadus virens) lives in Tropical rain forests of Cameroon, West Africa . It
also lives in neighbouring grass land. The border area where forests and
grassland meet, is called ecotone. The birds living in these two habitats are quite
different in their beak, legs, body size and body weight.
Greenbu l from ecotone could still mate with both rain forest and grassland
counterparts. But change s introduced in the gene poo ls are more than the gene
flow between the two species . The divergence in their gene pools overshadows
the effect of gene flow between them and each population becomes a new
species .
EXAMPLE 2: Parapatric speciation is described in land snails of the genus Partula
on the island of Moorea near Tahiti. Eleven species of Partula are described
from Moorea, even though the island is only 15 kilometers wide and there are no
geographic barriers. These species fall into two groups. Snails belonging to
Partula suturalis complex, are both sinistral and dextra l individuals. The dextral
and sinistral forms occur in different areas , but in certain areas, these exhibit
restricted hybridisation, while in some other areas , the reproductive isolation is
in the process of being established. The areas of reproductive isolation are
represented by jagged lines and are not isolated by any geograp hic barrie rs.
These species have evolve d on this Moorea Island only and in the absence of
geographical pressures.
11.9 A LLOPARAPATRIC SPECIATION
Endler ( 1977) proposed the term alloparapatr ic spe cia tion for a modified
allopatric speciation. Initially, speciation begins in allopatric populations which
are geographically separated. In later stages these populations become
parapatric.
Areas of reproductive
Sinistral forms isolation being established

642. Moorea Island
FIG. 11.14 : Para patric species in Partula snails on Moo rea Island .
11 .10 SYMPATRIC SPECIATION
11.10.1 Definition
Sympatric (same country) speciation refe rs to the origin of new species due to
the appearance of some biological barrier in the individuals of an initially
randomly mating popu lation that live in the same geographic area .
11.10.2 Characteristics of Sympatric Speciation
• Sympatric specie s occur in the same geographic area.
• Populations are not separated by any physic al barrie r.
• There are no physical barriers to prevent free gene flow (as
10
allopatric
speciation) between members of the populations.
• Sympatric populations may be isolated by preferences for different habitats or
niches.
11.10.3 Mechanism of Sympatric Speciation
Sympatric speciation occurs by genetic divergence in the gene pools and reduced
gene flow between members of populations. In sympatric speciation, the gene
flow between the members of a single population in a given area may get
reduced by:
• Ecological isolation or habitat differentiation and disrupti ve selection.
• Polyploidisation or change in chromosome number.
• Chromosomal aberration or change in chromosome structure
•
Introgressive hybridisation.
'M·'I=-'''J Mechanisms of Sympatric Speciation
Process
1 . Ecological isolation
and disruptive selec
tion
Explanation

643. Natural selection disrupts population's gene pool for adaptation to different
habitats or to resources.
2 . Polyploidisation (i) Autopolyploidy (ii) Allopolyploidy
3 . Aneuploidy
(Chromosomal aberration)
Change in chromosome number establishes genetic or reproductive isolation
- Polyploids with duplicate
chromosomes from the
same species

.
- Polyploids with duplicate sets of chromosomes from different
species.
Change in chromosome number by translocation and inversion
Examples
( i) Speciation in cichlid fishes of Large Lake of cameroon.
(ii) Speciation in Fruitfly Rhagoletis pomonella . Particularly common in plants
Maiden hair fern individuals
Speciation in wheat
Origin of species of Drosophila
1. Sympatric Speciation by Ecological Isolation and Disruptive Selection A large
geographical area is never uniform. It is divided into habitats or niches or the
microenvironments. The sympatric popu lations , though not separated
physically, may be isolated by preferences for different habitats or for different
resources, because of genetic factors. Gene pools once separated, continue to
diverge by accumulation of different gene mutations, or by natural selection that
favours different gene combinations in different habitats .
A few examples of speciation by ecological isolation of sympatric populations
are: EXAMPLE I: Speciation in CichIid fishes in Cameroon: In large lake of
Cameroon different species of Cichlid fishes have evolved based on their feeding
habits:
• bottom feeder cichlids

644. • surface feeder cichlids
• cichlids living near roots of aquatic plants.
B ecause of living in different habitats, memb ers of different cichlid popul
ations do not come in contact (ecologi cal isolation ). Due to absence of mating
and gen e exchange, these populations become reproductively isolated into
separate species.
EXAMPLE 2: Sympatric Speciation in Fruitfly Rhagoletis: Fruitfly Rhagoletis
pomonella is a parasite on American Hawthorn s (Cratageus) and lays eggs on
its fruits. It was also found to infest apple trees introduced in America.
The two ecologically iso lated populations of fruitfly living and laying eggs on
Hawthorn 's fruits and on apples were found to have substantial genetic
differences . The reproductive isolatio n between them is maintained because:
• To lay their eggs, the flies prefer fruits of the same type in which they are
developed.
• Time of maturation of apples and Hawthorn fruits is different. Therefore, the
time of sexual matu rity of these flies differs. This prevents interbreeding
between them.
Tetraploid parent

645. ~
,
p.~ ~
/ I Meiosis\~/ I Meiosis\-.

646. ~~~~ Y~"".\~)(§)(~~
, Diploid;ametes I ' Haploid~am etes '
(two copies of (one copy of each each chromosome) ~ chromosome)
§\§ ~
F, offspnng is c:::;:,c::>
~
triploid (three copies ~
of each chromosome) ~
/ I Meiosis\
~
~(S;;;l~\~ ~O~\&
~-----v-
y
~I
M ost gametes produced by the triploid hybrid
are not viable because
they have an incorrect
number of chromosomes
FIG. 11.15: Tetraploids are reproductively isolated from diploids. because their
triploid offspring are sterile as they produce gametes with inviable number of
chromosomes.
Allopatric, parapatric, and sympatric speciation form a continuum, differing in
their initial level of gene flow (m) between diverging populations:
• In allopatric speciation, the initial level of gene flow is minimal or absent i.e.,
m =O.
• In sympatric speciation, the initial level of gene flow between the members of
two populations is maximum l.e., m =0.5.
• Intermediate cases of gene flow Le., m => 0 and < 0.5 represent parapatric
speciation.
Peripatric speciation is regarded as a hypothetical form of speciation. It is also
called transilience or speciation by peak shift. It requires both genetic drift and
natural selection.
Prolonged ecological isolation establishes full-fledged reproductive isolation.

647. 2. Sympatric Speciation by Change in Chromosome Number (Polyploidisation)
New species can arise in a sympatric popu lation nearly instantaneously throug h
change in chromosome number. This is called polyploidisation. This may occur
by
polyploidy, aneuploidy and by translocation.
Polyploidy arises due to doubling of chromosomes when an error occurs during
first phase of meios is (nondisjunction of chromosomes). The gametes formed
are
diploid (2n) . These gametes, on uniting with normal haploid gametes (n)
produce
triploid zygote (3n). These triploid organisms do not produce functional gametes
becau se of irregular pairing and segregation during meiosis. As a result, the
tetraploid
and diploid populations become reproductively isolated and a tetraploid forms a
new species.
New species may arise either by autopolyploidy (the chromosome duplication
in a single species) or by allopolyploidy (the chromosome duplication of hybrid
between two different species). Examples of origin of species by these methods
are quite frequent in plants but a few in animals. Many of our valuable cultivated
crop plants , such as Wheat, Oats , Cotton, Tobacco and Sugarcane have evolved
by
sympatric speciation. This type of origin of species is also called ca ta clys mic
or
explosive type of evolution.
EXAMPLE 1: Origin of Wheat: Three spe cies of whea t ha ve arise n by
allopolyploidy and autoallopol yploidy:
(a) diploid einkorn wheat having 7 pairs (14) of chromosomes. (b) tetraploid
emmer wheat (hard wheat) having 14 pairs (28) of chromosomes,
and
(c) hexaplo id vulgare wheat (soft wheat) whic h has (42) 2 1 pairs of
chromosomes.
EXAMPLE 2: Raphanobrassica: Hybr idisation followed by the dou bling of
chromosomes (i.e., by allopolyploidy) resulting in the origi n of new species has
been prove d by experiments . The famous experimental crosses were made by
Karpechenko between radish and cabbage . Both of them possess 18
chromosomes

648. but belong to diffe rent genera. aturally their chromosomes are unlike and are
unable to pair at meiosis. Per chance a few male and female gametes were
formed containing 18 chromosomes so that they gave rise to plants with 36
chromosomes. These new tetraploid plants were fertile, because both radish (9R)
and cabbage chromosomes (9C) were represented twice in these plants (18R and
18C). They could pair easily. These tetraploid hybrid plants were a mixture of
characters from radish and cabbage. But they were unable to interbreed with
either of the parent plants and formed a new species .
EXA
IPlE 3: Drosophila: Different species of Drosophila have arisen due to
aneuploidy. Translocation and inversions introduced changes in the number and
appearance of chromosomes of ancestral species Drosophila virilis which has 6
pairs of chromosomes.
(a) Drosophila melanogaster and D. americana have 4 pairs of chromosomes.
(b) D. virilis has 6 pairs of chromosomes.
(c) D. pseudoobscura and D. persimilis possess 5 pairs of chromosomes. (d) D.
willistoni possesses 3 pairs of chromosomes.
The chromosomal co mposition ofD. viri/is is regarded to be of ancestral type
which possesses five pairs of rod-shapd and one pair of dot like chromosomes.
The chromosomal comp lement of D. pseudoobscura and D. persimilis could be
derived from the complement of D. virilis by a trans location between X-
chromosome and one of the autosomes so that these possess a pair of V-shaped,
three pairs of rod-shaped and a pair of dot-like chromosomes. The chromosome
complement ofD. melanogaster could be derived by two trans locations between
two pairs of autosomes resulting in the formation of two pairs of V-shaped, a
pair of rod-shaped and a pair of dot-like chromosomes. (Refer fig. 6.22)
3. Speciation by Hybridisatio n or Recombina tional Speciation
H ybridisation with polyploidy produces new specie s but hybridisation may also
give rise to hybrid species with diploid number of chromo somes. Certain hybrid
genotypes of F
1
recombinant offspring (F
1
hybrids) are found to be fertile and
better adapted in certain features than either of parent species. These F
I
hybrids
are reproductively isolated from parent species but produce fertile offspring
when interbreed among themselves. These genotypes increase and form a
distinct species. This is called reco mbi national speciation.

649. Speciation by hybridisation ha s been described in animals by Mallet (2007) and
in plants , sunflower by Rieseberg ( 1997). Three species of sunflower native to
West America are Helianthus annuus, H. p etio /aris and H. anomalus. Loren
Rieseberg and coworkers found that hybridi sation between H. annuus and H. p
etio/aris has given rise to H. anomalus and two other species, H. paradoxus and
H. desertico/a.
The recombinant species grow in differen t habitats than either of the parent ,
flowers later, and have unique morphological and chemical features. Thus
hybridisation can contribute to the origin of new species with novel
morphological and ecological features by generating novel gene comb inations.
11.11 CONSEQUENCES OF SPECIATION
Speciation is essential for maintaining diversity in the living world and for the
evolution to continue. For sexually reproducing organisms, every branch in the
phylogenetic tree of life, represents a speciation event. As a result of speciation,
populations become reproductively isolated and capable of pursuing independent
evolutionary path for higher taxa like genera, families, etc. Eldredge and Gould
(1972 and 1993) and Stanley (1979) proposed that speciation is necessary for
morphological divergence and evolution to occur. Mayr (1954) suggested that
founder events trigger rapid shifts in genetic equilibrium and that most
evolutionary changes are triggered by peripatric speciation.
Futuyma (1987) and Eldredge et. at (2005) emphasised that speciation because
of reproductive isolation enables morphological differences between populations
to persist. The gene pools of local populations diverge rapidly as a result of
genetic drift and natural selection. Sooner or later when they come into contact
because of breakdown of geographic or ecological barriers they have already
developed reproductive isolation due to accumulation of genetic differences.
Reproductive isolation is essential, because due to interbreeding much of the
genetic divergence accumulated may be lost, unless reproductive isolation is
established.
11.12 RATE OF SPECIATION
Rate of speciation differs in different lineages . For example, hundreds of species
have evolved in Drosophila. whereas only one species of horseshoe crab exists
for more than 300 million years. Speciation rate is expressed in terms of time

650. for speciation and as biological speciation interval.
1. Time For Speciation (TFS) is the time required for the establishment of
complete reproductive isolation once the process of divergence of gene pools of
populations started.
2. Biological Speciation Interval (BSI) is the time period between the two
consecutive speciations or between two sequential forks in the phylogenetic tree.
It includes time for speciation and also the time before the process of speciation
begins again .
• In case of speciation by polyploidy (i.e., sympatric speciation), the waiting
time for the speciation to occur is very long. But once started , the reproductive
isolation is ~c h i eved very fast.
• Sympatric speciation is faster if it is driven by ecological or sexual isolation .
• Allopatric speciation could be slow or rapid , depending on the strength of
pre-isolating mechanisms and occurrence of genetic variations.
11.13 FACTORS RESPONSIBLE FOR VARIATION IN SPECIATION
RATES
A number of factors influence speciation rate. These are:

652. 1. Environmental Changes: Reg ular fluctuating changes in climate cause
fragmentation of populations of a species that live in a formerly continuous
habitat. These changes introduce geographical or ecological isolation of the
populations.
2 . Ecological Specialisation: The divergence in the gene pools of popu lations of
a species increases rate of speci ation if their habitat is divided. The speciation
will be slow in populations occupying relati vely continuous habitats.
3. Species Richness: The larger is the number of species in a lineage , the more
rapid is the speciation.
• In Sym pat r ic Speciation by Polyploidy: Presence of large number of species
provides more chances of hybridi sation between them.
• In Allopat r ic Speciation: Presence of large number of related species in a
given area or habitat will include more species with bisected range of
distribution and therefore, more chances of allopatric speciation.
4. Dispersal Ability: Dispersal helps animals to venture new areas and evolve
into new species due to founder effect. Poor dispersal abilities are unlik ely to
establish new populations because the individuals fail to cross even narrow
barriers.
5. Population Bottlenecks: The specia tion rate is more in populations whic h
often pass through a bottleneck because their gene poo ls show more dive
rgence.
6. Sexual Selection: Animals with comp lex sexual behaviour exhibit higher rate
of speciation, because they show great preference for selection of mate.
11.14 THEORIES OF SPEC IATION
1. Classical Theory of Gradualism
According to t his theory, speciation is a microevolutionary event resulting from
the gradual accumulation of numerous minute gene differences over a long
period of time under the influence of natural selection. This is most commo n
and widely accepted theory.
2. Stochastic or Catastrophic Mode of Speciation
Th is theory emphasizes the role of chromosoma l aberrations (rearrangement)
producing reproductive isolation and conseq uently resulting in speciation. This
theory was proposed by Mayr.

653. 3. Founder Flush Speciation Theory
H ampton Carson believed that cycles of disorganisation and reorganisation of
ge- nomes are the cause of speciat ion. According to the founder flush theory, a
sing le fertilised female can act as a founder and colonise an isolated territory
which was not occupied by its members previously. If conditions are favourab le,
the population founded by this single individual will undergo a flush (the rapid
expansion).

655. Afte r several generations, when population growth outstrips the environment's
capacity, it causes population crash. This may result either in large scale random
deaths or emigra tion and dispersal of individual s of the population. A few
survivors rebuild the population . The cyclic events of population flush and crash
may be repeated severa l times. During this period , gene pool of the popul ation
acquires changes in adaptation to the environment and becomes much different
from the gene pool of the parental population making it reproductively isolated.
According to Carson's theory, the genet ic changes are brought about in two
ways :
I . The original foundin g of the population by a very few individuals which
establish allele frequencies different from those in the ancestral population.
2. Selection is reflexed during flush period. If descendants of the founder can
invade a new niche, they expand in a flush.
According to Carson, some blocks of genes on chromosomes remain lightly
linked while others do not undergo recombination becau se of some selective
advantage. New genotypes produ ced within these closed areas have a reduced
fitness. During flush period, genotypes produc ed by recombination in closed
regions of genome may survive because of no stress of selection. But after a
crash, the survivor's reshuffled genome is exposed to selection to produce new
combinations of open and closed gene groups adapted to the environm ent.
EXAMPLE: The evolution ofDrosophila species in the Hawaiian Islands appears to
have evolved by founder flush speciation. The relationship between these
species can be traced by mapping the location and frequency of inversions in the
banded polytene chromosomes of larval salivary glands.
A g roup of Drosophila, the planitibia complex, has three species: D. planitibia,
D. heteroneura and D. silverstris . These have the same basic set of chromo
some inversions. D. planitibia is found on the old Northwest Island of Maui. The
other two species, D. heteroneura and D. silves tris occur on Hawaii Islands. It is
presumed that a fertili sed female of an ancestral stock of D. planitibia on Maui
migrated to Hawaii Islands by crossing the Alenuihana Chann el. Subsequent
flush-crash cycles led to the evolution of the present day Drosophila species on
Hawaii Islands.
4. Statispatric Speciation

656. Mi chael White described speciation in the flightle ss grasshoppers in Australia
by chrom osomal aberration such as translocation . This type of quantum
speciation was called statispatric speciation. This model has also been applied to
the developmen t of closely related species of mole rats differing in chromosome
number.
A ccording to this model of speciation, a translocation arose by chance in a small
population. The heterozygotes for translocation were less fit than the homok
aryotyp es because of abnormal behaviour of chromosomes during meio sis.
May be that homokaryotypes for translocation are more fit. They spread by
partiall y replacing the ancestral population. This grows into a semispecies and
then to species .

658. • Allopatric speciation
• Cladistic species concept
• Founder flush speciation
• Gradual speciation
• Parapatric speciat ion
• Geographical races
• Vicariance
KEY TERMS
• Alloparapa tric speciation •
• Clines
• Geograph ic isolation •
• Peripat ric speciation •
• Populat ion crash •
• Phenetic species concept •
• Stochastic mode of speciation Biological species concept
Demes
Quantum speciation Sympatric speciation Statispatric speciation
REVIEW QUESTIONS
I . Discuss the merits and demerit s of morphological, biological and
phylogenetic species concepts .
2. (a) Compare and contrast morphological and biologica l species concept. (b)
'Occurrence of sibling species supports biological species concept. ' Justify. (c)
Mention the limitations of biologic al species concept.
3. Define "species" in genetic terms. Describe the mechanism of speciation using
the following terms: Mendelian population , gene pool, genetic equilibrium,
sexual recombination, mutation , inheritabl e variation s, natural selection
(differential reproduction) and reproductive isolation.
4. Write an essay on the origin of species.
5. Summarise the various factors involved in speciation.
6. Explain the factors responsible for geographic speciation .
7. Discuss briefly the problems related to speciation.
8. Write short notes on:
(a) Peripatric speciation

659. (b) Vicariance and allopatric speciation
(c) Founder's effect
(d) Stochastic model of speciation
(e) Ecotypes
9. How populations can become geneticall y isolated from each other if they
occupy different geographi c area?
10. Explain why allopatric speciation is also called geograp hic speciation ? II .
Give various theories to explain mechanism of speciation.
12. With suitable examp les explain founder flush theory.
13. Differentiate between gradual speciation and quantum speciation.
14. Describe agents of allopatric speciation.
15. (a) Differentiate between allopatric, peripatric, parapatric and sympatric
speciation . (b) Which of these speciation s will be fastest and why?
16. Discuss speciation in warblers in North America . What type of speciation
has occurred in them and what factors are responsible for their speciatio n?
17. (a) What do you mean by a founder flush and popu lation crash theory of
speciation? (b) Who proposed this theory?

661. 18 . Summarise differences between allopatric and sympatric speciation. Which
type of speciation is common, and why?
19. (a) Describe two mechanisms that can decrease gene flow in sympatric
populations. (b) Describe how an error during meiosis could lead to polyploidy.
FURTHER READINGS
1. Abbott, R.1., 1992. Plant Invasion, Interspecific Hybridisation and the
Evolution of New Plant Taxa, Trends Ecol. Evol, 7: 401-405.
2. Agarwal, A.A., 2005 . Natural Selection on Common Milkweed (Ascl epias
syriaca) by a Community of Specialised Insect Herbivores., Evol. Ecol. Res., 7:
651-667.
3. Barraclough, T.G., and A.P. Vogler, 2000. Detecting Geographical Pattern of
Speciation from Species-level Phylogenies. Am . Nat ., 155: 419-434.
4. Bartolome, c., and B. Charlesworth, 2006 . Rate s and Patterns of
Chromosomal Evolution in Drosophila ps eudoobscura and D. miranda.
Genetics, 173: 779-791.
5. Coyne , lA. and H.A. Orr., 2004. Speciation, Sinauer, Sunderland, MA.
6. Dean, M.M. Carrington, and S.1. O'Brien, 2002 , Balanced Polymorphism
Selected by Genetic versus Infectious Human Disease , Annu. Rev. Genomics
Hum. Genet., 3: 263-292.
7. Endler, J.A., 1977. Geographic Variation, Speciation, and Clines, Princeton
Univers ity Press, Princeton, Nl .
8. Feder, J.L., and 8 others ., 2005. Mayr, Dobzhansky and Bush and
Complexities of Sympatric Speciation in Rhagoletis, Proc. Nat. Acad. Sci, USA
102 (Supp!.) 6573-6580.
9. Futuyma, 0.1., 2008. Sympatric Speciation: Norm or Exception? pp. 1136-
1148. In K.1. Tilmon (ed.), Specialization, Speciation, and Radiation:
Evolutionary Biolo gy of Herbivorous Insects, University of California Press,
Berkeley.
10. Gavrilets, S. and Hastings, 1996. Founder Effect Speciation: A Theoretical
Reassessment. Am . Nat., 147: 466-491.
II . Gould , S.1., 2007 . Punctuated Equilibrium, Belknap Press of Harvard
University Press, Cambridge, MA.
12. Grant , P.R., B.R. Grant, 2008, Ho w and Why Species Multiple: The
Radiation Darwin :s or Fin ches. Princeton University Press, Princeton.
13. Gray, D.A., and W.H. Code , 2000 . Sexual Selection and Speciation in Field
Cricket s, Proc. Natl. Acad. Sci. , USA, 97: 14449-14454.

662. 14. Harrison, R.G.(ed.) ., 1993. Hybrid Zon es and the Evolutionary Process,
Oxford University Press, New York.
15. Liou, LiW, and T.D. Price, 1994. Speciation by Reinforcement of Premating
Isolation. Evolution, 48: 1451-1459.
16. Lynch, M., 2007 . The Orig in of Genom e Archit ecture, Sinaur, Sunderland,
MA. 17. Lynch, M., and J.S. Conery, 2000. The Evolutionary Fate and
Consequences of Duplicate Genes. Science, 290: 1151-1155
18. Mayr, E., 1982b. Processes of Speciation in Animals. In C. Barigozzi (ed.),
Mechanisms of Speciation, pp. 1-19. Alan R. Liss, New York.

664. 282 ~ Evolutionary Biology
19. Michel , A.P., J. Rull , M. Aluj a and J.L. Hawthorn-infesting Rhagoletis
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Feder, 2007. The Genetic Structure of Popul ations in Mexico: Implications for
S ympatri c Speciation . Mol. Eco!.. 16: 2867-2878.
20. Morjan, c. L., and L.H. Rieseberg, 2004. How Species Evolve Collectively:
Impli
cations of Gene Flow and Selec tion for the Spread of Advantageous Alleles.
Mo!.
Ecol.. 13: 1341- 1356.
2 1. Nei, M., T. Maruyama, and R. Chakraborty, 1975. The Bottleneck Effect
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Variability in Populations. Evolution, 29: 1-10.
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(cd.),
Species and Speciation. 2 1-29. Transversal Museum Mono graph No.4, Pretoria,
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Evol.. 16: 351-3 58.
25. Rundle , H.D., and P. Nosil, 2005. Ecological Speciation , Ecol. Lett.. 8: 336-
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27. Servedio , M.R.M.A.F. Noor., 2003. The Role of Reinforcement in
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the Predominant Biogeographical Force in Madagascar? Only time will tell.

665. Annll. Rev. Eco!.
Evo!. Syst.. 37: 405-431.
DOD

667. 12
Genetic Drift and Gene Flow
12.1 RANDOM GENETIC DRIFT OR SEWALL WRIGHT EFFECT
According to Hardy Weinberg principle, in infini te ly large , randomly
mating, panmictic populations, allele frequencies either remain con stant in
absence of mutations and selection or exhibit changes under the influence of
natural selection. But all natural pop ulations are finite in size and most of them
are quite sma ll. In small populations, the random loss of individuals and alleles
present in them produces large fluctuating changes in the allele frequencies from
one generation to the next. In one generation, the frequency of a particular alle le
may increase far beyond the normal range and may become extremely low in the
next generation just by chance. Some alleles may even get lost by sheer chance.
The se fluctuations in allele frequencies in small popu lations are non-
directional, non-adaptive and unpredictable and are called genetic drift.
Definition
Genetic drift can be defined as ' the nondirectional, random fluctuations in the
allele frequencies in the gene pool of small populations, occurring by chance and
not in adaptive direction.'
12.2 THEORY OF GENETIC DRIFT
Theory of genetic drift was developed by American geneticrst Sewall Wright in
1930s and by the Japanese geneticist Motoo Kimura ( 1950s) , to treat the effects
of sampling error in small populations. It is also called Sewall Wright Effect or
scattering of variability. It refers to the 'random fluctuations ' in allel e
frequencies in a small population from one generation to the next.
Because of genetic drift , the freq uency of harmful alleles may go up and rare
advantageous alleles may get lost mere ly by chance. The allele frequency
changes
284 Iil Evolutionary Biology

668. by geneti c drift are unpredictable. These can be compared to picking up
jellybeans from a bag and by chance , grabbing only green and yellows at one
time, orange and blue second time and orange , green and yellow the third time.
It means genetic drift cause allele frequency changes due to sampling variation.
Imagi ne a hypothetical population of 10,000 organisms. An allele in this gene
pool is represented in l out of 10 organisms. It means this allele is present in
1,000 organism s and has allele frequency 10 percent. Imagine that a disease
killed halfof the popul ation. The number of surviving organisms in the
population will be 5,000 and I in 10, i.e., about 500 of them could be expected to
have this allele. Say, by chance 550 organisms with this allele were killed. The
surviving population would have 450 organisms with this allele. The frequency
of this allele in the surviving population drops down from 10 to 9 percent.
In a sm all population of 10 organisms, this allele will be present in one
organism only. Though the frequency of the allele in the population is 10 percent
but it is present only in single organism. Per chance , if this organism dies
without producing offspring, the allele frequency drops from 10 to zero. This is
an example of genetic drift because:
• The allele has been eliminated by chance alone.
• The adaptive value of allele has not influenced the selection or elimination.
12.3 SALIENT FEATURES OF GENETIC DRIFT
12.3.1 Genetic Drift as Sampling Error
Genetic drift result in the fluctuations of allele frequencies due to variable
sampling from the gene pool in each generation and is just a chance event.
Sampli ng error occurs during fertilisation, where fusion of male and female
gametes is a chance phenomenon. It introduces changes in allele frequencies in
the gene pools of populations. The sampling error may occur, because:
• One or a few copies of an allele may not be present in the gametes that fuse to
form zygotes .
• Organisms that reach reproductive age may lack that allele.
• Organisms with that particular allele may fail to reach the reproductive age.
In all these situations, allele is not contributed to the gene pool of the population
and its frequency reduces or becomes zero.
Sampling error does occur in all populations, small or large. It is particularly

669. important in small populations where sampl ing error tends to be high in
percentage.
12.3.2 Random Fluctuations in Allele Frequencies
Genetic d rift cause allele frequencies to change at random over the time.
Because of genetic drift, frequency of an allele may increase one year but may
decrease in the subsequent year. This change from year to year is not predictable.
Therefore,
Genetic Drift and Gene Flow Iil 285 genetic drift is nondirectional, nonadaptive
and unpredictable, unlike natural selection that tends to preserve genes of
adaptive value.
12.3.3 Genetic Drift Tend to Reduce Genetic Variability
In small populations, due to genetic drift the gene frequencies continue to
fluctuate until one of the allele is lost and the other is fixed. This leads to
homozygosity in small populations. The heterozygous gene pairs tend to become
homozygous for the allele either dominant or recessive.
It means genetic drift reduces genetic variability in small populations by
eliminating one of the two alleles either new or the old one irrespective of its
utility. This is also called decay of variability. For example, if allele AI is
continuously eliminated over generations its frequency will come down to zero
(pAl == 0) and allele will be lost. If this allele AI continues to be lucky for
several generations, its frequency might drift to one (pA
I
== I). In either case ,
whether an allele is lost or fixed, the population moves towards homozygosity
and genetic variability in small populations tends to decrease.
12.3.4 Genetic Drift and Non-adaptive Traits
Genetic drift tend to preserve or eliminate alleles without distinction.
• Alleles that are neither harmful nor beneficial may be lost or fixed in the gene
pool of a small population entirely by chance.
• Genetic drift may fix slightly harmful alleles in small populations.
• It can also cause certain beneficial alleles to get lost.
Thus small populations can retain nonadaptive or neutral traits.
12.3.5 Genetic Drift and Divergence Between Populations
A large population (metapopulation) frequently comprises many independent

670. small subpopulations, called demes . Initially, all the demes have a common gene
pool and capable of interbreeding. But being isolated into separate groups, the
members of different demes do not exchange genes and their gene pools get
isolated. Merely by chance, the frequency of certain alleles may increase in some
demes and may decrease in some others . Consequently, different alleles are
fixed or lost in different demes of a population and their gene pools evolve
different allele frequencies.
In some cases isolated populations or demes come to possess some unusual
characteristics not present in the large parental population.
12.3.6 Genetic Drift and Fixation of New Mutations
Genetic drift tend to eliminate one allele and fix the other one, irrespective of its
dominance or recessiveness or advantageous or nonadvantageous nature . It
means in small populations, a new mutation has 50 percent chances of either
being lost or be fixed.
12.4 GENETIC BASIS OF RANDOM GENETIC DRIFT
In demes of limited size, random genetic drift arise by chance deviations, which
cannot arise in a large population. For example, if we contrast two populations of
two extreme sizes, say population A consisting of 5,000 breeding individuals and
population B only 50, the gene pool of each population contains equal number of
L and I. Their gene frequencies are represented by p and q:
pL = ql = 0.5
The extent of deviation of gene frequency from the original 0 .5 in the next
generation can be measured mathematically in terms of standard deviation.
For diploid parents standard deviation is determined as the square root of the
product of original gene frequencies (p x q) divided by the number of genes
available. The number of available genes will be double the number of breeding
individuals and N is number of individuals in a population.
Standard deviation c = ) P2:q
1. In large population: In a large popula tion with 5000 individuals, the standard
deviation (c) would be as follows :
0.5 x 0.5
Standard Deviation (c) =
10,000
../0.000025 = 0.005

671. Therefore, in a large population , the gene frequency ofL and I genes will
fluctuate around 0.5 ± 0.005 i.e., between 0.505 and 0.495. It means when
population size is very large, there is very little drift in gene frequency over a
long period of time and most frequencies cluster around the initial value of 0.5.
2. In small population: In population B with 50 individuals, the standard error
would be = 0.05 and in the next generation the gene frequencies will change to
0.45 and 0.55 either way. This amounts to 10% change in the gene frequency. It
means the standard error in a large population is negligible (.005) whereas in a
small population it is significantly high (.05).
Fixation of an Allele in
Small Populations
Repetition of similar standard deviations in succeeding generations in large
<I>
.-N =1,000
c:
<I>
Cl
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u-..;::;
c: '"
~"3
~"3
~a.
-c:
<1> '
~o N=500
a;
0:
F ixation Elimination
FIG. 12.1: Distribution of gene frequency in small and large populations when
selection is zero. In large populations (N = 1,00,000 and N = 10,000), the gene
frequency is maintained at 0.5. So, it is impossible for genes to become fixed or
eliminated. In a small population (N = 1,000 or 500), the allele either undergoes
elimination (q = 0) due to genetic drift or it is fixed (q = 1) (After Wright ,
1931).
populations results in an insignificant change. But in small populations repetition
of standard deviations gradually alters the genotype to the extent that one of the
genes is completely eliminated from the gene pool. In the absence of
spontaneous mutation, when one allele (either normal or mutant) is eliminated

672. from the gene pool and the other allele is fixed, the population and the gene is
said to be fixed because all the individuals will now be homozygous.
A large population is actually divided into a large number of demes of small size,
all with initial gene frequencies p and q. The percentage of these lines in which
an allele is either fixed or lost is directly proportional to the starting allelic
frequency. For example:
1. If p = q = 0.5 in all demes, it is expected that 50 percent of the lines will fix
one allele, and 50 percent will lose the same allele or fix the alternative allele.
2. If p = 0.7 and q = 0.3, it is expected that 70 percent of the lines will fix one
allele AI and 30 percent the allele A2•
12.5 GENETIC DRIFTS IN REAL POPULATIONS
Experiments conducted in the laboratory support the concept that generic drift
changes gene frequency and causes fixation or elimination of alleles .
12.5.1 Warmick Kerr and Sewall Wright's Experiment on Drosophila
Wannick Kerr and Sewall Wright worked with fruitfly, Drosophila melanogaster,
having straight/bent or forked bristles on the body. The difference in bristle
phenotype is controlled by a single gene pair.
They took 96 cages and placed four adult females and four adult male flies in
each. The frequency of two alleles in the experimental population was 0.5 each.
The two alleles do not affect fitness of flies. The flies bred in each cage
independently.

674. Four males and four females were randomly selected from each of the 96
offspring populations and were allowed to produce next generation.
Thi s procedure was repeated until all the 96 populations had passed through 16
generations.
After 16th generation, the 96 populations fell into three categories as follows:
(i) All Drosophila had only forked bristles in 29 populations. (ii) All Drosophila
had only straight bristles in 41 populations.
(iii) Both alleles for forked and straight bristles were present in flies in 26
populations.
Warwick Kerr and Sewall Wright concluded that genetic drift has fixed one
allele or the other in total (29 + 41) = 70 populations of Drosophila out of 96
populations. It means genetic drift decreases genetic variation within the
populations but increase genetic divergence or differences between the
populations but not in a particular direction.
12.5.2 Lamotte's Studies on Snails
L amotte studied the frequ ency of banded and band less sna ils of Franc e in 826
population s. He grouped the colonies according to their size, small one s
containing 500 to 1,000 snails, intermediate 1,000 to 3,000 and large population
with more than 3,000 snails. Gene-B produces band less condition and its rece
ssiv e allele-b cau ses banded character. The frequency of gene-B wa s found to
be :
(a) In large populations the frequency of gene- B ranges between 1.0% and 30%.
(b) Frequenc y of the same gene in sma ll po pulations ranges between 0% to
100% ind icatin g that in some popul ation s one all ele and in others its alterna
tive allel e has reached fixation .
12.5.3 Moody's Experiment
A laboratory experiment wa s de vised by Moody (1949) to explain the operation
of gen etic drift . Let us assume a small population formed of 12 individuals with
genotypes 3 MM : 6 Mm : 3 mm . Let each indi vidual be represented by a pair
of beads tied together as follows:

675. T wo red beads MM
Two blu e beads mm
On e red and one blue Mm
To represent the small population of 12individuals, 3 pairs of red bead s (3 MM),
3 p airs of blue beads (3 mm) and 6 pairs of red and blue beads (6 Mm) are
placed in a box . These are thoroughly mixed. Then pairs are taken out at
random, two at a time. The two pairs withdrawn at a time co nstitute a mating.
Like this six matings are arranged. Say, the six matings obtained in one
experiment are as follows:
I. MM x MM 2. MM x mm
3. MM x Mm 4. Mm x Mm
5. Mm x mm 6. mm x mm
Let us presume, each mating produces two offspring. In a cro ss between MM x
Mm :
In its two offspring, the parent M m co ntribution of M gene or m gene depends
purely on chance. To permit chance to operate in deci ding whether in these two
cases parent M m will contribute M or m, a bowl containing red and blue beads
in

677. FIG. 12.2: A cross between MM and Mm.
equal number is taken. One bead is drawn from the bowl at random without
looking at it.
(a) If bead is blue, it means Mm parent will contribute gene m and the offspring
will be mm.
(b) If bead is red, it means Mm parent will contribute gene M and the offspring
will be MM.
This drawing is done twice, once for each of the two gametes. The same
procedure is repeated for all crosses. Let us presume that by the end of first
generation, the F) offspring are produced in the ratio of 1 MM : 10 Mm : 1 mm.
The experiment may be extended for F
2
generation. The results show that the
gene frequency of M and m does not show steady increase or decrease but
random fluctuation. Sometimes, red beads (M) increase and sometimes blue
beads (m) increase, and sometimes one of the colour disappears completely in
the offspring.
Founder effect and bottleneck effect generate genetic drift in small populations
in nature .
12.6 FOUNDER EFFECT OR FOUNDER PRINCIPLE
When a few individuals or a small group of individuals from some large
population invade a new or isolated geographical region, these become the
founders or founder members. They establish a new population of their own .
These founders being few in number carry only a limited portion of the parental
gene pool. The descendents of the founders form the founder population or
marginal isolates. The allele frequencies in the founder populations will be
different from those in the source population due to sampling error.
A change in allele frequencies that occurs when new population is established in
a new area is called founder effect.
Parent population Migration New population arising from founder members

679. Migration
FIG. 12.3: Founder effect.
F ound er effect migh t occur when a few members of a population are blown by
a storm to a new island or small groups of people leave their home and form new
settlement. The new popul ation s are exposed to chance events amplifying some
traits and diminishing or eliminating others.
12.6.1 Significance of Founder Effect
• The isolated population s differ from source populati on in the frequencies of
their alleles.
• The genes or chromosome arrangements carri ed by the found ers have chance
of becoming established in new populations.
• Presence of high frequency of certain inherited disorders amon g isolated hum
an populations occurs due to found er effect and inbreeding.
Founder effect is well illustrated by colonisation of an island or other isolated
habitats.
12.6.2 Examples of Founder Effects
EXAMPLE 1: Founder Population of Green Iguanas on Ang uilla Islands: A
founder event occurred on the island of Anguilla in the Caribbean when in 1995
some green iguanas were transported by hurricane from islands of Guadeloupe.
They multiplied and formed founder population on Anguilla Islands . The allele
frequencies in green iguanas of these founder populations were found to be
different from parent population of Guadeloupe Islands, from where they
arrived.
EXAMPLE 2: Evo luti on of Pictu re-winged Drosophila Spe cies on Hawaiian
Isla nds: By carefully analysing the banding patterns of salivary gland
chromosomes Carson has shown that more than 100 native 'picture- winged'
species of Drosophila, residing on Hawaiian Islands have evolved as a result of
foun der events. Presumably, each island was infiltered by a few individuals,
whose descendants evolved into different species. For example, 40 species
unique to Maui Island complex are derived from only 12 founders: 10 from
Oahu and 2 from Kauai and each single founder provided a unique chromosome
arrangement.
EXAMPLE 3: Founder Effect on Human Population on Tristan Islands: The

680. small human popu lations at Tristan da Cunha (a small group of islands in
Atlantic Ocean, midway between Africa and South America) suffer from retin
itis pigmentosa blindness. This is caused by a recessive gene which develops in
homozygous condition. These human populations developed from few founder
couples that migrated from the main population. The frequency of blindness in
these founder populations is 10 times more than in the main population because
of inbreeding and founder effect.
EXAMPLE 4: Amish People in Lancaster County in Pennsylvania : People in
Amish community many among themselves. They have high incidence of certain
hereditary abnormalities such as short limbs, fused wrist bones, extra fingers and
heart disease. These abnormalities are due to founder effect and the genetic drift
lead to homozygosity.
Populations present on isolated islands in mountain valleys and caves, etc.,
represent founder populations evolved from few founder members and exhibit
founder effect.
12.7 BOTTLE NECK PHENOMENON
A large population formed of thousands or even more indiv iduals may
experience an extreme reduction in size due to disease outbreak, natura l
catastrophes such as floods , storms or fires thereby only a few individuals could
survive. The individuals form the progenitors for the future generations of the
popu lation in that area. The number of individuals may increase in the next
generation or may decline after one or two generations. This yearly or seasonal
phenomenon of cyclic fluctuation in population density also leads to periodic
squeezing of some of the genes in a gene pool in random fashion . The severe
drop in population size in a given area is called bottlen eck phenomenon.
Alleles of one gene in original population

682. Chance ev ent (environmental calamities, founder's effect or bottleneck effect)
reduces the frequencies of alleles
Survivor individuals with changed allele frequencies
A new population with new combination originates and differs from parent
population
FIG. 12.4: Bottle neck effect.
This ter m bottleneck bhenomenon was introduced by Stebbins . It represents 'a
phenomenon of severe, temporary reduction in population size (population
crash) followed by steep increase (population flush) causing great fluctuations in
allele frequencies.'
Population bottleneck leads to genetic bott leneck. It is sudden reduction in the
number of alleles in a population. Drift in allele frequencies occurs during
genetic bottleneck and reduces genetic variability in small populations that
persisted in the original population for generations, The reduced variability in
the gene pool of population makes its gene pool different from sister 's gene
pool.
Fluctu ations in population density are very common. These may be annual ,
long term or sporadic. Whatever may be their nature , fluctuations provide a
pivotal point for the change in gene frequencies and introduce genetic
modifications or gene elimination in the gene pool of the popu lation from which
these have arisen. When cycles are at their peak, too many individuals are
present, the population spreads out far and wide and is exposed to different
microenvironments. Therefo re, it has more chances to accumulate mutations in
its gene pool. On the contrary when the population is at an ebb, the number of
individuals may reduce to the extent that the small demes constituting the
population become isolated and restricted in distribution. These are then exposed
to random genetic drift resulting in the fixation of certain genes. This period of
crisis with low population density represents a ' bott leneck' and the changes
occurring in the gene
constitute the ' bottleneck effect'. This
frequenci es during this bottleneck period may result in the elimination of certain
alleles. The lost allele s may be different in different demes depending on chance

683. .
Examples of Bottleneck Effect
EXAMPLE I: Bottleneck Effect in Prairie Ch ickens
A large population of great prairie chickens iTympanuchus cupido) lived in
prairies. Once prairies were converted to farmland, these chickens got
exterminated due to loss of habitat. By 1993, only two small popu lations of
great prairie chickens remained in Illinois. Each population had 50 chickens
only. This population had lost nine alleles present in the original population. This
is because of bott leneck effect. Genetic drift in this population fixed some
negative alleles for low egg hatching and low levels of genetic variations .
EXAMPLE 2: Pingelapese Blindness in Pingelapese People
Pingelapese blindness is an autosomal recessive blindness in which a person
suffers from colour blindness, nearsightedness and cataract. Pingelapese people
living on Micronesia Islands of Eastern Carolina show 5 percent instances of this
disease . Elsewhere, only lout of 50,000 to 1,00,000 people are affected by this
blindness. The presen t pingelapese population evolved from a group of 20
people that survived a typhoon. This bott leneck population had some recessive
characters which appeared in the descendents due to homozygosity caused by
interbreeding and fixation of recessi ve alle les by genetic drift.
EXAMPLE 3: Bottleneck Effect in Cheetah
Cheetah population in the world is suffering from population bottleneck. Until
10,000 years ago cheetahs were widely spread out. But presently, they are
represented by two isolated small populations living in South and East Africa.
Members of each population are genetically so much alike that scientists believe
that they have developed from only very few founder members . In such a small
population genetic drift resulted in the loss of genetic diversity.
12.8 GENETIC DRIFT AND EVOLUTION
The role that genetic drift actua lly plays in the evolution of organisms
Original population with 25 different alleles of a particular gene.
Cheetah populat ion was affected by changing habitat and massacre by hunters.
Repopu lation
occurs. Only four
different alleles remain.

684. FIG. 12.5: Bottleneck effect in Cheetah.

685. • •
~

686. •••
•
Larger samples (1-5)

687. ••
Smaller samples (A-F)
~ Mating within this sample
population may eventually
result in the loss of the
allele represented by 0 and
the . allele becomes fixed
FIG. 12.6: Role of genetic drifts in the fixation of an allele and elimination of the
other one .

689. in nature has been the subject for debate , but there seems little doubt that it
contributes to genetic divergence and evolution of gene pools of small
populations. Because in nature :
I. Most breeding populations of animal s are usually small.
2. Even a widely ranging, broad based population is isolated into small
subgroups, called demes, either on account of ecological or geographical
discontinuities, homing instinct or territoriality. The size of these small dernes or
subpopulations is such that they appear to be affected by chance events
underlying genetic drift.
3. Seasonal, annual or cyclical fluctuations are observed in population size in
may species. For example:
(a)Huge populations of insects that appear in the warm spring and summer
months experience high mortality during cold winter months. The few that
survive "the following spring form a much smaller gene pool. (b) Because of the
limited size of the small breeding populations, the gene pool of their new
generations may not be the representative of the parental gene pool because of
chance or the action of genetic drift. The altered gene pools in due course of time
lead to the formation of new species. Thus genetic drift in small isolated
populations leads to:
(a)Variations among populations or demes by fixing or eliminating certain
alleles.
(b) Fixation or elimination of gene mutations.
(c)Fixation of unfavourable, neutral or favourable characteristics in populations.
(d) Establishing reproductive isolation between different demes of a large
population and origin of new species.
Genet ic drift basically gives a head start to the founders of the isolated marginal
population and accelerate speciation, because these marginal isolates are not
likely to be well adjusted to the new habitat. These are exposed to strong
selection for adaptations. Carson described the selection in marginal isolates as
homoselection. This leads to speciation. For example, the first finches that
arrived in Galapagos Islands by chance, had been much different from finches on
the mainland. Every marginal isolate on each island gradually evolved into a
new species by the initial interplay of genetic drift and the operation of natural
selection at a later stage.
Genetic Drift Versus Natural Selection

690. I. Genetic drift operates in small populations, whereas natural selection operates
in large populations.
2. Genetic drift is nondirectional, random and unpredictable. Natural selection
operation is directional and predictive.
3. Genetic drift does not support adaptive changes in the frequency of alleles.
Natural selection favours in the frequency of those alleles that provide
reproductive advantage and leads to adaptive evolution.

692. 4. Genetic genetic drift is non-selective. Fluctuations in allele frequencies caused
by
drift occur by chance . Natural selection favours gene combinations that are most
suited for the given environment. The gene frequenci es do not fluctuate but
change in a specific direction.
5 . Genetic drift leads to the loss of genetic variation and favour s homozygosity
by fixing or eliminating the other allele . Natural selection supports
heterozygosity in the populations because of heterozygous superiority. It
encourages and operates on genetic variability.
6. Geneti c drift basically gives a head start to the founder s of the isolated
marginal population and accelerates speciation, because these marginal isolates
are exposed to strong selection for adaptations and adj ustment to the new
habitat.
12.9 GENE FLOW
12.9.1 Definition Populations of a species typically exchange genes with one
another to lesser or greater extent. This exchange of genes among demes of a
species is called gene flow.
Gen e flow among populations is the cohesi ve force that keep s together the
geographically separated populations into a single evolutionary unit, the specie s.
The populations of a large or widely distributed specie s may be distributed
either as discrete populations on different islands or continuously. In
continuously distributed populations, the members get isolated by distance. Gene
flow in either case is reduced.
12.9.2 Primary Effects of Gene flow in Populations
In general , gene flow among populations:
• Reduces genetic differences between populations.
• Increase s genetic variation within populations.
1.0
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0

695. 10
If there is gene flow (m igration) among populations, they will all eventually
reach the overall average allele frequency.
Equilibrium
Frequency = p
20 30 40 50 Number of generations
FIG. 12.7: Gene flow between populations leads to converge their allele
frequencies
In mea ns, in absence of evolutionary forces , gene flow between populations
tends to make their gene pools identical and establish genetic homogeneity.
Gene flow therefore, causes populations to converge in their allele frequencies.
(Fig. 12.7).
12.9.3 Rate of Gene Flow Among Populations
The rate of gene flow (m) among populations depends on the following factors:
• The proportion of gene copies of the breeding individuals in each generation
that have entered that population by immigration from other populations
• The rate of migration of individual s, or dispersal of their gametes, larvae ,
seeds or pollens from different populations. For example, animals like land
snails , salamander, and wingless insects generally move little and are divided
into relatively small , genetically distinct populations, have much less gene flow.
• Gene flow is greater among more mobile organisms, such as monarch butterfly
and many marine invertebrates with plankton ic larvae because they are carried
long distances by water currents.
• The pattern of geographic distribution of popu lations.
12.9.4 Gene Flow and Distribution Pattern of Populations
Patt ern of gene exchange between different populations of a species is
influenced by the distribution pattern of these popul ations and subpopulations.
These model s are of following types :
1. Island model: According to this model the population of a specie s is divided
into a series of ideal subpopulations having local effective size N. They
exchange individual s at the rate of m. Each subpopulation co ntributes an equal
number of genes to a migrant pool and the total proportion of individuals in a

696. subpopulation from the migrant pool is m per generation and the rest (I-m ) are
drawn from the local population . Th is mode l is ca lled the islan d mod el of mi
gration (Fig. 12.8). More spec ifically, in island model of migration, each
subpopulation contributes an equal number of genes to the migrant pool.

698. 2. Stepping Stone Model: In natural
FIG. 12.8: Pattern of exchange among five subpopu lations (A, B. C, D, E) under
the island model of migration . Each subpopulation of local effective size N
exchanges
migrants with other subpopulation with equal probablity.
po pulations, migration is usually greater between subpopulations that are closer
to each other. This rules out equal probability of gene exchange among all pairs
of subpopulation with island model of migration.
Th e stepping stone model of migration was introduc ed by Kimura and Weiss
(1964) to consider short- range and long-range migration s. This follows linier
distributi on of subpopulations (F ig.12.9).
Populations having one-dimensional linear structure show linear stepping stone
model. It occurs along a river, river valley, or a mountain ridge.
3. Continuously Distribution Model: Some populations enjoy continuous
distribution across large landscape s, such as coniferous trees across boreal
forests. They are not subdivided into discrete subpopulation by any geographical
barriers. However, such continuously distributed popul ations also exhibit gene
flow limited to relatively short distances. Because of restricted gene flow, the
gene pool of its individuals becomes increasingly different as the geographical
distance betwe en the individu als becom es greater. Thi s was describ ed as
isolation by distan ce by Wright (1943 ).
FIG. 12.9: Pattern of exchange among five subpopulations under single
dimension stepping stone model of migration.
• Bottleneck effect
• Founder effect
• Homoselection
• Sewall Wright effect
KEY TERMS
• Effective population size
• Gene frequency

699. • Sampling error
• Population crash
• Fixation
• Genetic drift
• Scattering of variability
• Population flaush
REVIEW QUESTIONS
I. Justify that a small number of founders can cause a radical change in the
genotype in a new population.
2. How can we calculate the effect of genetic drift on gene frequency?
3. What are expected consequences of dividing the large population of a species
into a number of isolated panmictic subpopulations?
4. Discuss maj or roles of genetic drift. Illustrate your answer with suitable
examples.
5. What will be the evolutionary effect of interbreeding between demes and a
total large population?
6. Define the following :
(a) Deme (b) Founder effect (c) Bottleneck effect
7. How does population size influence the role of genetic drift?
8. Discuss genetic basis of random genetic drift.
9. Discuss role of genetic drift as one of the evolutionary forces.
10. How does genetic drift differ from natural selection?

701. FURTHER READINGS
I. Allison, A.C., August 1956 "S ickle Cells and Evolution ." Scientific
American . 2. Hedrick, P.w., 2000. Genetics oj Populations, Second Edition,
Jones and Barlett ,
Bo son, MA.
3. Hughes, C., and R. Eastwood, 2006 . Island Radiations on a Continental Scale
:
Exceptional Rates of Plant Diversification after uplift of the Andes. Proc. Natl,
Acad.
Sci., U.S.A. 103.
4. Kimura, M., 1983. The Neutral Theory ojMol ecular Evolution, Cambridge
University
Press, Cambridge, England.
5. Mayr, E., 2001. What Evolution Is. With a forewood by Jared Diamond . Basic
Books,
New York.
6. 0 ' Brien, S.J; Wildt, D.E. May 1956 "The Cheetah in Peril" and Bush, M.
Scientific
American.
7. Pagel, M. (ed . in chief), 2002. Encyclopedia oj Evolution, 2 Volumes., Oxford
University Press, New York.
8. Provine, w.B., 1986. Sewall Wright and Evolutionary Biology, The University
of
Chicago Press, Chicago.
9. Wright, S., 1978. Evolution and the Genetics ojPopulations: Vol. 4. Variability
Within
and Among Natural Populations. University of Chicago Press, Chicago . DOD

703. 13
Natural Selection in Action
13.1 CONCEPT OF NATURAL SELECTION
The evo lutionary agent s discussed in the previous chapter bring in changes in
the frequen cies of alleles and genotypes in Mendelian populations. The new
genotypes that produ ce more efficient adapti ve relationship with the
environment, ensure better survival and comparati ve reproductive success are
favoured by natural selection. In genetic term s, natural selection favours
individuals that contribute more offspring to the next generation and ensures
perpetuation of those genotypes and alleles which change the gene pool for more
efficient adaptive relationship with their environment. This result s in •descent
with modification' .
13.1.1 Natural Selection Defined
Na tural selection is the force which ensures differential contribution of alleles of
a gene to the gen e pool of a population in the next generation and maintains
polymorphism in the genotype s of individual s generation after generation.
13.1.2 Essential Features of Natural Selection
Based on the above definition, natural selection has two essential features:
13.1.2.1 Reproductive Success or Differential Reproduction
Differential multiplication of gene s and gene combinations means that the
individuals which are
which are
best adapted to their environment, produce more offspring than those less
adapted. The y contribute proportionately more alleles to the gene
po ol of subsequent generations. In case differential reproduction continues for
several generations in a population, the alleles present in those individuals which
produce more offspring increase in frequency over time. The change in the gene
frequency

705. and genot ype frequency in the gene pool of a population by natural selection
can be illustrated by following example:
300 Ii] Evolutionary Biology
EXAMPLE 1: When allele A makes an organism more efficient in reproduction
than its allele a, the frequency of allele A gradually increases in the gene pool
generati on after generation and the frequency of allele a gradua lly decreases.
Thus, natural selection is a creati ve force and is achieved either by increased
. rate of reproduction or by the decreased vulnerability to environmental agents
responsi ble for mortality.
Which Individuals Produce More Offspring
• Individuals which are best adapted to the environment, l.e., which have
adaptive or beneficial mutations.
• Individuals in which positive selection pressure due to the presence of
beneficial mutations is more than the negative selection pressure of harmful
mutations.
• Individuals which have better chances of sexual selection.
• Organisms who are able to overcome the physical and biological
environmental factors.
13.1.2.2 Encouragement to Beneficial Genes
Natural selection encourages conservat ion and multiplication of those genes and
genotypes that assure the highest level of adapt ive efficiency to the population
in its exi sting environm ent. It mean s when two or more gene comb ination s
are present, selection favours increased reprodu ction of the gene combination
which is most suitable under the existing environmental conditions. Therefore,
natural selection bri ngs about impro ved adaptive rela tion s between organisms
and env iro nment. How natural selection favours beneficial genes and gene
combinations can be illustrated by following exampl es:
1 + 9 3+9
(1) Parental
generation (Variants: 1/10 =10%) 9+9 (2) Offsp ring in
First generation (Variants: 3/12 = 25%) 27 + 9
(3) Offspring in Second generation

706. (Variants: 9/18 = 50%) (4) Offspring in Third generation
(Variants: 27/36 = 75%)
FIG. 13. 1: Sp read of genetic variability by differential reproduction.
EXAMPLE 1: Evolution of Green-coloured beetles from Red beetles. In the
population of red beetles, a green beetle appears due to colour mutation. The
green body colour is heritable and has survival advantage. It is difficult for
crows to spot green coloured beetles on green leaves while the red coloured
beetles on green leaves are spotted even from a distance. As a result, red
coloured beetles are eaten away by crows more often and green beetles escape.
Gradually, the number of green beetles increases in the beetle population, and at
one time,
the en tire beetle popul ation has only green beetles. The natural selection has
encouraged the increase of alleles for green colour as agains t red because green
beetles could conceal them on green leaves more effectively from their predator
crows than the red beetles which crows cou ld locate from a distance.
EXAMPLE 2: In a population ofDrosophila me!anogaster, all the females (whet
her white-eyed or red-eyed) prefe r to mate with the red eyed males, but if
whiteeyed males are the only mates available, they are compelled to mate with
them. It means
A B
FIG. 13.2: A. Red beetle; B. Green beetle (Green-coloured beetle is at an
advantage of not being spotted out in the green background).
white-eyed gene is eliminated from the population as a result of selection, which
acts through mating preference. In addition, selection produces an adaptive
improvement, since red-eyed males have better environmental relation. Natural
selection is, therefore, a creative force in evolution as it favours efficient gene
combinations.
E MPLE 3: C ha nges in Beak Size, Beak Shape, and Bod y Size in G alapagos
Finches.
Peter and Rosemary Grant studied changes in the beak size, beak shape, and bod
y size in the popu lations of Darwin's finches on the Daphine Major Island of
Galapagos Islands from 1973 onward. There was drought on the island in 1977.
Grant's team compared the population of finches before and after the drought

707. and found that:
• Birds that survived the droug ht had deeper beaks than the birds that died,
because during this draught period finches could get only tough shelled seeds
which could be cracked by large and deep beaks.
• In 1983, when environment changed and small, soft seeds were available in
plenty, small finches with small , pointed beaks became more numerous.
At molecular level , the recent work of Abzhano v et.a! on the development of
beak size and shape in finches at Galapagos Islands showed that variation in
beak size is controlled by the degree of expression of a single gene, Bomp-4. The
expre ssion of this gene is influenc ed by the type of food availabl e.

709. FIG. 13.3: Natural selection has favoured survival of green coloured beetles and
has led to evolution of green body colour in beetles.
302 ~ Evolutionary Biology 13.2 SALIENT FEATURES OF NATURAL
SELECTION
Some of the special feature s of natural se lection are:
1.Natural selection acts on individuals but the evolutionary changes occur due
to changes in the allele frequencies in populations. For example, in
Microbacterium
tuberculosis the individual bact erial cells did not change during the evolution of
antibiotic resistant strain, when rifampi n was introduced to their environment.
They had the same rifampin resistant polymerase allele. But the frequency of
this
antibiotic resistant allele increa sed in the bacterial population over generations
and
changed the characteristic of the population. In other words, an individual's
allele
frequenc ies cannot change over time but allele frequencies in the gene pool of a
population change generation after generation with the change in environment
due
to selection pressure of the environment.
2. Evolution by natural selection is no t goal directed. It means natural selection
is non-p rogressive. Adaptive modifications do not occ ur because organisms
need
them . Rather they are already present in the organ isms and are retained because
they
increase survival and fecundity for reproduction. For example, there was an
increase
in the size of beak of ground finches on Daphne Major Island of Galapagos
during
drought period because they could survi ve by feeding on hard shelled seeds .
But
the aver age beak size in finch population declined when the island got torrential
rains, because the availability of food supported the survival of finches of
average
beak size. It means natural selection simply favours individuals that happen to be
better adapted. It is neither progressive nor goal directed.
3. Not all traits in the organisms are adaptive. Although, organisms are adapted

710. to their environment, adaptation is never perfect because the traits an individual
or
individuals of a population have , increas e survival. Natural se lection does not
lead
to perfec tion. Nonadaptive traits appear due to changes in DNA sequences
caused
by the redundancy of genetic code. These are described as gene tic con straints.
There are several reasons for the genetic constraints:
• Because of pleiotropy (i.e.. a single gene sometimes influences more than one
characters) selection on alleles of one trait causes a correlated selection or
increase of the alle les of other trait. This trait mayor may not have survival
value but persists in the pop ulation. Th is is called gene tic correlation.
• Lack of certain genetic traits or absence of genetic variations also makes
adaptations imperfect. For example, birds can sense mag netic field and see UV
light but man can not. Though these traits will be beneficial for huma n beings
but the requisite genes for these traits are not present.
Hence , a variety of unuseful or harmful traits persist in popu lations .
13.3 NATURAL SELECTION IN NATURE
13.3.1 Industrial Melanism
The industrial melanism in peppered moth, Riston betularia, provides the best
example of directional selection. In early nineteenth century, there was a
dramatic rise of
Grey Black
peppered
pepperedmoth moth
( invisible (visible on on
grey tree black tree trunk) trunk)
Grey Black peppered peppered
moth moth (visible (invisible on on black tree black tree
trunk) trunk)
Tree trunk before melanism Tree trunk after melanism
FIG. 13.4: Role of natural selection in the directional evolution of melan ic
variety of peppered moth (Siston betularia) due to industrial melanism.
indu strialisation in Europe. The black sooty smoke covered the forests and
fields. This changed the usual colour of tree trunks from mottled greenish-grey
to black . The wing colour of typical peppered moth was mottled grey that

711. blended perfectl y with lichen-covered tree trunk s and protected it from the
enemies. Until 1845, only light-coloured moth s were known in England. In
1845, the first dark-coloured peppered moth (melanic form) was seen in the
region east of Manchester. This variant was named Biston betularia carbonaria.
During next 50 years, dark individu als gradually increased from less than I to
about 99% in the vicinity of industrial areas. The reason for this striking increase
in the number of melanic variety was explained by E.B. Ford and H.B.D.
Kettlewell. Ford found that the caterpillars of melanic variety (Carbonaria) were
more vigorous and viable, capable to withstand the environm ental hardships.
But Kettlewell showed that moths of melanic variety were at a disadvantageous
position and could not survive in the nonsooty forests, because the birds could
locate them on lichen s and could eat them . So they were maint ained at a very
low frequency. With the elimination of lichens in sooty atmosphere, the
carbonaria were cryptically coloured and blended with the tree trunk . Therefore
, the melanic variety became abundantly distributed in due cour se of time.
Natural selection has operated in the direction of eliminating gene for light-c
olour and gradual increase of gene for dark pigment.
13.3.2 Resistance to Pesticides (DDT) in Insects
When DDT was introduced to control mosquitoes, it proved to be a successful
insectic ide. But , now it has become ineffective against mosqu itoes. It can be
explained as below:
The origin al popul ation of mosquitoes had some DDT-resistant individuals. In
the absence of DDT, such DDT-resistant individual s had no additional
adaptability or survival value over DDT sensitive mosquitoes. Natural selection
favoured them only when DDT was sprayed on a large scale. Therefore, DDT-
resistant genotype became more and more numerous. Over a period of time , the
entir e population became DDT-resistant type. Likewi se, new insecticide s were
developed, but they remained effective for some time and insects developed
resistance to them also.
30 4 ~ Evoluti onary Biology
13.3.3 Antibiotic Resistance in Bacteria
P enicilin was the first antibiotic discovered. It was widely used against bacterial
infection. Soon it was found to be ineffective again st many bacteria. It was
replaced by chloromycetin (chloroamphenicol) and then by a third antibioti c

712. ciproftaxacin. Reduction in effectiveness of an antibiotic is due to the
development of resistance in bacteria against that antibioti c. The antibiotic
resistance in bacteria arose due to gene mutation. Such antibioti c-resistant
bacteria survive and multiply to produc e resistant strain.
13.3.4 Antibiotic Resistance in Mycobacterium tuberculosis
In 1 980s, bacteria causing tuberculosis (Mycobacte rium tuberculosis) was
treated with antibiotic r ifampin. But in 1993, a new strain of M. tuberculosis
was found, that was resistant to rifampin and other antibiotics and TB becam e a
global threat. Scientists found that the genom e of resistant strain of M.
tuberculosis developed a point mutation in gene r poB, where a cytosine was
replaced by thymine and the normal codon TCG mutated to TTG . The
polymerase produced by mutated gen e ha d leucine instead of serine at l5yd
amino acid in the polypeptide chain. The drug rifampin was unable to bind to
mutant RNA polymerase and M. tuberculosis became resistant to the drug.
S uch antibiotic-re sistant bacteria survived and multiplied to produce more and
more bacteria of resistant strain. The normal bacteria non-resi stant to antibiotics
died off.
Resistance to a wide variety of insecticides, fungicides, antibiotics, antiviral
drugs and herbicides has evolved in some insects, fungi, bacteria, viruses and
plants, and such resistant varieties have been favoured by natural selection.

714. M utation (resistant to
penicillin) Penicillin containing
medium
CJ CJ CJ CJ CJ
CJ
c=:J
c=:J
Mut ants survive penicillin treatment
Pen icillin sensitive bacteria
(All are
killed by
penicillin)
.... ''----'' Population of mutant bacteria (all resistant to penicillin)
FIG . 13.5: Selecti on of penicill in-resistant bacteria by natural selection.
13.3.5 Evolution of Drug Resistant Bacteria
Evolution of methicillin-resistant Staphylo coccus aureus (MRSA) is another
an example of natural selection. In 1945, more than 20% of Staphylococcus
aureus bacteria were found to be penicillin-resistant in American hospitals,
because they were producing enzyme penicillinase to destroy penicillin. When
more powerful antibiotic methicillin was used against them, within two years
methicillin-resistant strain of S. aureus appeared. These bacteria were able to
synthesise a cell wall around them that was not affected by methicillin. They
survived methicillin treatment and became increasingly more numerous
spreading MRSA more rapidly.
Initially , MRSA bacteria could be controlled by antibiotics other than
methicillin. Later on, some MRSA strain s became resistant to multiple
antibiotics because of gene exchange with members of their own and other
species. The present day multi-drug resistant strains of Staphylo coccus aureus
are presumed to have evolved as MRSA strains and became resistant to different
antibiotics by exchanging genes with other similar bacterial populations.
13.3.6 Sickle Cell Anaemia

715. Sickle cell anaemia is a genetic disorder of human beings, found specially in
Blacks from Tropical Africa. In a sickle cell anaemic person, normal
haemoglobin (HbA) is replaced by Hb
s
whose oxygen-carrying capacity is less
than HbA. The RBCs in sickle cell anaemic persons become sickle-shaped in
venous blood owing to the lower concentration of oxygen. This causes rupture of
RBCs and severe haemolytic anaemia. Individuals homozygous for abnormal
haemoglobin (HbSHb
S
) die at an early age. In heterozygotes (HbAHbS), the
RBCs containing HBs allele
become sickle-shaped and unable to bind oxygen, but RBCs with HbAallele
remain normal. These heterozygotes (HbAlHbS)
and homozygotes (HbA/HbA) have
normal life expectancy.
The question arises that why has
this character not been eliminated from
human population by natural selection?
The geographical distribution of sickle . ,,
cell anaemia provides answer to the
FIG. 13.6: Malaria infested belt of the world.
above
question. It is found in tropical
Africa where malaria is widespread . Malarial parasites that live in RBCs are
unable to grow in sickle-shaped RBCs . It means individuals heterozygous for
sickle-celled gene are able to cope with malarial infection whereas the normal
persons with normal RBCs suffer from severe malarial infection in malaria
infested areas . This shows that natural selection favours the sickle-celled
character in malaria-infested areas and the gene controlling it is fixed in such
populations by natural selection, because of its survival value in malaria-infested
regions. Therefore, this character is found in blacks living in malaria infested
belt of the world (Fig. 13.6).
13.3.7 Malaria in Relation to G-6PD Deficiency
G-6PD deficiency is a X-linked hereditary disease in humans. It represents the
deficiency of an enzyme glucose-6 pho sphate dehydrogenase. This enzyme is
essential for the regeneration ofNADP (nicotinamide adenine dinucleotide
phosphate) from its reduced form NADPH , formed due to electron transfer
during glucose metabolism.

716. Persons suffering from G-6PD deficienc y suffer from haemolytic anaemia and
prolonged neonatal jaundice. Due to haemolysis of RBCs, malarial parasites are
unable to survive and multiply in the RBCs. Therefore, the decreased activity of
G-6PD within the RBCs provides resistance against malarial fever. In malaria
infested regions like sub-sahara region, this feature proves to be beneficial and is
favoured by natural selection. Therefore, fifty perc ent population of Negroes
and populations derived from their ance stors livin g in malaria infested areas
suffer from G-6PD deficiency.
13.4 DEMONSTRATION OF ROLE OF NATURAL SELECTION
13 .4.1 Lederberg's Replica-Plating Experiment to
Demonstrate Role of Natural Selection
Joshua Lederberg and Esther Lederberg conducted experiments to demonstrate
genetic
basis of drug-resistant mutations in the bacteria, Escherichia coli.
Orig inal plate
Velvet disc
-~SE~
Velvet disc --P«:J...>'
In Med
i
um
with
g
\
g
In Streptomycin free Medium Strept omycin

718. ~
- ReplICa plate from
Replica plat e from

velvet disc to growth
vetvel disc to growth
7~= (~-:nB
ining med ium lack ing
penici llin
+
Growth 01on
ly Growth 01all
streptomycln bacterialresistent
Medium with streptomyci colonies
colonies
(Plate 2) (Plate 3)
FIG . 13.7: Lederberg 's replica plating experim ent revealing that drug resistant
mutation in bacterial cells is not induced by the drug but was present prior to
exposure of bacteria to drug .
Colonies of bacteria were grown on streptomycin-free agar medium in a
petridish by inoculating suspension of bacteria on an agar plate. This formed the
' master plate ' of bacterial culture. It contained several bacterial colonies. Each
colony of bacteria represented a clone .
Replicas of master plate were prepared on new plates in the following manner.
The agar surface of master plate was pressed gently on a piece of a sterile velvet.
Some bacterial cells from each bacterial colony clung to the fine fibres of velvet.
These were then transferred to new agar plates, where these grew into new
colonies.
Lederberg tried to culture bacteria on agar plates containing streptomycin or
penicillin. Most colonies of the master plate failed to grow in the medium
containing antibiotic. The few colonies that could be formed were resistant to
antibiotic. Fig. 13.7 shows that only two colonies were drug-resistant. These two
antibiotic resistant colonies appeared at the same position in all replica plates .
The above experiment indicates that some bacterial cells had acquired penicillin
resistant mutation even before they were exposed to penicillin. Natural selection
has selected and supported the progeny carrying the beneficial mutation.
13.5 WORKING OF NATURAL SELECTION
The working of natural selection i s exceedingly complex because it acts at all
the levels of organisation and upon all stages of life cycle of an organism.
Examples of levels at which natural selection makes differential discrimination

719. are: Intermolecular, intergene, interchromosome, intergamete, inter-individual,
interdemic, inter-racial, interspecific and inter-community. It may be caused by
differential viability, differential mortality, differential fertility, or differential
natality. Selection creates new adaptive relations between population and
environment by favouring some combinations and constantly moulding and
modifying the gene pool.
Working of natural selection can be discussed under following heads:
13.5.1 Selection for Single Gene Traits
The natural selection operates for traits determined by a single gene pair. The
selection may be against or for a dominant or a recessive allele . For
understanding this selection, we need to know following terms :
I. The survival rate of each genotype is defined as the fraction of that genotype
that contributes to the next generation. It is repre sented by A.
2. The relative fitness (W) of each genotype is its survival rate expressed as a
fraction of maximal survival rate. The relative fitness of optimal genotype is
considered equivalent to 1.00 and relati ve fitness of genotypes are calculated as
fraction or percentage of optimal genotype.
3. The selection coefficient (8) of each genotype is defined as I minus its relative
fitness (i.e 1-W).
Selection for single gene traits may operate against dominant or recessive alleles
or against homozygotes or heterozygotes.
(a) Selection against Dominant Alleles (i.e., Selection for recessive alleles):
Selection against dominant genotypes leads to the elimination of dominant
alleles and complete fixation of the recessive ones. Selection against dominants
operates rapidly. In case of dominant lethal traits, the dominant lethal alleles are
eliminated in a single generation of organisms because with lethal trait either
homozygous or heterozygous fail to survive.
(b) Selection against Recessive Allele (i.e., Selection for dominant alleles):
Selection against recessive traits and recessive alleles proceeds very slowly. It
leads to total elimination of homozygous recessive genotypes but recessive gene
is retained in heterozygous organisms. The recessive gene is never lost
completely because when recessive trait becomes rare, selection becomes weak.
The only way these

720. FIG. 13.8: Different stages in the life cycle of organisms at which selection
operates.
can be lost is through genetic drift .
(c) Selection against Homozygotes (l.e., Selection for heterozygotes): The
selective superiority ofheterozygotes leads to equilibrium in which both alleles
in the population are fixed.
(d) Selection against Heterozygotes (i.e., Selection for both homozygotes): In
this situation both the alleles are favoured by selection. The natural selection
may eliminate either of the allele or one of the allele may get eliminated by
genetic drift.
13.5.2 Selection for Multigene Traits
In case of multigenes, selection operates against each pair. The genes which do
not contribute to any phenotypic expression are not selected.
13.6 COMPONENTS OF NATURAL SELECTION OR LEVELS OF
NATURAL SELECTION
Evolution by natural selection depends on changes in allele frequencies
determined by the components of fitness of genotypes. In sexually reproducing
populations, selection may elicit changes at genic level acting directly on the
alleles or on the
phenotypes of diploid organisms at different stages of life cycle and even at still
higher levels of biological organisation. In any of these stages, selection effects
range from complete lethality, sterility or reduction in fitness. The selection
operates at the following levels:
I. Genic selection
3. Zygotic selection
5. Interdemic selection 2. Gametic selection 4. Sexual selection
6. Species level selection
13.6.1 Genic Selection
Genic selection is a selection for individual alleles . Normally, heterozygous
individuals produce equal number of gametes with one of the two alleles. In
some cases one allele is transmitted to more than 50% of the viable gametes of
heterozygous individuals. This is called segregational distortion or segregational
advantage. It means one allele is overrepresented and other one

721. underrepresented.
Segregational distortion during meiosis may be one of the factors of gametic
selection, in which gametes carrying particular gene show preferential survival.
13.6.2 Gametic Selection
Factors that influence the survival and mating capacity or fertilisation success
are responsible for gametic selection. Gametic viability or gametic survival
depends on the alleles it carries and fertilisation success of a gamete depends on
the presence of allele or alleles that influence the movement of male gamete
towards female gamete (ovum) . Gametes with alleles that provide better
survival and better fertilisation success introduce more alleles to the gene pool.
For example, if a gamete possessing allele al is more successful reproductively
than the gamete possessing allele a2, more at gametes will fuse to form zygotes
than a-. If the frequencies of at and a2 are P and q respectively and the fitness of
more successful allele al is taken to be 1, the fitness of relatively less successful
allele a2 is reduced by factor s (selection coefficient) and will be (1 s) . The
effective gametic pool from the above population will have gametes with
following frequencies :
Frequency of at gametes = p
a2 gametes = q (1 s)
If selection coefficient for a2 is one (i.e.,
S
= I), the value of q will be zero. It
means a2 is a lethal allele, but if value of s is less than I, a2 will persist in the
population. Hence, a2 allele becomes reduced in each generation until it
disappears completely and allele at becomes fixed.

723. The gametes are haploid and effect of selection on haploids is more rapid and
direct than on diploids. The deleterious recessive alleles are rapidly eliminated
and lethal recessive alleles are eliminated just in one generation.
31 0
~
Evolutionary Biology
13.6.3 Zygotic Selection
In higher animals and plants, selection takes place primarily on the diploid or
zygotic and postzygotic stages. It is influenced by following three factors:
1. V iab ility or Survival of Individual O r ga nisms: The probability of survival
of adult organisms up to reproductive age ensures contribution of their alleles or
genotypes to the gene pool of population in the next generation. It means those
genotypes of adults are selected and favoured by selection which add more
offspring to the next generation.
2. Mati ng Success: It is represented by the number of mates obtained by an
individual, i.e., how many females a male can mate with and introduce its genes.
The mating success forms the basis of sexual selection.
3. Fecu ndity: It is the average number of viable offspring per female. It depends
on the number of viable eggs or ovules formed in a female and on the genotypes
of both males and females, which may display reproductive compatibility at
different levels.
The diplo id organisms will show three possib le genotypes for a single allelic
pair :
Q I Q b Q l Qz
and
Qz Qz·
The effectiveness of selection will depend upon the
degree of dominance. If dominance is compl ete and selection occurs against
recessive
Qz Qz,
the heterozygotes retain the recessive or even lethal recessive
gene for several generation s in the population.
• In case of complete dominance and the selection acting against recessive
homozygotes, the change in freque ncies per generation will be very small and
the recessive allele will not be lost easil y, i.e., it will persist for many
generations in heterozygotes.
• In case of partial dominance, the homozygote of recessive allele is not
eliminated by lethality and both the alle les are fixed in the population.
• In case of codominance, the selection becomes very fast because it acts against

724. homozygous lethals as well as against heterozygotes. The heterozygotes also
exhib it the deleterious trait.
• Most recessive alleles are insensitive to selection and continue to stay in the
gene pool of populations because in heterozygotes, they are protected from
selection.
• In case selection acts against dominant allele, the recessive allele accumulates
in the gene pool, because deleterious dominant allele is subjected to selection in
both homozygous dominants and heterozygotes . It means frequency of a lethal
dominant allele falls to zero in a single generation .
• In case where heterozygo te confers some ad vantage to the fitness of an allele,
both alleles remain in the population and balance is achieved between the
relative frequencies of two alleles. Ford has described such a situation as
balanced polymorphism. If adaptive value of both homozygotes is far less, the
homozygous genotypes are eliminated at each reproductive cycle and both

726. the alleles are supplied by heterozygotes and continue to stay in the population.
This is due to heterozygote superiority.
• In the absence of dominance, the deleterious alleles are exposed to selection,
allowing rapid changes in allele frequencies.
13.6.4 Sexual Selection
Sex ual selec tion operates in sexually reproducing species for the greater mating
success through direct com petition between the members of same sex in relation
to the propagation of the species. It can also be described as intrasexual
selection. It is different from natural selection. Whilst natura l selection depends
on the success of both sexes, at all ages in relation to the general conditions of
life, sexual selection operates on sexually mature adults and between the
members of the same sex of a population.
There are fo llowi ng forms of sexual selection:
• Contest between males for access to the desired fema les for successful mating
.
• Female's preference for some male phenotypes over others .
• Competition between fema les to charm supermales
Choice of mate is exercised by females, hence males compete among themselves
to
woo the females for mating. Therefore, the courting ma les of many species of
anima ls have elaborate morphological features or exhibit bizaree courtship
behaviours. Males display bright colours, long feathers or fins or long and
elaborate horns etc. Some males exhibit consp icuous disp lays, fights, defending
large territories, constructing nests, or producing pheromone molecules and
sounds to attract females. Such structures and behaviours appear to have evo
lved throug h selection caused by fema le's choice of mates. Males with such
features enjoy higher reproductive success.
13.6.5 Interdemic Selection or Group Selection
A deme is a local population of widely distributed species.
lnterdemic selection is group selection of subpopulations within the metapopula
tion of a species. A metapopulation
consists of a set of local populations,
among which there is gene flow and
exhibit patterns of extinction and

727. recolonisation. Inter de mic selection
operates at two levels:
• At the level of individual geno
types i.e within the populations.
• At the level of genotypes among
the popu lations . lnterdemic se-

729. lection occurs when populations FIG. 13.9: Two male big horn sheep fightingdie out or give rise to new
popu
to woo a female sheep during mating season.
312 ~ Evolutionary Biology
l ations at diffemet rates, depending on whether they have traits that are
beneficial or harmful to the popul ation as a whole. It has led to the evo lution of
altruistic behaviour. An altruistic trait benefits the group as a whole , but is
selectively neutral or deleterious to the bearers. In altruism an individual
sacrifices its well being for the good of oth er memb ers of its spec ies. Such a
trait is selected for until
FIG. 13.10 : A male peacock displaying its large showy ta il feathers for
attracting the peahen for mating.
every individual within the population has this trait.
13.6.6 Species Level Selection
It is selection rate of spec iation. It ass umes that an evo lutionary lineage that
form s new species rapidly is less likely to become extinct than is a lineage that
speciates slow ly. If these characte ristics of species are geneti cally determined,
the selectio n operates at species level.
13.7 THE RESULTS OF NATURAL SELECTION
The end result of natural selection is rep lacement of relati vely disadvantageous
alleles by more advantageous one s. Th is is visible in the form of adaptive
evolution.
(A) (B)
G roup selection favors the "altruistic" genotype.
-
Time
100008000
+ I + +
2 00
+

730. +
~
3 000
A + +
4 0000
A ltruistic behavior will evolve because group selection favors it (i.e., more
"selfish" populations go extinct) .
The ·se lfish" genotype has a higher individual fitness.
00008000
+ + +r><.~Gene flo
08880 between
+ +~ population
00088
+ + + + +
o
W ithin-population selection favors the "selfish" allele and increases it more
rapidly than whole-population events , so the "selfish" allele will become fixed.
FIG . 13.11: Difference between group and individual selection.
Each circle represents a population of a species

732. An advantageous allele may be fairly common in initial stages, in case it is
neutral under previous environmental circumstances, and has proved to be
advantageous in changed conditions. Howe ver, in any population, a new allele
or newly arisen mutation , whether advantageous or neutral , is rarely
represented initially. Similarly, a disadvantageous allele is rare in the gene pool.
The allele frequen cy of advantageou s alleles or mutant s gradually increases
and gets fixed in the population.
A n advantageous allele whether dominant or recessive gets fixed, but a
recessive advantageous allele takes very long becau se homozygous recessives
are very few. Once establ ished , it attains fixation rapidly and deleteriou s
domin ant is eliminated.
13.8 MODELS OF SELECTION
N atural selection can alter the frequenc y distribution of alleles and the heritable
traits in different ways, depending on which phenotypes in a population are
being favoured. Based on the types of results produ ced and the genotypes fixed
or favoured by natural selection, following model s of selection have been
recognised:
I . Directional selection
2. Stabilising selection
3. Disrupti ve selection
4. Cyclic selection
13.8.1 Directional Selection or Progressive
Selection
Dir ectional selection occurs when selection favours individuals of a population
exhibiting one extrem e of a phenotype and produces a regular change in one
direction in respect to a certain character. It favours a nonaverage or an extreme
phenotype and pushes the population 's phenotype frequency curve in the
direction of that character.
Dir ectional selection is progressive selection. It removes more individuals from
one end of the normal bellshaped curve of variability distribution and adds
toward s the other end and thereby alters the mean value of the trait in the
population in a particular direction. If directional selection operates over many

733. generation, an
Mean
New
\
mean
I
I
I
I
I
FIG. 13 .12: Directional selection: In directional selection, one of the extremes
of a characteristic is better suited to the environment , meaning that individuals
at the other extreme are selected against (eliminated) . Over succeeding
generations , the coloration of the population moves in a direction-in this case
toward darker coloration.

735. 314 ~ Evolutionary Biology
evoluti ona ry trend within the popul ation results and often continues for many
generations. But the evolutionary trend may be reversed when environment
changes and a different phenotype is favoured , or it may get halted when
optimum is reached.
13.8.1.1 Salient Featues of Directional Selection
• Directional selection operates when environment is changing in one direction.
• It favours accumulation of those mutations that increase fitness of the
population in the changing environment.
• It favours non-average or specialised phenotypes and eliminates the normal or
average individuals .
• It brings about progressive evolution.
13.8.1.2 Examples of Directional Selection
EXAMPLE I: Industrial Melanism: The industrial melanism in the peppered
moth, Biston betularia, provides a well-stu died exam ple of directional selection
from nature . In the early part of nineteenth century there was a dramatic rise of
industrialisation in Europe. The black sooty smoke fell upon the countryside and
covered forests and fields with soot. This changed the usual colour of tree trunks
from mottled greenish - grey to black.
The wing colour of the typical nonmelanic peppered moth was mottled grey that
blended perfectly with lichen-covered tree trunks on which moth rested during
day. These were protected from the enemies. Until 1845, only light coloured
moths were known in England. In 1845, the first dark coloured or melanic
variety of peppered moth appeare d in the region east of Manchester. This
variant was named Biston betularia carbonaria. During next 50 years, the
frequency of dark individuals gradually increased from less than I to about 99%
in the vicinity of industria l areas. In fact, the number of melanic type reached 90
percent or more in most British industrial areas. The reason for this striking
increase in the number of melanic variety (carbonaria) was worked out by E.B.
Ford and H.B.D. Kettlewell.
Kettlewell photogra phed the light and dark coloured forms of peppe red moth
against dark sooty and light, lichen covered backgroun ds (Fig. 13.13)of tree
trunks. When light coloured moth rests on tree trunk, it is concealed on the light,

736. lichen covered tree twigs but is clearly visible on a sooty black tree trunk.
Similarly, dark moths are concealed on the dark sooty tree twigs, but were
detected by insect-eating birds while resting on light, lichen-covered trees. Ford
found that the caterpillars of melanic variety (carbonaria) were more vigorous
and viable, capable to withstand the environmental hardshi ps much better than
the wild type.
(a) In non ooty atmo phere: Kettlewell proved that, though having greater
survival value, the melanic variety could not survive in the nonsooty forest,
because the birds could locate them on lichens. So they were devoured by birds
irrespective of their number.

738. (b) In sooty atmospheres With the elimination of lichens, the carbonarias were
cryptically coloured and the light coloured moths were not. Therefore, the
melanic variety became abundantly distributed in due course of time.
In non-polluted or nonindustrial areas of England and in Northern Scotland, the
light - coloured variety remained more abundant showing role of selection. The
enforcement of 'Clean Air Act ' in 1950, led to gradual reduction of soot in the
atmosphere. In 1970, L.M. Cook reported a significant increase in the frequency
of light coloured peppered moths in Manchester area.
EXAMPLE 2: Resistance of Insects to DDT: The development of resistance to
DDT or other insecticides by mosquitoes and houseflies is also an example of
directional selection.
EXAMPLE 3: Resistance of Bacteria to Drug: Cavalli and Maccacro
demonstrated the roleof directional selection on colon bacteria, Escherichia coli.
By transferring E. coli popu lation to increasing concentration of drug,
chloramphenicol, they raised populations of E. coli having 250 times increased
resistance to this drug . The mutant resistant forms have better chances of
surviva l than the nonresistant bacteria.
EXAMPLE 4: Evolutionary trends in the fossil records indicate directional
selection. (See evolution of horse or evloution of man)
13.8.1.3 Conditions for Directional Selection
Under natural conditions, direction al selection operates under two different sets
of conditions:
I . In a population subj ected to a progressive or directional change in the
environment (illustrated by Biston betularia ).
2. In a population which migrated into a new territory having progressively
altered environmental conditions. For example, at the end of ice age many
species migrated northward into the regions , left bare by the retreating glaciers.
These must have been subjected to increasing cold, and an increasingly extreme
cycle
Blended with Generation 0 Several Generations later
the light o '
tree tn

740. Merge w ith the dark coloured tree trunk
FIG. 13.13: Photograph of dark and light forms of peppered moth (Siston
betularia) clinging A . to a light coloured , lichen covered oak tree ; B. to a sooty
blackened oak tree.
of short days and long nights of winter, and the long days and short nights of
summer and became adapted to the extremes.
13.8.2 Stabilising Selection or
Normalising Selection
St abil isi ng selec tion fav ours the ave rage or interme dia te ph en otypes and
eliminat es both extreme variants that fall towards both ends of bell-shaped
curve of variability distribu tion of phenotyp es. This type of selection acts in the
abse nce of environmental changes for a long period.
Va ria tion introduced in the populations by genetic recombination, mutation and
migration in a constant environment are eliminated because the individuals are
already adapt ed to the given enviro nment. The gene pool of a population is
genetically constant and maintains the adaptiveness in natural popul ation s.
Stabilising selection Mean
lil
c
1
.
Q
~
0>
C
0>
Cl
>.
c
CIl
E
O>
E
1=
13.8.2.1 Salient Features of Stabilising Selection

741. • Stab ilising selection operates in constant or unchanging environment.
• It introduces homogeneity in the populations i.e., reduces varia tion and tends
to maintain the status quo for a particular phenotype.
• It favour s average or interm ediate values in individuals and eliminates
overspecialised as well as specialised or less adapted individuals. It means in an
unchangi ng environment, individuals with extreme values of any trait are at a
disadvantage than the individual s with average measurements.
• It checks accumulation of mut ati ons in the gene pool of population, as these
may lower the fitness of species in unchanging environment.
FIG . 13.14: In stabilizing selection , individuals that possess extreme values of a
characteristic (here, both the lightest and the darkest colors) are selected against
and
die o r fail to reproduce . Over suceeding generations. an increasing proportion
of the
popoulation becomes average in coloration . The changes in the bell- shaped
curve for the distribut ion of
measurements of the phenotypic traits produced by stabilising selection .
• It tends to arrest variance and evoluti onary change, but maintain s adapti
veness.
• Stabilising selection operates rarely because the environment is rarely constant.
• Stab ilising selection is also called centripetal selection.
13.8.2.2 Examples of Stabilising Selection
EXAMPLE 1: Land snails: An example of stabilising selection was described by
British biologi st, W.F.R. Weldon in 190 I. Exact measurements were made of
the shapes and angles formed by the inner whorls of the shells in a population of
land snails. The inner whorls are formed during growth in the youngest age and
once formed they do not change . Weldon discovered that the snail s that
survived successfully for longer periods had measurements neare st to the mean
for the population, whereas the most abnormal individuals with very large or
very small inner whorl tend to be eliminated.
EXAMPLE 2: Sparrows: A well known example of the direct action of
stabilising selection was provided by H.C. Bumpus in 1899. He found that about

742. 136 sparrows were injured or killed in a severe sleet storm in Rhode Island and
the birds that were killed by the storm
average. Of the 72 birds
had abnormally long or short wings as compared to that survived, the majority
possessed characteristics
close to the mean value , i.e., had normal wings and normal body proportion.
This indicate s that the individuals too different from the average tend to be
eliminated during catastrophic events .
EXAMPLE 3: M ortality in Babies: The birth weight of human babies provide s
another example influenced by stabilising selection. The optimum birth weight
favoured by stabilising selection is around 7.3 (i.e., 6.6-8.8) pounds. Newborn
infants less than 5.5 pound s and more than 10 pounds have the highest mortality
rate. The curve for mortality is virtually the complement of the curve for
survival.
Mean birth weight
~
20
Optimum
100

744. birth weight
70
~
50 15
30
"U
(l)
c
n o
20
(l)
;:t
~ 3
:;
0
~ 10 10
::l
a.
7
e!.
'0 ~ c 5
~ 5
3Q.
2
01 I I I I I I I I I I I j
o
1 2 3 4 5 6 7 8 9 10 11
Birth weight (pounds)
FIG . 13.15: Body weight of human infants at birth shows stabilizing selection .
Babies that weigh more or less than average are more likely to die soon after
birth than babies with weights close to the population mean birth weight (around
7 pounds) .
EXAMPLE 4: Red Checkered Moth: In another experiment, the stabilising action
of natural selection on a population was demonstrated by E.B. Ford. In red
checkered moths (Panaxia dominula) , an abnormal mutant variety was obtained
by artificial selection. This variety was released in a locality in Britain where
moths of this variety did not exist earlier. Being sedentary these were not able to
interbreed with wild moths of the species. After five years, the descendants of
these moths had normal wing colouration. This indicates that natural selection
had favoured genetic combination for normal wing colouration.
The stabilising selection leads to the reduction in the variance of characteristics
and the population becomes more homogeneous.

745. 13.8.2.3 Conditions for Stabilising Selection
Stabilising selection operates only under unchanging environmental condition s.
13.8.3 Disruptive Selection or Diversifying Selection
In di srupti ve selection, two or more extre me phenotypes are favoured over an
intermediate phenotype in a previously homogeneous population. It indic ates
that the extreme values have the highest fitness and contribute more offspring to
the next generation and the intermediate or mean values are relati vely
disadvantageous . Disrupti ve se lection is essentially se lec tion for di
versification w it h resp ect to a particular character in a population having rel ati
vel y homogeneous gene poo l.
13.8.3.1 Salient Features of Disruptive Selection
• Disruptive selection operates in heterogeneous environments when more than
one different environmental factors are operating again st th e same phenotype s.
• It also occurs in populations that occ upy areas with differ ent types of
resources for their utili
f--Mean
Intermediate phenotypes eliminated
Extreme phenotypes conserved
~-!-\,....-Eliminated
+-:-+
FIG . 13.16: Disruptive selection: In disruptive selection, individuals with
average phenotypes are selected against and eliminated and the extreme
phenotypes are favoured. As a result a
homogeneous population splits into two or more subpopulations.
sation. Organisms best ada pted for each type of resource survive and diversify.
• It pushes the phenotypes within a population away from the mean, favourin g
the values at the extremes of the variability distribution curve.
• It splits a relatively homogeneous popualtion into two or more adaptive forms.
As a result, the unimodel distribution curve for a character is changed to bi or
trimod el curve of variabi lity distribution .
• It tends to increase variance within the popu lation .
• Disrupti ve selection occurs when a poulation is subj ected to divergent

746. selection pressures in different parts of its area of distribution.
• Disrupti ve selection maintains balanced polymorphism in popul ation s based
on frequency depend ent adjustments to the heterogeneous environm ents.
13.8.3.2 Examples of Disruptive Selection
EXAMPLE 1: Sunflower Population: Stebbins and coworkers studied disruptive
selection in a population of sunflowers in the Sacromento Valley of California
over a period of 12 years. In the beginning, the genetically variable population of
these sunflowers was a hybrid between two species. After five years this
population splitted into subpopulations separated by a grassy area. One of these
subpopulations occupied a relatively dry site and the other occupied
comparatively wet site. During the next seven years , the size of the population
fluctuated greatly in response to differences in rainfall. The differences between
the two subpopulations in response to their own environmental changes were
seen , making them markedly different. This shows disruptive selection occurs
when a population is subdivided into a number of small subpopulations each one
having a slightly different gene pool.
EXAMPLE 2: Mimetic Butterflies: In shallow tail butterfly, Papilio dardanus of
Africa , the males have yellow and black wings with tails. They are similar to
swallow tail butterfly of United States . Their females are without tails. These
females show different forms of mimicry each one accurately mimicking a
different distasteful species of butterflies and all of them occur in the same area.
Mimicry in females is associated with adaptive value of protection during egg
laying.
EXAMPLE 3: Bill Size in Black-bellied Seedcracker Finch: The bimodel
distribution curve of bill sizes in black-bellied seedcracker, Pyrenestes ostrinus
(The West African Finch) , illustrates the role of disruptive selection. In West
Africa , seeds of two types of marshy plants (sedges) are the main source of food
for finches. Birds with large bills can readily crack hard seeds of Scleria
verrucosa . Birds with small bill feed on soft seeds of S. goossensii. Young
finches whose bills deviate markedly from these two types and have
intermediate beak fail to survive because they can not use either kind of seeds
effectively.
EXAMPLE 4: Shell Colour in Limpets: Shell colour in limpets , the marine
mollusc, ranges from pure white to dark tan. They are attached to white goose-

747. neck barnacles or to tan-coloured rocks. The white or light coloured limpets
camouflage with white barnacles and tanned barnacles are indistinguished from
rocks. Hence, they are saved from predatory shore birds. Limpets of intermediate
shell patterns, being conspicuous are preyed by birds resulting in their gradual
disappearance.
Smaller-billed finches feed more efficiently on soft seeds
.-----'
Smaller-billed finches feed efficiently on soft seeds

748. 120 1
100
ills o
f intermediate
'"
80
"E
15
Birds with '0 60 widths can ot use either kind of (j; seed effici tly and survive
poorly.
.0
E
OJ
40
z
20
0
12 14 16 18
FIG . 13.17: Disruptive selection results in bimodal distribution. The bimodal
distribution of bill sizes in the black bellied seedcracker finch of West Africa is
formed due to disruptive selection , which favours individuals with larger and
smaller bill sizes over individua ls with intermediate-sized bills.
EXAMPLE 5: Shell Colour in British Land Snails: Cepaea nemoralis, the British
land snail has wide distribution and a wide habitat. It lives in grass fields, hedge
rows and in forests. In low vegetation areas, the snails with dark shells and
without light coloured bands are preyed by thrushes, while in forest areas snails
with light coloured bands are eaten up. Therefore, the population of snails is
divided into two different phenotypes living in two different habitats. The light
coloured snails with light bands survive successfully in grass fields and dark
snails without bands in forest habitat.
13.8.4 Cyclic Selection
Selection , whether stabilising or directional, is constant from one generation to
next if the selective environment is not fluctuating. But, when environment is not
stable between generations or between seasons, the optimum phenotype and also
the optimum genotype may show fluctuation because of the selection operating

749. in one direction in one generation or season , and in opposite direction for the
next. This type of selection is called cyclic selection.
Cyclic selection helps in maintaining genetic difference s in a population and
fixes all the alleles of the gene pool , because different traits will be
advantageous at different times under different seasonal conditions.
Simultaneous Action of
Three Types of Selections
Selection, whether directional or stabilising , may act in a constant fashion if the
environment is unchanged or uniform. When a population is sub- jected to
divergent or oscillating environments, different genotypes among the members
of a population are established. Since environmental cond itions keep changing,
the three types of selections do not remain separate, but combine in different
ways. The disrupti ve selection may
FIG . 13.18: Disruptive selection has favoured two extreme phenotypes of snails
with and without light coloured
bands on their shell.
be followed by directional selection and may further lead to stabilising selection.
F ox ( 1975) has described the action of all the three types of selections
simultaneously upon different phenotypes of a population of lizard in West
Texas. Fox collected a large number of individual s of the desert side-blotched
lizard and kept them in 2-hectare area and studied their scaly character, etc. He
observed that directional selection has acted on four of the scale characteristics
especially the number of supraocular scales . The disruptive selection has acted
on the number of interfemoral scales (A) in male lizards and stabilising selection
has acted on the number of femoral scales in female lizards (B) Fig. 13.19.
The balan cing selection favours the heterozygotes but it does not eliminate the
homozygotes with low adapti ve value. Rather it maintains the frequency of
different 50% c:
o c:c:
U
o
40%
t55
40
50% ,gQl
~
o
Qi
Ql Ql
In Qi Ql Qi
~
In Qi
In
40% ~ .E
30%~ ~
30
Ql
Ql

750. .E
4.ij
.0 ctl
In
Cl
Ql
30%
~

"E
'
c:
.0
ctl
s
-E
20
's
.~
~ 20% .~
~
20% iil
Ql In
ro
C
'0
C
E
Ql '0
10%
~
ci 10
10% ~ zci ~z Q..
L J~ I

~
3 4 5 6 7 8 9 1026 27 28 29 30 31 32 33 34 35 36 3738
A. Number of Interfemoral B. Number of in femoral scales in male lizards.
scales in female lizards
FIG. 13.19: Action of disruptive and stabilizing selection on the number of
interfemoral and femoral scales in the male and female lizards respectively in a
West Texas population (Adapted from Dillon, 1978).
genotype s more or less constant. For example, if there are three genotypes AA,
Aa and aa in a population, the frequency of gene A and a in the population is
maintained more or less constant for a significant number of generations. In
man, sickle cell anaemia is caused only in heterozygotic condition. In some
population s its frequency is maintained at 30 to 40 per cent. Even some-lethal,
semi lethal and subvital alleles are also maintain ed as a part of gene pool
because of their ability to confer selective advantage in heterozygotic comb
ination.
13.9 FREQUENCY DEPENDENT SELECTION
The frequ ency dependent selection can be described as a form of balancing
selection which maintains balanced polymorphism in sexually reproducing
animals and also maintains sexual dimorphism.
It has been demonstrated at the level of primary gene product s and also at the
level of sexual dimorph ism. The experimental popu lation of Drosophila had

751. two different alleles for an esterase or for alcohol dehydrogenase in a definite
proport ion. The viability of homozygous genotypes was observed to be
indirectly proportional to the gene frequency of the allele.
Th e above results can be explained by assuming that the environm ent, occupied
by the population , is heterogeneous and can be divided into subniches. These
may provide distinctly different microcl imate (temperature, humidity, etc.).
Each of these subniches is capable of supporting only a limited number of
individuals and is called the carrying capacity. If the carrying capacity of a
subniche is x for homozygotes AA and the frequenc y of gene A increases so
that the number of AA homozygotes is more than x , the selection will disfavour
A until such time as its gene frequency approaches the carrying capacity x , but
not below that. If because of some reasons the number of individuals with AA
falls below the carrying capaci ty x , there will be more opportunities for AA
homozygotes to survive. Different subniches will be characterised by different
gene frequen cies of various alleles. Such a system stabilises the genotypes
regardle ss of whether the heterozygote s were favoured or not.
13.10 HETEROZYGOUS ADVANTAGE OR HETEROSIS
13 .10 .1 Definition
The enhanced fitness of heterozygotes relative to homozygotes for characters
such as
longevity, fecundity and resistanc e to disease is called hetero zygot e superiority
or heterozygote advantage. This phenomenon is also called hetero sis or hybrid
vigour.
EXAMPLE :
The hybrid offspring of different species are frequently larger, produce
more seeds and are of great ecological vigour. Dramatic increase in agricultural
yield of hybrid corn and other grains, fruits and vegetables is the result of hybrid
superiority. In animals, however, hybrids are sterile, though in sibling species of
Drosophila, about 50 percent of genetic loci are represented by different alleles.
It means those species which are close ly related to form hybrids , carry different
alleles at about 50 percent of their loci.
13.10.2 Reasons for Hybrid Superiority
Two main th eories have been put forward to explain heterozygote superiority: 1.

752. Shephard has, suggested that hybrid superiority is because of pleiotropy i.e.,
products of most genes are involved in more than one physiological or
ontogenetic
process . Such forms are called allotypes. Selection favours allotypes that have
the
best all round effects. Usually the gene product of a given allele will be
dominant
in one proces s and recessive in another. In a heterozygote, the dominant effects
of
the allotype will be expressed and the less favourable recessive effects will be
suppre ssed .
2. Haldane suggested that the heterozygotes are biochemically more versatile
than
homozygotes. During the course of its life an individual has to cope with more
than
one external environments. Naturally, a heterozygote, slightly different
genetically
and producing two allotypes is better off than a homozygote. An individual
might
just survive a sequence of environmental catastrophes, if it has different enzyme
s
to cope with different environmental catastrophes.
This shows that heterozygotes are more fit than homozygotes and that the
individuals
heterozygou s for more loci would be more fit than those heterozygous for fewer
loci.
13.11 BALANCING SELECTION AND BALANCED POLYMORPHISM
Selection that fa vours heterozygotes is called bala nci ng selection. It maintain s
a high degree of balanced genetic polymorphism in a population (as opposed to
directional selection which leads to transient polymorphism). Balanced
polymorphism is the persistence of different genotypes through heterozygote
superiority. In balanced polymorphism, the genetic polymorphism is maintained
unchanged for a number of generations.
EXAMPLE:
In human population, the presence of sick le cell gene over the
generations is an examp le of balanced polymorphism . Sickle cell heterozygotes
with HbA/Hbs alleles survive the infection with malarial parasite more

753. successfully than normal HbA/Hb
A
homozygotes. The homozygotes Hbs/Hb
s
develop sickle cell disease and do not survive . The simple reason for the
persistence of gene Hbs in notable frequencies in geographical areas where
malaria is endemic is because it provides protection against malaria. This
example shows that even lethal alleles may stay in a population if they confer
some heterozygous advantage.
Fr equency dependent selection also operates at the level of sexes in a
population. The individuals intermediate betwe en two sexes (intersexes) are
selectively disfavoured. One sex does not replace the other. Rather, one sex (say
female) is in scarcity, the chances of other females reproducing are increased,
i.e., their Darwinian fitness or adaptive value increases.
The gen e frequency depend ent selection maintains a constant frequency of the
particular gene or genes in a population. At equi librium, the alleles are
selectively equivalent or neutral and selection stops operating at the locus in
question.
"£1:".'1' Frequency dependent Selection on an Esterase locus in
Drosophila
melanogaster. The Adaptive value 'w' varie s according to the Gene Frequency at
the beg inning of Experiment (Adapted from Kojima and Yarborough, 1967). Aa
ptiv e Values (w) of the three gen otypes Percentage of F at the start
of experiment
70
50
30
15
FIF Genotype FIS Genotype SIS Genotype
0.44 0.68 1.00
0.77 0.94 1.00
1.00 0.89 0.83
0.97 1.00 0.40
"'1, -11::&''') Characteristics asso ciated wlth r- and k- selection Characteristic
1. Climate

754. 2 . Competition
3. Resources
4. Resource utilisation
5. Population Growth
6. Survival
7 . Life span
8. No. of offspring
9. End result
rs election k-s election 1. Occurs in variable or unpre
dictable climatic changes
1. Occurs in fairly constant or
relatively narrow cl imatic changes
2. Density dependent
3. Limited
4. Slow and uniform
5. Efficient resource utilisation
6. Survival rate is uniform and rate of mortality constant 2. Density independent
3. Abundant
4. Rapid
5. Rapid
6. High mortality at younger stages and high survival at later stages
7. Relatively short
8. Large
9. Increased productivity helps rapid multipl ication and spread of population.
7. Long
8. Few
9. Increased efficiency for better survival.
13.12 r-SELECTION AND K-SELECTION Organisms exhibit two alternative
strategies for increasing their populations. These are r-selection and k-selection
strategies.
13.12.1 r-selection
Pop ulations that increase rapidly due to high fecundity exhib it r-selection, such
as bacteria, diatom s, plant weeds, parasitic animals and lower invertebrates.
Populations show rselection only when they are subjected to rapidly changing

755. environments, highly fluctuating food resource s, have low population densities
and have plenty of opportuniti es to increase and spread. Such populations produ
ce more offspring and show little or no parental care.
r-selection is mainly concerned with rapid population growth by produ cing
more offspring and early sexua l maturity.
13.12.2 k-selection
Populations that occupy more or less uniform or predictable environments and
have population sizes close to the environmental carrying capacity (k), exhibit k-
selection. In k-selected populations, there is density dependent competition for
food, nesting and other resources. The k-selection increases viability of
individuals and their ability for ecological intr specific competition. Such
organisms exhibit delayed reproductive maturity, produce less number of
offspring and greater parental care.
Thus k-selection is more concerned with the survival success of the population
than their increase as in r-selection. However, r and k-selections are not strict
alternatives. In some populations, a compromise between the strategies is also
observed.
13.13 SELECTION PRESSURE OR SELECTION INTENSITY
Selection intensity (I) is defined as the difference in survival rates between
optimal (So) and suboptimal (S.) phenotypes, multiplied by the frequency of
suboptimal phenotypes (f.) in any population, i.e.,
I = (So S.) x f.
Selection intensity = {survival rate of optimal
- survival rate of suboptimal} x frequency of suboptimal From the above
equation, following conclusions are evident:
(a) When selection pressure is zero , all phenotypes are optimal, i.e., frequency
of
suboptimal phenotype (f.) is zero. These conditions are usually found when an
organism is introduced into a new environment.
(b) When selection pressure is one, all phenotypes are suboptimal and survival
rate is zero (So S. = 1). The frequency of survival of suboptimal is zero.

756. (c) Existing natural populations are exposed to selection pressure between these
two extremes.
(d) Increase in the value of f. and (So S.) increases selection pressure.
13.13.1 Factors Increasing Selection Pressure
1. Population Size: When population increases in size, certain environmental
factors become limiting, such as food availability in case of animals and
intensity of light in case of plants . This causes competition for resources among
members of the same population. Individuals with characters of competitive
advantage will utilize resources, survive and reproduce while those with
characters of competitive disadvantage may die before reproducing. Both
environment and population size operate simultaneously to produce selection
pressure.
2. External or Environmental Factors: These may include increase in the number
of predators, pathogens, food, light, water, mineral salts , changes in the climatic
conditions, change in habitat, seasonal variation, etc. All these factors exert new
selection pressures.

758. 326 ~ Evolutiona ry Biology
13.13.2 Result of Selection Pressure
1. Th e selection pressure is a mean s of increasing or decreasing the spread of
an allele within the gene pool of a population or species. These changes in allele
frequency lead to evolutionary change.
2 . This may cause over spec ialisation in individua ls for a particular mode or
modes of life.
3. Increased uniformity and dependency by a species and its overspeciali sation
may lead to the extinction of that spec ies.
It means, selection pressure is a conservative mechani sm that selects pheno
types best adapted to the prevailin g environmental conditions.
13.14 SELECTION AND REPRODUCTION 13.14.1 Natural Selection and
Self Reproduction
Lit erally, if process of self reproduction is perfe ct, there is no room for natural
selection, but the proc ess is never absolutely accurate and deviatin g mutants
appear from time to time. If the mutant from reproduce themselves less
efficiently than the ancestral units, these are eliminated. But if mutati on forms
are more efficient, they win over the ance stra l form. Moreo ver, if mutant s can
maintain themselves in an environment unsuitable for the ancestor, the ancestral
and mutant forms may continue to exist side by side. Therefore , natural
selection in self reproducing system may lead to diversific ation.
13.14.2 Selection in Asexual and Self-fertilising Organisms
Indi viduals either reproducing asexuall y or sexually by self-fertili sation
usually pass on the same genotype to their offspring. Such offspring constitute
"pure races", unless mutation s intervene the geno type of such organisms and
their offspring remain the unchanged. When a mixture of pure lines is exposed to
natural selection, the multiplication of some lines may be favoured and other
lines may be discrim inated. Harlan and Martini planted a mixture of eleven
varieties of barley in seve ral regions of United States having different climates
and other environmental conditions. After seve ral years, their mixture was
planted and harvested in the same regions. The proportional varieties of the
mixture was found to have altered considerably.
13.14.3 Selection in Sexual Cross -Fertilis ing Populations

759. In sexually reproducing and cross-fertil ising populations no two individuals are
genotypically identical except the identical or monozygot e twins. Every cross
breeding or interbredding population posse sses genes in different frequencies.
Natural or artificial selection may increase the frequencies of some gene s and
decrease the frequen cies of others in the gene pool of the population. Since new
genotypes are constantly produced by cross-fertilisation , selection has an
opportunity to exerci se its effect and a new materia l to work upon.

761. 13.14.4 Selection and Hybridisation
Success of selection depends upon the genetic variability present in the
population to which the selection is applied. In sexual and self reproducing
populations there are no chances of variations except mutations. Therefore,
selection in such populations can only isolate the best genotypes already present
at the beginning of natural selection. In sexually reproducing cross fertilising
animals hybridisation regularly introduces new genotypes and new combinations
of genes and provides more chances for the natural selection to operate upon.
13.15 SELECTION AND MUTATIONS
If selection against an unfavourable recessive character were to continue over a
long-period of time , the recessive gene might become eliminated entirely from
the population. However, recurrent mutations do add additional recessive alleles
periodically to the gene pool of population and the recessive gene is never lost
completely. It means forces of selection pressure and mutation pressure are
antagonistic and oppose each other so that an equilibrium between these
opposing force s is established.
The equilibrium frequency of a mutant allele of mutation frequency and
selection coefficient. the equilibrium for allele frequency also increases, but
when selection coefficient increases , the equilibrium allele frequency decreases.
Also the modifier genes (alleles) present at other loci change the degree of
dominance of a deleterious dominant or partially dominant gene .
in a population is the function When mutation rate increases,
13.15.1 Effect of Selection on Genotype Frequency Favouring the Dominant
Allele
In order to find out frequency of A
2
genes in the next generation, add one half of
the contribution of heterozygotes A IA
2
plus the contribution of A
2A 2
and divide
by new total i.e.,
q2(I s )+ pq l-sq 2
ts, is new frequency of gene A
2
)
Here s represents selection disadvantage of type A2A 2•
The change in frequency (b.q) which has resulted in one generation as a result of

762. selection will be the difference of new and the old gene frequency of gene A
2A 2
i.e., b.q = ql q
By substituting the value of ql
Sq
q2(I
=
s) + pq
q
q sq' -

764. Co nditions of dominance and selection
1. No dominance selection against gene A
2
2. Com plete dom ina nce selection against A
2
A
2
Initial frequency and fitness of genotype Change of frequency t.q of gene A2
A , A
1
A, A
2
A
2
A
2
[Y, 2pq, q2
1, 1-1 /25, 1-5
1/ 2sq (1 -q) 1-sq2
1, 1,1-5 1,1,1-5
sq2 (1-q) 1_sq2
3. Complete dominance selection against A, 1- s, 1- s, 1
5q2 (1 -q) 1-5 (1 -q)
2
4 . Overdominance selection against A,A, and A~2 A l
1
-
5
"
1
,
,
5
2
pq(5,p -s2q) 1- s,p2- S2p2
If the above equation is simplified
sq 2(1_q)
Sq = I _ sq' but I q = p
Sq
=
_ sq2p 1_ sq' If sq is small , the denominator is essent ially equal to one and the above formula can be
further simplified to
Sq = -sq2p
With same type of reasoning it is possible to calculate the effect of selection on the frequ ency of the
dominant gene A10 or of both A I and A
2
, when homozygous or hetero zygous.
If s, p or q is small , selection will act very slowly and that selection is most effective at intermediate gene
frequency. It becomes least effective when the mutant is at a very high or very low frequency. Further
selection against a recessive mutant is highly ineffective when that mutant is at a low frequency, because the
gene is carried by both homozygous and heterozygous individuals and full force of selection acts only on
homozygous individuals A
2A 2-
Since defective homozygous individuals are less frequent than heterozygous
individuals, a large proportion of deleterious recessive allele is not exposed to selection.
If selection is directed against a deleterious dominant, the gene is expre ssed and exposed to selection both
in homozygous as well as in heterozygous condition. If individua ls possessing the dominant deleterious gen
e do not leave offspring, the gene will be eliminated except for new mutati ons. It means selection is most

765. effecti ve for domi nant mutations.

767. 13.16 SELECTION AND VARIATION
Homozygously uniform populations offer no opportunity for selection and there is no noticeable
evolutionary change in allele or genotype frequency. This means genetic variability and change in the
frequencies of alleles or genotypes is essential for fitness selection.
13.16.1 Fisher's Fundamental Theorem of Natural Selection
It has been observed that the greater is the genetic variability upon which selection for fitness acts, the
greater is the expected improvement in fitness. Based on these observations, Fisher (1930) proposed the
fundamental theorem of natural selection. It states that in mathematical terms the fitness of a population
should increase at a rate that is proportional to the genetic variability or genetic differences in fitness
present in the population.' For example, if individuals of a population in a particular environment were
completely homozygous for all genes, selection for fitness in the changed environment would be unable to
produce any genetic improvement. It means genetic variability must be maintained for population to survive
.
If s, p or q is small, selection will act very slowly and that selection is most effective at intermediate gene
frequencies. It becomes least effective when the mutant is at a very high or very low frequency. Further
selection against a recessive mutant is highly ineffective when that mutant is at a low frequency, because the
gene is carried by both homozygous and heterozygous individuals and selection acts only on homozygous
individuals A
2A 2-
Since defective homozygous individuals are less frequent than heterozygous individuals, a
large proportion of deleterious recessive allele is not exposed to selection.
When selection is directed against a deleterious dominant, the gene is expressed and exposed to selection
both in homozygous as well as in heterozygous condition. If individuals possessing the dominant deleterious
gene do not leave offspring, the gene will be eliminated except for new mutations. It means selection is most
effective for dominant mutations.
13.16.2 Red Queen Hypothesis
Populations tend to change genetically in the direction of enhanced fitness for their environment. Recurrent
genetic variability in factors such as differential mortality, differential fecundity and differential mating
success and gene frequency changes are constant features of all evolving populations facing changing
environment.
Van Valen (1976) proposed that Red Queen Hypothesis is applicable to all evolving populations. This states
that adaptive evolution of one species of a community causes a deterioration of the whole environment for
all other species. As a consequence, because of environmental changes, species continually face new
selective challenges, and constantly confront recurring threats to fitness. In order to survive (to stay in the
same place), they need to overcome the competitive challenge. This constant arms

769. race of populations with the changing biological environment is described as Red Queen reign. It leads to
increasing morpho logical complexity in living organisms.
13.17 SELECTION AND ADAPTATIONS
___(THE BALDWIN EFFECT} _
Natural sel ection acts to bring about adapta tions. It has been found that the adaptations can arise as a
direct response to the environment or to the needs of organisms (Lamarcki an theory) . Some recent
experiments conducted by Waddington favour his conception. This is known as Baldwin effect. Waddington
exposed a number of wild type Drosophila to temperature shock during development. As a result of heat
treatment, these flies developed cross-veinless wings. The cross-veinless condition was not produced by the
mutation caused by heat treatment, because such flies did not posses s a cross-veinless mutation. It means it
was a direct effect of the environment. Such an environmentally induced condition that simulates the
phenotype of a genetic mutant is known as phenocopy. The experiment was repeated for several
generations by breeding the cross-veinless flies together and treating their offspring with heat shock. After
15 generations, the heat treatment was discontinued, but it was found that cross-veinless flies still cont
inued to appear.
At first th is result was considered with the Larmarck's theory of inheritance of acquired characters. But
actually the wild type individuals used here were found to possess some genes, polygenes or multiple factors
that could produce the cross-veinless phenotype only under the unusual environmental conditions provided
by the temperature shock . The selection over a number of generations has increased the frequency of these
genes in the population to such an extent that the individual genotypes now carried enough genes to
produce cross-veinless phenotype. This indicates the response of individuals to new environmental pressure
could not be incorporated through selection into the population as a whole.
• Allele frequency
• Balancing selection
• Centrifugal selection
• Differential reproduct ion
• Disruptive selection
• Frequency dependen t selection
• Heterosis
• k-selection
• MRSA (Mithicillin Resistance staphylococcus aureus)
• Red Queen Hypothesis
• Selection coefficient
KEY TERMS
---------_._--
• Balanced polymorphism
• Baldwin effect
• Centripetal selection
• Directional selection
• Fisher 's fundamental theorem
• Genotype frequency
• Heterozygote advantage
• Lederberg replica plating
• r-selection
• Relative fitness
• Segregational load

770. • Stabilishing selection
• Survival rate

772. REVIEW QUESTIONS
I. Justify the statement ' natural selection is differential reproduction. ' 2. (a) How are selection and fitness
related?
(b) Why is selection more effective against an alle le in haploids than in diploids?
3. How would you define fitness? Equate fitness with survival.
4. Why is selection generally less effective in diploids against rare deleterious recessive alleles than against
deleterious recessive alleles?
5. Why does heterozgote superiority lead to allele frequency equilibria?
6. What are different types of natural selections? Explain with a suitable example the role of stabilising
selection.
7. Differential between different types of natural selection. Under what circumstances each one of them
operates? Discuss simultaneous action of the three types of selections .
8. Write short notes on: I. Heterosis
4. Selection coefficient 2. Directional selection 5. Segregational load 3. k-selection 6. Phenocopy
9. How can natural selection act to promote the establishment of reproductive isolation mechanism and how
it will contribute to the formation of new species? 10. What selective conditions can explain balanced
polymorphisms and persistence of harmful alleles in popu lations?
II. Selective success for increased fitness depends on genetic variability. Can an increase in fitness occur in
the absence of new mutation? Explain.
FURTHER READINGS
I. Endler, lA., 1986. Natural Selection in the Wild, Princeton University Press, Princeton, NJ.
2. Fisher, R.A ., 1930. The Genetical Theory of Natural Selection, Chlarendon Press, Oxford, U.K. (2nd ed)
1958, Dover, New York.
3. Gillespie, lH., 2004. Popu lation Genetics: A Conc ise Guide , 2nd ed. , Johns Hopkins University Press,
Baltimore, MD .
4. Hallgrimsson, B. and B.K. Hall (eds .), 2003 . Variation: A Central Concept in Biology,
Elsevier/Academic Press, Burlington, M.A.
5. Hartl, D.L., and A.G. Clark, 1997. Principles of Population Genetics, 3rd ed., Sinauer Associates,
Sunderland, M.A.
6. Hedrick, P.w., 2000 . Genetics of Populations, 2nd ed., Jones and Bartlett, Sudburry, MA.
7. Majerus, M.E.N., Oxford, U.K.
8. Orzack, S.H. and 1998, Melanism: Evolution in Action. Oxford University Press,
E. Sober. (eds.), 200 I. Adaptation and Optimality, Cambridge University Press , Cambridge.
9. Pagel, M. (ed. in chief), 2002. Encyclopedia of Evolution, 2 volumes, Oxford University Press, New York.
10. Hall, B.K. and 8. Hallgrimsson, 2008. Strickberger's Evolution . 4th ed., Jones and Bartlett Publishers,
Sudbury Massachusetts.
DOD

773. 14
Evolution of Genes and Genomes
MOLECULAR EVOLUTION
At the molecular level, biological evolution is the result of changes in the genetic makeup of populations
over generations. The total number of genes present in all the members of a population is called its gene
pool and the entire complement of DNA sequences in a cell or in an organism is its genome. With the
advancement of technologies in molecular biology and biotechnology, evolutionists are able to study
evolutionary changes in the genes, genomes and gene pools of populations and to explain how organisms
undergo mutations and get adapted to new environments or avail new resource. To trace phylogenetic
relationship among organisms molecular comparisons are carried out at the level of DNA structure, whole
genome composition and amino acid sequences in individual proteins.
• Differences in amino acid sequences in individual proteins provide information about the evolution of
individual genes .
• Comparison of amino acid changes in orthologous proteins (proteins from different species that are
sufficiently similar) can help in determining phylogenetic distance between species .
• Homologies between DNAs from different sources can be determined by measuring degree of DNA
hybridisation.
• Nucleotide changes in 58 ribosomal RNA can be used to develop a phylogenetic tree.
• The rate of molecular changes over the time for some genes can be used as molecular clock. Different
parts of the genome have different clocks .
14.2 MOLECULAR PHYLOGENIES
Molecular phylogenies include 'the study of phylogenetic relationship in the organisms of different groups.
This relationship is based on molecular information obtained by
334 ~ Evolutionary Biology
comparing sequences of amino acids and their molecular configuration in different proteins and by
comparing sequences of nucleotides in DNA and RNA molecules.
14.3 PROTEINS AND PHYLOGENETIC RELATIONSHIP
Sequenci ng of amino acids in proteins by biochemical methods, comparing such sequences for the same
protein in different species and calculating the number of mutations necessary to convert one amino acid to
another has helped in constructing molecular phylogeny of present day living forms .
14.3.1 Haemoglobin Phylogeny
Haemogl obin, the iron-containing protein present in RBCs of vertebrates is very importan t for the
transport of oxygen and for the survival. A number of haemoglobin-like molecules are found in a wide
range of organisms from inverte brates to vertebrates and in plants, fungi and bacteri a. Moreover,
haemoglobin-like molecu les, such as myoglobin, occur in other body tissues also.
Huma n haemoglobin (HbA) molecule is a tetramer, formed of two a and two p polypept ide chains (2a 2P).
Some adult haemoglobin has 8 chains instead of p-chains i.e., 2a 28 while foetal haemoglobin (HbF) has
two y-chains instead of p chains (i.e., 2a 2y). In some
E
chains are present instead of p chain. Foetal
haemoglobin carries almost 30% more oxygen than adult haemoglobin. The study of amino acid sequence
in these five haemog lobin chain s (a, p, y, 8 and E) shows that all these globin genes arose by the

774. duplications of common ancestral locus.
The o riginal haemoglobin molecule was a unimer and chemically related to myoglobin , i.e., initially both
of them were identical. Myoglobin in tissue cells evolved into oxygen stor age protein and haemog lobin in
blood evolved into a transport protein for oxygen transport and graduall y changed into a tetramer.
Haemoglobin molecule differs in different species. It means:
• Different kinds of globin chains arose during evolution due to mutation in the globin genes.
• Each particular globin chain followed its own evolutionary path because of independent gene mutations.
• Evolution involved changes in the amino acid sequences in globin chains in different species and groups.
Haemoglobin is a highly conserved protein molecu le. It is possible to establish a time scale for the
evolution of different haemoglobin chains. Linus Pauling has estimated that one amino acid subst itution
has occurred every 7 million years during haemogl obin evolution.

776. 1. M yoglobin differs from all haemoglobin molecules at more than 100 sites. This indicates that myoglobin
and haemoglobin separated at least about 650 million years ago by the first duplic ation of myoglobin
locus. This one chain haemoglobin is still found in primit ive fish, the cyclostomes.
2 . The a chain differs from p chain at 77 sites. It is estimated that a second duplication of the same locus
led to the differentiation of a and pgenes about 500 million years ago, when first bony fishes appeared.
3. During mammalian evolution some 200 million years ago, p locus duplicated again to give rise to y gene
that codes for foetal haemoglobin.
4 . pchain differs from y chain at 39 sites but from delta (8) chain only at 10 sites.
5 . The delta gene (8) evolved only about 40 million years ago by the most recent duplication of p locus .
This occurred prior to the
Ancestral chain
1.0 Billion years ago
200 Million years ago
40 Million years ago
Myoglobin Alpha Gamma Beta Delta
a y p 1>
FIG. 14.1: Phylogenetic tree showing molecular evolution of haemoglobin chains from myoglobin chain.
The circles and years represent the time when ancestral genes duplicated and mutated.
separation of old world monke ys, apes and humans .
14.3.2 Evolution of Cytochrome c
Cy tochrome c is another evolutionary conservat ive molecule that acts as a respiratory enzyme. It is formed
of slightly over 100 amino acids. It has 104 amino acids in vertebrates and a few more in certain lower
forms.
1. Som e amino acids in cytochrome c have not changed at all, e.g., amino acids at 20 specific posit ions are
unchanged from mould to man.
2. One region of cytochrome c formed of eleven amino acids (in position from 70 to 80) has remained
unchanged in all organisms.
3. The cytochromes of closely related vertebrates either do not differ or differ in only a few amino acid
residues. For example, cytochrome c of man and chimpanzee have identical composition.
4. The greater are the phylogenetic differences, the more is the difference in cytochrome composition. For
example, human cytochrome c differs from cytochrome of Drosophila in 24 amino acids and from
Neurospora in 40 amino acids .
Such variations in amino acid sequences can be explain ed on the basis that some of the substitution
mutations are neutral , i.e., neither they have any evolutionary advantage nor disadvantage. So they are
preserved by chance (genetic drift).
14.3.3 Evolution of Mammalian Antidiuretic Hormone
A ntidiuretic hormone, vasopressin, is an octapeptide produ ced by hypothalamus. It promot es reabsorption
of water by kidne y tubules. It occurs in three forms in vertebrates:
Lysine Vasopressin

777. (Pig)
(Birds. Mammals) Oxytocin
Arginine Vasopressin (Mammals)
Third amino
acid
Oxytocin

778. Birds. Mammals) /
Arginine Vasotocin
(Fish, Amphibians, Birds)
FIG . 14.2: Phylogenetic relationships of octapeptide hormones in vertebrates.
1. Ar ginine vasopressin is mammalian antidiu retic hormone. It is found in most mammals includ ing huma
ns.
2. Lysine vasopressin is found in pig.
3. Arginine vasotocin is present in amphibians, reptiles and birds. This hormon e is also present in bony
fishes but is not antidiuretic.
It means vasopress in appeared in vertebrate phylogen y much earli er and acquired antidiuretic function
later. The functional variants of vasopressin evolved by single base substitution in DNA molecule that codes
for arginine vasotocin. The mammalian arginin e vasopressin is formed by the substitution of third amino
acid isoleucine (lieu ) by phenylalanine. In pig, the 8th amino acid arginine is replaced by lysine.
Oxytocin, another octapeptide is secreted by hypothalamus. In mammal s it acts as lactogenic hormone. But
this hormone evolved in fishes million years ago from arginine vasotocin by a single amino acid
substitution. The arginine of vasotocin is replaced by leucine in oxytoc ine. Origin of oxytocin reveals that a
new produ ct may arise by the modification of a pre-existing product and a signle amino acid substitution
may cause marked change in function.
14.4 ORIGIN AND EVOLUTION OF NEW GENES
G enes in a species genome can originate or modify either from pre-existing genes in the same genom e or
from gene transfer from the genome of different species. The various mechani sms assoc iated with the
evolution of genes include: gene dupl ication, lateral gene transfer, exon shuffling, gene chimeri sm,
retrotransposition and motif multiplic ation.
14.4.1 Gene Duplication and Multigene Families
M any genes get dupli cated either in the form of a large blocks of chromosomes, or as parts of whole
genomes by polyploidy. In gene duplic ation a particul ar gene locus gets dupl icated and the two genes
with identic al base sequences coexist in the same chromosome. The repeated duplications of same gene
loci have resulted in gene families or multigene families (the group s of related genes within an organism's
genome). Such duplicate genes are called homologous genes. Based on molecular phylogenies homolo gous
genes are of two types: Orthologous genes and Paralogous genes.
I . Orthologous Genes (Gr. Orthos = exact): The se are two or more homologous gene loci of the same gene
family, present in the gene pool of different species. They show homologous relationship in nucleotide
sequences. Ortholo gous genes arise from a common ancestral gene and diverge only after the speciation or
phylogenetic splitting of the organisms. For example, cytochrome c genes in humans and dogs are
orthologous .
T hough these genes serve the same function , the gene's sequence in humans has become different from that
in dog only after their divergence.
2. Paralogous Genes (Gr. para = in parallel): These are two or more homologous genes or gene loci
present in the same genome or in the members of same species . They show homologous relationship in
their nucleotide sequences and arise by gene

779. (a) Evolution of orthologous gene by speciation (b) Evolution of paralogous genes by duplication within a
species
An cest ral gen e Ance st ral gen e Ance st ra l spec ies
...
Sp ec ies
C
...
Gen e duplic ation and div erg en ce
Orthologous genes
Two new spe cies : Species A Species B Two pa ralogous genes Species C after many generation s
FIG. 14.3: Difference in the origin of orthologous and paralogous genes .

781. Gene duplication
A
A, 6,
II
Unequal pairing and cro ssing over
Gen e fusion
FIG. 14.4 : The results of unequal crossing over for three gene segments on a chromosome because of
unequal pairing between homologous chromosomes.
dupl ication. They can diverge within the same species because they are present in more than one copy in
the genome. For example, more than 1,000 copi es of olfactory receptor genes are present in humans and
mice . They confer sensitivity to a wide variety of odors.
14.4.1.1 Mechanism of Gene Duplication
Duplication of gene or gene s can occur:
(i) due to unequal crossing over during proph ase I of meio sis (ii) slippage during DNA replicati on
(iii) due to transposable elem ents which can provide homo logous sites for non sister chromatids to cross
over.
14.4.1.2 Significance of Gene Duplication
Gen e duplication has played an important role in evolution because it increases the numb er of genes in the
genome . Since one locus is necessary to produce the polypepti de chain, the additional locus becomes extra
. But it may prove valuable in protec ting the individual against any harmful mutation in that locus. A lethal
Norma l gene Normal gene Mutation only
lr Mutat ion
""-"'~~~
Gene duplication and mutation
Ar Mutation
Abnormal pro tein Normal protein IAbnormal prote in (inconsequential)
Ad ditional mutations and selection
I Three mutations
... 1 2 3
in duplicate gene
Selection against defective gene A
Two genes with different functions B
FIG . 14.5 : Duplication of a gene followed by random mutations , and natural selecti on can lead to two
genes with different funct ions A. In the absence of gene duplication, a mutat ion
would be lethal if the mutant gene produces a harmful or non function al protein ; B. With mutations in the
dupl icate gene , a new protein is produced with a modified function.
Evolution of Genes and Genomes Iil 339
mutation can remain in the genom e of an individual , in case it occurred in the duplicat ed locus, because
the normal origin al gene would continue to synthesise the essential polypeptid e.
In case a number of mutations occur in the dupl icated locus it would produce a polypeptid e much different
from the original polypepticde. In due course of time, the two polypeptide chains, each with its own gene
evolves. When a gene is represented by two or more loci lying close together on one chromosome, they form
gene cluster. The cluster of same gene loci is called gene fa mily.

782. 14.4.1.3 Examples of Gene Duplication
EXAMPLE
1: Hae moglobin M uItigene Fa mily
In human beings a gene clu ster has a l and az genes lying side byside . They arose by tandem duplication
and produce al and az polypeptide chains . The b gene clu ster has seven such genes located one after the
other. In this cluster 5 beta genes are active and two inactive . The five active genes are beta (b), delta (d),
two gamma (Gg and Ag) and epsilon (e). The two inactive genes are represented by psi (y). This cluster of b
genes spans 60,000 nucleotides on chromosome II. The a gene cluster is located on 16th chromosome and
has seven genes (Fig . 14.6 A).
Evolution of Human Globin Gene Family
Th e clusters of a -globin and Pglobin genes show how duplic ation has led to the evolution of several genes
with related function s. The ancestral globin gene underwent duplication and divergence into the a and P-
globin genes about 450-500 million years ago. Each of these genes duplicated seve ral times and the copies
accumulated mutations independently over numerous generations. As a result, a and P gene families evo
lved. Random mutations in these genes over the time destroyed their function and produc ed pseudogenes.
A. A lpha Globi n gene Clust er (Chromosome 16)
Inact ive
,-"-----.,
yb
G
9
A
d b
2
e 9
aJaJ CD
B. Beta globin gene cluster
(Chromosome 11 )

784. 60 50 40 30 20 10 o
! ! ! I 1 ! !
Kb
FIG. 14 .6: Two families of human haemoglobin genes occur as clusters on two different chromosomes. The
a-cluster has around 30,000 base pairs on chromosome 16 while b-c1uster has 60,000 base pairs on
chromosome 11 .
Ancestral globin gene
Q)
:§
e
III
c
.2
"'5
Further duplications
g and mutationsw
1; 1jI 1jI", 1jI. , a , a , 1jI
T
y

A
Y1jI1\21j1" Ii
aglobin gene family pglobingene family on chromosome 16 on chromosome 11 FIG. 14.7: Evolution of the
human p-globin a nd bglobin gene families from a single ancestral globin gene. Genes represented by green
boxes are pseudogenes (nonfunctional).
The sim ilarity in the am ino acid sequences of different a -g lob in and P-globin polypepti des supports the
evolution o f gene families by duplication and mutations. Fig . 14.8 shows the estimated time wh en different
g lo bin genes di ve rged and the estimated number of nucl eotide replacements that caused am ino acid
changes in each po lypeptide.
The cl ustered organi
sation of P-g lobin genes in five di fferent primates (including man) and in rab
bit is shown in Fig. 14.9. The ge nes marked with~ represent ps eudogenes that are non-functional.
(
81)
Epsilon cha in(1:) Epsilon chain(1:)
Epsilon chain(1:)
Epsilon chain(1:)
Gamma chains Early
(' y, Gy)
vertebrate Deltachain (S) myoglobin-like
molecule
Beta chain (p)
Myoglobin 700 600 500 400 300 200 100 0
Millions of years ago
FIG . 14.8: Phylogenetic relationships between globin-type proteins found in humans showing the estimated
times when they diverged from each other and the estimated
number of nucleotide replacements that occurred for amino acid changes in each branch of the lineage.
O ccurrence of two immediately adjacent functional genes is a common feature of ge ne cluster. Presence of
multimers and arising by gene duplication by th e aggregation of polypeptide chains, is considered to be an
important event in protein evolutio n.

785. EXAMPLE 2: Evolution of Multigene Family for Enzyme Lactate Dehydrogenase
The enzyme lactate dehydrogenase (LDH) is used in glycolysis for the lactatepyruvate reaction. It occurs in
multiple molecular forms as isozymes in different species and even in different tissues of the same organism.
LDH is a tetramer having 2H and 2M subunits. There are two corresponding genes associated with their
synthesis. The subunit M encourages the accumulation of lactate from pyruvate, while subunit H minimises
or does not promote conversion of pyruvate to lactate. The occurrence of these subunits in different
combinations in different tisues is an evolutionary modification which enables the tissue cells of the heart
muscles and body muscles to utilise pyruvate according to their need.
A s ingle amino acid mutation caused the conversion of LDH to MDH malate dehydro genase. This change
has resulted by the substitution of arginine for glutamine at the I0 2nd polypeptide position, but the two
units have oppo site functions.
EXAMPLE 3: Evo lution of Genes with Nove l Functions
The duplicate copy of the gene in some cases has undergone modification to produce a proteins of an
entirely unrelated function. Genes for lysozyme and a -lactalbumin are such orthologous genes. Lysozyme is
an enzyme that hydrolyses bacterial cell walls and protects against bacterial infection. a-lactalbumin is
nonenzymatic and is associated with milk production in mammals.
Birds have one gene for lysozyme only but mamm als have both the genes. Findings sugges t that the
lysozyme gene dupli cated in mammalian lineage after both bird and mamm alian lineage got separated
from reptil es. Subsequ ently one copy of the duplicated gene mutated into a -lactalbumin gene.
14.4.2 Origin of New Genes by Exon Duplication and Exon Shuffling
M ost genes of multi cellular eukaryotes are discontinuous, in which functional sequences of nucleotides (ex
ons) are interrupted by nonfunctional se que nces (i ntrons). N ew genes may arise by the dupli- cation or
by reshuffling and assembling of different exons from two or more pre-existing genes.
A n exon codes for a polypeptide chain or the dom ain which forms a specific structural or functional
region of a protein .
E
Gy Ay Ii P
Humans
[8J._........
-.J
Great apes r-~~~~~", (gorilla)
Old world
....._.. ...-.[8]
monkeys
(baboon)
New world
monkeys
(monkey)
~:'----- P ros im ia n s (lemur)
Lagomorphs (rabbit)
FIG. 14.9 : Clustered organisation of p-globulin genes in five different primates including man and rabbit.

787. 342 ~ Evolutionary Biology

789. Genes
Monomers
H
H
H
H
Pure Mixed multimers Pure multimer multimer FIG. 14.10: Multiple molecular composition of enzyme
lactate dehydrogenase.
1. Exon Duplication: It result s in a protein that contains a second copy of the encoded domain. This
change in protein structure could change its function by increasing its stability or enhanc ing its binding
ability. Quite a few protein coding genes have multipl e copie s of related exons.
2 . Exon Shuffling: It is the mixing and matching of different exons either within the gene or between two
differen t (nonallelic) genes. This may arise due to errors during meiotic recombination and could lead to
new genes with novel functi ons. New genes are formed by the addition of exon or exons either to the
beginning or end of ancestral gene. For example, tissue plasminogen activator (TPA) protein is an
extracellular protein that helps to control blood clotting. It has four domains of three types, each encoded
by a separate exon. The se exons are derived from three other proteins (Fig . 14.11).
M anuyuan Long and colleagues (2003 ) estimated that 19 percent of exon s in eukaryoti c genomes have
evolved from pre-existing exon s via exon dupli cation and exon shuffling.
14.4.3 Origin of New Genes by Retrotransposition or
Gene Chime rism
A ch imeric gene is formed by the fusion of exons from two or more different ancestral genes. These arise by
relocation of wandering DNA segments, called transposable elements or transposons and
retrotransposons.
Tr ansposons are DNA segments that move within the genome by "cut-and-paste" mechanism, while retrot
ransposons move to new places by "copy-and-paste" mechanism . They move by means of RNA
intermediates. A transposon produces a mRNA
which is rever se tran scribed into eDNA with the help of enzyme reverse transcriptase. The e DNA is
inserted into another location in some other gene. Retrotr ansposon gene is not interrupted by introns and
is functional. These elements can contribute to genome evolution by several ways, such as:
recombi
• by promoting
n ation
• by disrupting
genes or the
elements
':tel; ':tel;
Epidermal gro
wth
factor gene with four
':tel; ':telil ,,,
,,,,

790. EGF exons
.r>:«,
Exon
Exon
--._ --... ';... _ _ shuffling dupJLcation
,,, I, ,,
Fibronectin genewithmultiple
,
F'exons
--------
.......... .._----Exon
shuffling TPA gene as It exi sts tod ay structural control
FIG . 14.11 : Evolution of a new gene TPA by exon shufflin from three ancestral genes.
• by relocating individual exons or entire genes to new locations in the genome Approximately I percent of
human gene s are intronles s and have originated by
r etrotran sposition of ancestral genes having introns. Many retrotranspositions have resulted in the origin
of non-functional pseudogenes. Also, most of the G-protein coupled receptors (GPCRs) genes are intronless
in humans and mammal s whereas in many invertebrates these genes have introns.
14.4.4 Origin of New Genes by Motif Multiplication and Exon Loss
Multiplic ation of specific motifs (repeated nucleotide sequences) within genes also produce new genes with
new function s. Motif multiplication is described in Antarctic fishes which have antifre eze glycoproteins
(AFGPs). The gene for the synthe sis of
New copy of Retrotransposon
retrotransposon I,
/)'
Formation of a
single-stranded
RNA intermediate

792. Insertion
FIG. 14.12: Retrotransposon movement.
(A) AFGP gene
SP 11
AFGP polyprotein
~------~v~-----------'
ThrAlaAla repeats (antifreeze domain)
12 13
....... .......
"<:- A CAGCGGCA Thr Ala Ala
11
........ ...... ...
E2
... ...... ...
ofI]c-
--
87 ThrAlaAla
----'---__ I
FIG . 14.13: Evolution of anti freez e glycoprotion (AFGP) genes in antarctic noto then ioid fishes from
ancestral tryps inogen gene by the repetition of three codons segment for am ino acids Thr-ala -ala.
thi s protein is derived from trypsinogen gene by retaining a segment prior to exon 2 (E2) in the beginning
(i.e., at the 5' end) and exon 6 (E6) and stop codon at the end (i.e., at 3' end). The repeated segment of
threonine-alanine-alanine (i.e., thrala-ala) motif in the middle of chimeric gene is for the antifreeze domain
of the protein.
A eDNA for protein domain of unknown function (DUF 122xc) was found in 34 different human genes. It is
represented by around 50 copies in human genome but by less than 12 copies in primates. It is estimated
that amplification of this motif took place approximately 6 Myr after the divergence of humans from apes.
14.4.5 Origin of New Genes from Noncoding Regions
Nove l genes may arise de novo from noncoding regions of the genome in related species. Levine and
colleagues (2006) have found five genes in Drosophila melanogaster. These genes are not present in other
related Drosophila species.
14.4.6 Origin of New Genes by Horizontal or Lateral Gene Transfer
Hor izontal or lateral gene transfer is the transfer of genes or hereditary material between unrelated
organisms. It is different from vertical transmission from parents to offspring. Genetic material has often
been transferred across different lineages in the history of life. For example:

794. • Horizontal gene transfer is supposed to have occurred in Archaea and Bacteria resulting in the origin of
Eukaryotes.
• Plasmid-mediated horizontal gene transfer has produced many baterial clones and forms like Escherichia.
Salmonella and Shigella.
• Entamoeba histolytica (a eukaryote) is supposed to have received fermentation enzyme gene from Archaea
. This has enabled the parasite to survive successfully in anaerobic environment.
• Lateral transmission of genetic material is being carried out successfully by biotechnologists to produce
genetically modified organisms .
Lateral gene transfer has added new genes to the genome of prokaryotes and eukaryotes which has
occurred from the earliest time.
14.5 REGULATORY GENES AND EVOLUTION
The molecular geneticists have identified two classes of genes :
1. Structural Genes: These genes are associated with the actual synthesis of
specific enzymatic proteins and structural cellular proteins. These have qualitative
action. Change in the structural gene introduces change in the sequence of amino
acids in the protein.
2. Regulatory Genes: Such genes regulate the functioning of structural genes.
Each structural gene may have one or even several regulatory genes . They have
quantitative control on the production of cellular proteins . Any change in the structure
and functioning of regulatory genes leads to change in the rate of synthesis of
protein by a structural gene.
Simple gene arrangements such as deficiencies , duplications, inversions, transloca-
tions and transpositions can change regulatory control over structural genes. According
to King and Wilson, human and African apes display about 99 percent similarity in
amino acid sequences in most of their proteins , yet they are so different that they
have been placed in two separate taxonomic families (Hominidae and Pongidae) . It
means regulatory events during development can easily be modified by mutational
changes in the regulatory genes. Scientists believe that such regulatory mutations
have played larger role in the morphological and functional differentiation of species
than many-many change s in the structural genes. Some evolutionists believe that
macroevolut ionary changes such as origin of new genera , families and orders also
takes place by mutations in the regulatory genes .
14.6 NUCLEIC ACID PHYLOGENIES
14.6.1 Restriction Fragment Length Polymorphisms (RFLPs)
Comparative DNA ana lysis is carried out by using restriction enz ym es that recognise specific palindromic
sequences . For examp le , restriction endonuclea s e Eco RI isolated from E.coli recogn ises pa lindromic
sequence

796. G C
I .,1. I I I I
A A T

T
t I in DNA strands and produces cleavage between G and A
In
C T T A A G
both strands.
Since, DNA molecules from different animals differ from each other in nucleotide sequence, they differ in
the number and placement of sites recognised by Eco RI. Thus, if DNA from different animals are subjected
to the action of restriction enzyme Eco RI, each type of DNA is fragmented into segments of different
lengths.
It means, if DNA from different animals are subjected to the action of restriction enzyme, Eco RI, each type
of DNA is fragmented into segments of different lengths . There are different kinds of restriction enzymes,
each with a different target site. A DNA molecule subjected to a battery of different restriction enzymes
produces DNA fragment s of different lengths and with specific staggered ends, called restrictionfragment -
length polymorphs (RELPs). The comparison of these RELPs helps in estimating genetic variation in
populations and each unique pattern is designated as a haplotype.
Restric tion site maps can also be constructed for different DNA fragments obtained by using different
restriction enzymes.
Based on restriction site maps and DNA-DNA hybridisation, phylogenetic map of human s, chimpanzee,
gorilla and orangu tan and old and new world monkeys has been developed.
14.6.2 Nucleotide Sequence Comparisons and Homologies
Phylogen etic relationship can also be determined by comparing known nucleotide sequences from different
organisms. This helps in comparing changes between protein coding and noncoding DNA sequences and in
determining the synonymous and nonsynonymous nucleotide substitutions in coding regions.
With t he discovery of polymerase cha in reaction technique (peR technique) complete genome sequences of
2,05,000 living organisms and some fossil forms have been obtained. By comparing the data evolutionary
digvergence is determined and evolutionary tree has been plotted. Mitochondrial DNA (mt DNA) sequences
are compared to establish phylogenetic relatio nship in different groups of vertebrates, mammals or
humans.
14.6.3 Combined Nucleic Acid-Am ino Acid Phylogenies
Based on the nucleotide sequences from 5S rRNA and amino acid sequences from c-type cytochromes a
comprehensive 'Tree of Life' was presented by Barnabas and coworkers (1982). From this it has been
concluded that:
• Eukaryotes diverged from early prokaryotic form and the gene for cytochrome c got incorporated into
eukaryotic nucleus after the inclusion of mitochondrion organelle into the eukaryotic cell.

798. • Strong homologies have been traced between cynobacteria and plant chloroplasts. This indicates that
eukaryotic plant cells evolved by the incorporation of cynobacteria.
14.7 GENOME AND PHYLOGENETIC RELATIONSHIP
Genome i s the complete genetic complement of an organism. It includes total number of nucleotide
sequences both coding and non-coding in a cell of an organism. The basis of changes at the genomic level
are mutation, increase in the number of genes and in the arrangement of genes. These many introduce
changes in structure or size of the genome and are responsible for genome evolution. The earlie st life-form
s had minimal number of genes, just sufficient for the survival and reproduction. One important event that
occurred during evolution is an increase in the genome size which provided extra genetic material for gene
diversification. Change in genome size may occur by:
• increase in the number of chromosomes (polyploidy or genome duplication or aneuploidy)
• change in the number of chromosomes by loss or fusion of chromosomes (inversion or translocation i.e.,
the structural changes in chromosomes
• change in the number of genes (gene duplication)
• change in the location of genes and in the linkage roups introduced by transposable elements (e.g..
location of a-globin and P-globin gene families on different human chromosomes)
• introduction of genes into the genome by hybridisation or by viruses.
With rapid advances in the genome sequencing, data collection, PCR (Polymerase Chain Reaction), etc.,
complete genomes of different viruses , bacteria, plants and animals are worked out. Therefore,
comparisons of genome sequences from different species has become a routine activity. It reveals about the
evolutionary history of life, from very ancient to the most recent forms. Genomic similarity indicates
phylogenetic or evolutionary closeness.
14.7.1 Examples of Phylogenetic Relationship
EXAMPLE 1: Analysis of human and chimpanzee genomes reflects the overall similarity between the two
and supports the concep t that chimpanzee and humans have diverged only about 6 million years ago and
are most close ly related among primates.
EXAMPLE 2: Comparisons of complete genome sequences of bacteria, archaea, and eukaryotes indicate
that these three groups diverged between 2 and 4 billion years ago (Bya) . This also supports the concept of
three domains of life.
14.8 CONVERGENT MOLECULAR EVOLUTION
Convergent molecular evolution represents evolution of similar sequences of amino acids in a protein of
unrelated groups of organisms to carry out similar function.
Apes & Man Human
Homo sapiens
c-
Common gibbon
LL Hylobates lar
Pygmy chimpanzee Pan paniscus Gorilla
Gorilla gorilla
Orangutan
Pongo pygmaeus
New world
- Monkeys Chimpanzee IPan trogiodytes

799. I
Siamang
H.syndactylus Old world monkey Cercopithceidae
w w ~ ~
~ o ~ 0 ~
Millions of years ago
~ o0
FIG. 14.14: A phylogenetic tree of huma ns, apes and old world monkeys and the dates of their divergent
based on DNA-DNA hybridisation.
These pro teins might have originally served different functions in different groups. Converge nt molecular
evolution occurs due to persistance of selection for the same function . For example, Ruminant artiodacty ls
(cows, etc.) and monkeys both have a lysozyme enzyme for the fermentation of vegetable matter in foregut.
In both groups the lysozyme has the same five amino acid changes. Even hoatzin bird, Opisthocomus, is
also found to have some of these lysozyme changes for fermentation of food in foregut. The three unrelated
animals have evolved these Iysozymes independently and show conver gent molecular evolution.
14.9 MOLECULAR CLOCKS OR EVOLUTIONARY CLOCKS
The conce pt of molecular clock suggests that the steady rate of change in nucleotide sequences in
DNNgene over time leads to evolutionary divergence between organisms. This provides a basis for
estimating time of divergence of their lineages.
The gre ater is the number of mutational differences between organisms, the greater is their evolutionary
distance . Also the rate at which mutational changes accumulate in the arrangement of nitrogenous bases
and consequently in the structure of proteins is constant over a period of time.
It has been discovered that over evolu tionary time, base-sequences in DNA have diverged resulting in the
modification of amino acid sequences in proteins. By comparing the amino acid composition of proteins in
present day organisms,
scientists can infer the molecular changes that occurred in past. The more early in the past an ancestral
stock diverged into two present day species, the more changes get accumulated in the amino acid sequences
of the proteins in two contemporary species. It means the number of amino acid modifications in the line of
descent can be used as a measure of time of the divergence of two species from a common ancestor. The
molecular clock or evolutionary clock at molecular level determines the rates at which many mutations
become fixed.
For example, changes in amino acid sequences of ubiquinon proteins, such as cytochrome c in different
organisms present in a uniform rate of evolution. Cytochrome c, a protein in respiratory chain , is present in
all organisms. The gene coding for this protein probably appeared very early in evolution and was favoured
by natural selection. As a result of few mutations, the sequence of some of its amino acids is altered.
Cytochrome c from horses and other mammals differs in 5.1 Amino acids. Since horses diverged from other
mammals about 90 million years ago, it means on an average one amino acid substitution has occurred
every 17.6 million years (i.e.. 90
-;- 5.1 = 17.6). The average amino acid difference between reptilian and mammalian cytochrome c is 14.8
and these two groups have diverged about 300 million years ago. Their average amino acid difference
would be 300 -;- 17.6 = 17.0 amino acids . At this rate of one amino acid per 17.6 million years, the plants
and the animals are estimated to have diverged at least 792 million years ago. This figure is close to 800
million years estimated by palaeontologists.

800. 14.9.1 Neutral Theory and Molecular Clock
According to Jack King and Thomas Jukes (1969) and Motoo Kimura and Ohta (1971) most mutations at
molecular level are adaptively neutral (the neutral theory
Human
Chimp
Gorilla
Gibbon 7 Rhesus
Aethiops
Rabbit
Rodent
-=========..22...
15
Dog
Horse
Donkey
16
Pig--------7':
Llama
Sheep
Goat

802. Cow
Kangaroo
FIG. 14.15: A phylogenetic tree of 17 vertebrate groups determined from
amino acid sequences in seven proteins . Each bifurcation or nodal point represents a common ancestor for
its two diverging branches.
of evolution). King and Jukes also conclud ed that most proteins have regions where substitution of many
amino acids can take place without changing the protein char- acter or function. They have no effect on
fitness and are not influenced by natural selection. Therefore, they are establi shed in the gene pool and are
expected to occur at a constant rate like a clock . The great variabi lity in primary structure of homologous
proteins from variou s speci es and also the rate at whch these molecular changes accumulate provide
evidenc e in favour of evo lution. These can be used in establishing phylog enies and also the timing of
lineage separation. Maxon and Wilson (1974) plotted the lineage of Hy/a frogs over 80 million years. The
lineage was based on the time scale and differences between the albumins of two lineages. The average was
estimated to be 100 immunological distance per 60 million years.
14.9.2 Assumptions of Molecular Clock
The concept of evolutionary clock is based on following assumptions:
I. The mutations at molecular level are incorporated at fixed or regular rates over a time.
2. The fixation of molecular mutations does not occur on their adaptive or selective value but on a fixed
rate.
3. The proportional rate of fixation of mutation in one gene relati ve to the rates of fixation of mutation in
other genes remains the same throughout any line of descent.
4. The lines of descent leading from a common ancestor to all its descendants have similar rates of fixed
mutation.
14.9 .3 Example of Molecular Clock
EXA IPLE: Fitch and Langle y studied the amino acid sequences for seven proteins in 17 vertebrate groups
and prepared the phylogenetic tree (F ig. 14.15). They compared the number of nucleotide substitutions that
occurred within a given time for all proteins collectively and also for each protein separate ly. The result s
showed that the rate at whic h proteins have changed together varie s significantly among the branches in
the different lines of descent. However, there is uniformity in the rate of molecular change over a time . Fig.
14.15 shows the mammalian phylogeny derived from the mutational dista nce data and provides an average
number of nucleotide substitutions at each branching point. This corresponds with the linear relationship to
time divergence.
14.9 .4 Problems of Molecular Clock
1 . All changes are not neutral. Their role of change is influenced by natural selection. Certain changes are
favoured while some are eliminated.
2. The structure of certain important genes is conserved over long period. Their mutation rate is very slow.
14.9.5 Significance of Molecular Clock
Molecular information about changes in the sequence of nucleotides in DNA and RNA or in the sequence of
amino acids in different proteins and the comparison of their rates help in evaluating relationship between
distantly related organisms.

803. These informations help in the preparation of phylogenetic trees and estimating proximate time of
separation of each group from the other
.
14.10 MOLECULAR EVOLUTION IN TEST TUBE
Evolutionists in recent times have tried to demonstrate evolutionary process in the laboratory at molecular
level by a technique called test tube evolution or evolution in test tube.
A strain of bacteria in a culture medium is exposed either to a new source of carbon (xylitol) or of nitrogen
(butyramide). The bacterial cells are unable to metabolise these resources properly and are unable to grow
in such culture media. When these bacterial cells are simultaneously exposed to the new nutrients and some
mutagen, adaptive mutations appear in certain bacterial cells and enable them to survive and multiply.
14.10.1 Enzymatic Evolution in E.coli
1 . Campbell and co-workers showed enzymatic evolution in E.coli. They exposed a mutant strain of E.coli
with a deletion of f3-galactosidase Z gene to lactose medium. Lactose can not be utilised by the strain
because of the absence of enzyme f3-galactosidase. Due to the presence of lactose, the colonies of E.coli
appear red on the indicator medium. Within one month, the Z-deficient strain gave rise to white colonies,
because they were able to use lactose. Further, these mutated bacteria were able to grow in medium
unsupplemented with other sugars. The final strain of E.coli had the lactose hydrolysing enzyme (ebg =
evolved f3-galactosidase) which was different from f3-galactosidase enzyme present in normal strain of
E.coli.
2 . Mills and co-workers have demonstrated the evolution of viral genomes and even smaller nucleotide
sequences under molecular selection pressures. They used RNA of a Qf3 virus. It is about 4,220 nucleotides
long and can replicate in test tube when the medium contains enzyme replicase and other chemical
components necessary for replication. Scientists found that for rapid replication, Qf3 RNA molecules
fragmented into small polynucleotide segments, having for sequence recognising replicase enzyme in
culture. These short independently replicating RNA molecules were about 220 nucleotides long.
3. Joyce and co-workers have demonstrated that RNA molecules half the size of Qf3 sequence have evolved.
They have their own signals for test tube replication. Stemmer showed that sex can be introduced into test
tube evolution.
352 ~ Evolutionary Biology
The advantage of test tube evolution is its ability to rapidly create entirely new molecules that can perform
new biological roles and has attracted many experimenters.
14.10.2 Significance of Test Tube Evolutionary Studies
Experimental molecular evolution is thus becoming a highly promising field for understanding how changes
on molecular level occur and lead to evolution.
• Evolutionary clock
• Gene duplication
• Multigene family
• Phylogenetic tree
KEY TERMS
• Gene cluster •
• Gene family •

804. • Orthologous genes •
• Polymerase chain reaction (p e R) •
• Restriction-fragment length polymorphism (RFLPs) • Gene conversion
Molecular phylogenies Paralogous gene
Pseudogenes
Test tube evolution
REVIEW QUESTIONS
I . What do you understand by molecular phylogeny? What are the advantages of using proteins and nucleic
acids in determin ing phylogenies?
2. Why have 5S RNAs been used for nucleic acid phylogenies?
3. (a) What are regulatory gene changes ?
(b) How can changes arise in regulato ry genes?
(c) Why such changes have more evolutionary impact than changes in the structural genes?
4. What are regulatory gene changes? Why are regulatory gene changes presumed to have greater
evolutionary consequences than changes in the structural genes?
5. Write a note on test tube evolution .
6. Describe with example multigene families and discuss their significance.
7. What do you understand by molecular evolution? Describe molecular evolution of haemoglobin in
vertebrates.
8. Describe how evolution of haemoglob in has helped in developing the evolutionary clock and phylogeny
of vertebrates.
9. Write short notes on:
(a) Structural gene
(b) Regulatory genes
(c) Gene family
(d) Test tube evolution
(e) Molecular clock
10. How has gene duplication contributed to change s in the genome?
II. (a) What is polymerase chain reaction?
(b) How are PCRs used in making phylogenetic determinations?
12. Give at least two examp les in which proteins and nucleotide sequences have helped in the
determination of phylogenies .
FURTHER READINGS
I . Adams, M.D., S.E. Eelniker, R.A. Holt, C.A. Evans et a\., 2000. The Genome Sequence of Drosophila
melanogaster; Science, 287, 2185-2195.
2. Aparicio S., J. Chapman, E. Stupka , N. Putnam , et a\., 2002. Whole Genome Shotgun Assembly and
Analy sis of Genome of Fugurubripes, Science, 297, 1301-1310.
3. Arbidopsis Genome Initiative, 2000 . Analysi s of the Genome Sequence of Flowering Plant Arabidopsis
thaliana , Nature, 408, 796-815 .
4. Ayala, F.1. , A. Escalante, C.O ' Huigin , and J. Klein, 1994. Molecular Genetic s of Speciation and
Human Origins. Proc. Natl. Acad. Sci., USA, 91, 6787-6794.
5. Ayala, F.1., Rzhetsky, and F.1. Ayala, 1998. Origin of the Metazoan Phyla: Molecular Clock s Confirm
Palaeontological Estimates. Proc. Natl. Acad. Sci., USA, 95, 606-611 .
6. Bennetzen, J.L., 2000. Transposable Element Contributions to Plant Gene and Genome Evolution. Plant
Mol. Bioi.. 42 : 251-269.

805. 7. Berezikow, E., F. Thuemmler, L.W. van Laake , I. Kondova , et a\., 2006 . Diversity of Micro RNAs in
Human and Chimpanzee Brain. Nat. Genet., 38, 1375-1 377.
8. Bharathan, G., B.1. Janssen, E.A. KelIogg, and N. Sinha, 1997. Did Homeodomain Proteins Duplicate
before the Origin of Angiosperms, Fungi and Metazoa ? Proc. Natl. Acad. Sci., USA, 94, 13749-13753.
9. Blencowe, B.1., 2006 , Alternative Splicing: New Insights from Global Analyses. Cell. 126,37-47.
10. Burger, G., M.W. Gray, and B.F. Lang, 2004. Mitochondrial Genomes: Anything goes. Trends Genet.,
19,709-716.
II. Clark , A.G. and Drosophila 12 Genomes Consortium, 2007 . Evolution of Gene and Genomes in the
Drosophila Phylogeny. Nature. 450 : 203-218.
12. Feng, D.F., G. Cho and R.F. Doolittle , 1997. Determining Divergence Times with a Protein Clock :
Update and Re-evaluation. Proc. Natl. Acad. Sci.. USA, 94, 13028- 13033.
13. Herrmann, 8., and S. Hummel (eds.), 1994, Ancient DNA, Springer-Verlag, New York.
14. Krings , M., A. Stone , R.W. Schmitz, M. Stoneking, and S. Paabo, 1997. Neanderthal DNA Sequences
and the Origin of Modem Humans . Cell, 90, 19-30. 15. Venter, lC., M.D. Adams, E.W. Myers, P.w. Li, et a\.,
2001. The Sequence of the Human Genome. Science, 291, 1304-1 35 1.
000

807. UNIT-IV
Basic Patterns of Evolution
Chapter 15. Patterns of Evolution Chapter 16. Microevolution and Macroevolution Chapter 17.
Adaptations

809. 15
Patterns of Evolution
Evolution is the process by which the population of a species splits to form two or more species. Starting
with one life form long, long time ago the process of evolution has generated the diversity of life forms on
earth. In an effort to organise these diverse living forms into groups on the basis of similarity, scientists
have generated the tree of life or the phylogenetic trees. The study of phylogenetic trees enables us to trace:
• The similarities between different organisms
• The relationship between different groups
• Evolutionary relatedness among different groups
• History of branching or diversification of various groups
• Patterns of branching or patterns of evolutionary changes
During the process of origin and evolution of different phylogenetic groups of plants and animals, different
patterns of evolutionary changes have been identified. These are discussed in this chapter.
15.1 SEQUENTIAL AND DIVERGENT EVOLUTION
Evolution has been described as the process of gradual modification in the living organisms (plants or
animals) so as to establish diversity in the world of living beings . Two fundamental patterns could be
envisaged in the process of evolution:
I . Minor changes in the gene pool of a population are passed on from one generation to the next, with the
result that no new populations are formed, but the descendent population is not genetically identical with its
predecessor. This is known as sequential evolution.
2 . The changes which result in sudden evolution of new populations, species, families , groups or classes
represent divergent evolution.
The sequential evolution is therefore, an example of random fluctuations over a long period of time without
producing new populations. Therefore, the changes
occurring on account of evolutionary forces like mutations, variations, natural selection and gene tic drifts
produce only temporary changes which fluctuate at random. For example, in human population, we find
that not even two real sisters or brothers are identical or resemble their parents, yet the changes do not
divide the individuals of a popu lation or race into subcategories. Secondly, these changes are not
directional.
The divergent evolution , on the contrary, is an example of directional evo lution. The changes occur in a
cumulative direction and result in the origin of new populations from the old ones. Therefore, the varied
groups of plants and animals either related or unrela ted provide an example of divergent evolution. It is the
divergent evolution which is more evident.
As a matter of fact, sequential evolution and divergent evolution are rather inseparable. Not even a single
population exclusively exhibits sequential evolution, because all populations diverge in due course of time
and split up into new populations. Moreover, the forces responsi ble for bringing about changes are rather
the same in both the cases except that they operate for a very long period and are assiste d by additional
factors .

810. Seque ntial evolution, though helps in understanding the operation of various evolutionary forces, does not
play any role in the evolution of new species or groups. It is, therefore, the divergent evolution as is seen in
fossil records, illustrates the results of evolution.
The s equential evolution is actually microevolution and the divergent evolution, in its simplest form
causing diversification or splitting of population, is nothing but macroevolution only. The fragmentation
and development of new popu lations from the existing popu lation is called speciation and usually leads to
the evolution of new spec ies.
Evolut ionary changes which are responsi ble for establishing the taxonomic categories above species level
represent macroevolution . It includes adaptive radiation of a population to different new habitats. The
megaevo lution includes those changes in the organ isation which enable the organisms to enter into a new
major adaptive zone.
15.2 PHYLETIC GRADUALISM AND PUNCTUATED EQUILIBRIUM
Speciat ion can happen at a rapid rate or gradually. On a global level, the life on earth has passed through
periods of bursts of speciation followed by long periods of relativ ely little changes. It means evo lution may
be gradual or punctuated.
15.2.1 Phyletic Gradualism
The per sistent accumulation of small changes within a lineage is described as ph yl etic gradualism. It
involves accumu lation of small changes over millions of years with in one lineage so that the parent
population passes through a series of intermediate stages, and the descendant po pu lation or populations
appear as distinct species differing from antecedent populations. The

812. transformation of a lineage over time is termed as anagenesis. For example, in Fig. 15.2, a new species-B
arises by slow and steady transformation of a large antecedent population-A.
According to Gould and Eldredge the phyletic gradualism is very slow and is unable to produce major
events of evolution.
Cyanobacteria can be taken as an example of gradualism since they have changed a little to adapt to
environmental changes.
15.2.2 Punctuated Equilibrium
George Gaylord Simpson (1944) suggested that the gaps in the fossil records represent the sudden
appearance of a new species from an ancestral form followed by periods of little change . In 1972, Stephen
Jay Gould and Niles Eldredge advocated that most evolutionary changes are rapid bursts of speciation.
They alternate with long periods in which individual species remain virtually unchanged. The long intervals
of geologic time in which very limited or no significant morphological changes occur represent 'period of
stasis' or 'period of equilibria'. The intervals interrupting the period of equilibria and marked by
conspicuous active evolutionary changes are called 'punctuations.' During these brief periods the lineages
actually branch into new lineages. These patterns of fast evolutionary changes were called punctuated
equilibria.
The fossil records reveal that both gradual and punctuated equilibrium contribute to evolution of lineages.
Gradualism is seen during anagenesis, while punctuated equilibrium occurs during cladogenesis.
The fossil records of bryozoa show punctuated equilibrium because this group remained unchanged for
millions of years and then suddenly branched to yield new species.
George Goylord Simpson described gradualism and punctuated equilibrium as bradyletic evolution (G.
brady. means slow) and tachytelic evolution (G. tachy, means first).
15.3 ANAGENESIS AND CLADOGENESIS
________________----1
Rensch (1954) had used the above terms
to differentiate various types of evolutions.

814. FIG.
15.1: A. Phyletic gradualism; B. Punctuated equilibrium .
-if.:Hi::&IO". Punctuated Equilibrium
G radualism
1. Gradualism occurs during anagenesis.
2. Morphological change is gradual and occurs continuously.
3. New species originate gradually.
4. An ancestral species can be transformed into a new species.
5. Phyletic replacement does not involve splitting of the lineage .
Punctuated Equilibrium
1. It occurs during c1adogenesis.
2. Morphological change is sudden and rapid and occurs only rarely.
3. New species originate abruptly.
4 . The sub populations of the ancestral species transform into new species.
5. Phyletic replacement involves splitting off of a new lineage/species, which then replaces its ancestor
abruptly.
Anagenesis: It represents progressive change in characters of a lineage through time. It result s in linear
succession of lineage through time , i.e., succession of one species by other in due course of time. It is cha
racterised by the replacement of one linea ge by another without any branch ing. Anagenesis represents any
kind of unid irectional evolutionary change whether it leads to a marked advanc e or not but not the
branching patt ern of lineage origin .
A nage nes is creates organisms with novel
characters and abilitie s, beyond those of the ir
ancestors (Fig. 15.2)
Cladogenesis: It represents divergen t
evolution or phylogenetic evolution in whic h
parental population or parental lin eag e
branch es into severa l lineages. Lin eag e 1
branches resulting du e to cladogenesi s Ul
'Cii
are ca lled clades. A clade is a groupQ)
c
that includ
es
common
a
ncestor
a
nd all
it
s
Q)
g de
scendants.
It

815. repres
ents
monophyletic
'0 co
evolution, i.e., a group of organisms arising
U
from one ancestor. This ensures rapid origin
of new spec ies.
In clad
istic
approach
,
the characters of
a
Clade
1

_

Anagenesis
clad or taxa are classified as follows:
1. Apomorphic Characters: These are FIG. 15.2: Diagram showing anagenesis the characters that are
derived by evolution (evolutionary advance) and c1adogenesis so they are also called derived characters.
(branching lineage),
2. Plesimorphic Characters: These are those characters that are shared with the ance stral species.
3. Synapomorphy: It is the posses sion by two or more related lineages of the same phenotypic character
derived from a different but homologous character in ance stral lineage.

817. 15.4 MONOPHYLETIC, POLYPHYLETIC AND PARAPHYLETIC EVOLUTION
;;';;''';'';''~'----
15.4.1 Monophyletic Evolution
Taxa who se members have descended from a common ancestor are called monophyletic. All members or
species of monophyletic taxon descend either from the same parents or same population or same species
i.e., the new species is the temporal extension of the parent species. For example, a class is monophylectic if
all its lineages ancestral to the class originated from the same family. The phenomenon of origin of
monophyletic taxa is called monophyletic evolution or monoph yly. (Fig . 15.3: Taxa 0 , E, F)
All vertebrates have a common ancestor. All the mamma ls also represent monophyletic group .
15.4.2 Polyphyletic Evolution
The members of a single taxon if descended from two or more a ncestral lineages through convergent or
parallel evolution represent a polyphyletic taxon. The origin of such a taxa is called polyphyletic origin and
their evo lution as polyphyletic evol ution or polyphyly (Fig. 15.3: Taxa B, C and D).
15.4.3 Paraphyletic Evolution
A group of organisms that has a common ancestor but does not include all the descendants of that ancestor
is called paraph yletic taxon. The origin of such a taxon is called paraph yletic origin and their evolution
as paraph yletic evolution. (Fig. 15.3 Taxon A)
Common ancestor of A, B, and C
C ommon ancestor of Band C
A paraphyletic taxon (pink) includes some
A but not all of the descendants of a single ancesto r.
B
Common ancestor
Commo n ances tor of D, E and F
Common ancestor of E and F
A polyphyletic taxon C
(yellow) contains organ isms that do not share the same common ancestor.
D
E
A monop hyletic taxon (green) includes all descendants of a single
F
ancestor and no other organ ism.
FIG. 15.3: Monophyletic, polyphyletic and paraphylelic taxa .

819. For e xample, class Reptilia is a paraphy letic group because it does not include all the descendants of its
common ancestor. The birds which are very close to crocodiles and can be placed with crocodiles in the
same taxon are placed in a separate class. Similarly, Gymnosperms also form a paraphyletic group.
~
5.5 DIVERGENT EVOLUT ION
,--_(A DAPTIVE RADIATION OR ADAPTIVE DIVERGENCE) _
Th e living organisms exhibit plasticity in their organisation i.e., organism s with same genotype can change
their phenotype in response to change s in the habitats . Because of this characteristic, organisms of the
same group or closely related groups appear very different when found in different habitats (divergence) .
This is called adaptive divergence or adaptive radiation.
15.5.1 Definition of Divergent Evolution
Adapti ve divergence or adaptive radiat ion is the rapid speciation and ecological diversification within a
single species or a single lineage of species in several specialised directions.
Adapti ve divergence occurs when organisms of a parental stock or of a lineage enter new adaptive zones
and each group gets adapted to survive in the new zone. The concept of adaptive radiation was introduced
by Osborn . The concept states:
"Eac h isolated region if large and sufficiently va r ied in its topography, soil, clima te and vegetation, will
give rise to a diverse fauna. The larger th e region and more diverse th e conditions, th e greater will be th e
var ieties of an imals found."
Ther efore, adaptive radiation is evolution in several specialised direction s starting from a common and
generalised ancestral type, or the entry of the organisms of the original stock to new adapti ve zones.
Examples of Divergent Evolution
1. Adaptive Radiation in Limb Structure of Mammals: The limbs in different groups of placental mamma ls
are modified for climbing, flying, swimming, tearing food, burrowing and running. But their ancestry can
be traced back to a primitive insect eating, five-toed creature that lived on land and walked with
pentadactyle, plentigrade flat feet. The radiation of modern mammal s into five different types of feet
occurred when they occupied different habitats.
• Arbo real or tree dwelling mammals developed grasping limb s. In sloths, monkeys and apes the limbs are
modified to have powerful grip of the branches. Their fingers are elongated for grasping and limbs are
adapted for swinging from one branch to another.
• Aer ial mammals, i.e., bats have their fore limbs modified into wings for flight.

821. Bones of Bones of Forelimb Hind limb
Upper Thigharm (1)(1)
Humerus --t----- Femur
elbow -+----Knee
Forearm
Radius - - - - ------\-+II'Hl...-+------Tibia
Shank
(2) (2) Ulna --------+---ltlt,I<t-t-------r ibula
Wrist Carpals (carpus) (9) Ta
rsals
Ankle (tarsus)
(9)
Metacarpals (5)
Metatarsals (5)
Hand
Phalanges 14 Phalanges
Foot
(14)
3
4
5
~ Digits (toes)
FIG . 15.4: Structure of prototype or generalised pentadactyle limb.
• In aquatic mammals, e.g., whales, porpoises and walruses , the limbs are modified into flippers that help
in swimming .
• In carnivorous and anteater mammals the digits of forelimbs are modified for tearing the prey.
• The forelimbs of fossorial mammals (e.g., mole) are spade-like . They are specialised for digging and are
poorly adapted for locomotion on ground.
• In cursorial mammals (like horses and deers) the limbs are modified for fast running over hard ground.
All the aforesaid limb structures are constructed on the same fundamental pattern as shown in Fig. 15.5 and
can be derived from the prototype , pentadactyle limb structure. In other words it could be said that all of
them represent evolutionary lines radiating out in various directions from the prototype limb structure. This
is known as adaptive radiation which represents evolution of new forms in several directions from the
common ancestral type (divergence).
2. Adaptive Radiation in Marsupials in Australia: In Australia, a number of marsupials evolved from the
ancestral stock, all adapted to new habitats. Because

823. (b)
(a)
2 Sloth and monkey
(grasping) (
d)

824. I

826. Ra dio-ulna (radius and ulna fused)
3
Displaced
carpal
2 3 4
3 FIG. 15.5: Adapt ive radiation of forelimbs in mammals.
of this, they developed appearan ce, structure of limbs and tails very different from ancestra l stock. (Fig.
15.6).
3. Adaptive Radiation in Darwin 's Finches: Different species of Finches on Galapagos Islands have
evolved from one mainland species present on West Coast
Patterns of Evolution fiJ 365
of South America. On these islands, finches got adapted to exploit different niches and habitats and evolved
into different species. Their beaks became modified to eat different types of foods available in different
niches .
Finches , in general, possess stout, conical beaks adapted for crushing seeds. But they have undergone
great diversification in their feeding habits and accordingly in the shape and size of their beak. (Fig. 15.6)
• The ground finches of the subgenus Geospiza exhibit great variations in their beak structure. Although,
chiefly seed crushers, the size of beak is correlated with the size of seeds they eat.
• Warbler finch has a slender warbler-like beak and is insectivorous in habit.
• Vegetarian tree finches have a short, thick and somewhat parrot-like beak and feed upon leaves, buds and
fruits.
• Woodpecker finches have stout and straight but long beak and are completely insectivorous. These search
bark and leaf clusters and bore into the wood like a woodpecker.
• Insectivorous tree finches have a beak very similar to vegetarian tree finch but they feed upon beetles and
other insects.
Tasmanian wolf
Sugar glider

828. Wombat
Marsupial rat
FIG. 15.6: Adaptive radiation in marsupial mammals in Australia .
• Ca ctus gro und finches have a long, somewhat decurved beak and a split tongue. It probe s the flowers of
prickly pear cactus for nectar and feeds upon the soft pulp of this cactus.
4 . Tooth Radiation in Mammals: Mammals possess heterodont dentit ion, i.e., incisors for biting , canines
for tearing and grasping and the premolars and mola rs adapted for grinding. The premol ars and molars
exhibit greatest structural modifications for different types of food:
• In insectivorous type (which feed on insects), the premolars and molars are low crowned with few simple
cusps, generally suited for crushing feeble prey.
• In carnivorous type these become high crowned with complicated cusps and with shearing structures
(carnasi al).
• In her bivorous forms, the premol ars and molars are either short crowned (br achydont) adapted for
succulent vegetation or long crowned (hypsodont) adapted for harsh grasses. Their incisors are sharp and
adapted for seizing and cutting the vegetation.
• The carrion-feeding form s have blunt teeth while in fish-eating mammals the teeth become simp lified and
prehensile.
• In toothed whales, the teeth have become secondarily homodont, i.e., these are practically all alike,
slightly decurved and adapted for grasping the prey.
• In sper m-whales or baleen wha les, the teeth are absent.
• In myr mecophagous forms, the teeth have disappeared and jaws reduced. They have developed a tubular
snout with a sticky tongue inside to feed on ants.
15.5.2 Conditions Responsible for Adaptive Radiation Major triggers for adaptive radiation can be
discussed under two major heads:
15.5.2.1 Entry into New Adaptive Zones and
New Ecological Opportunity
Adaptiv e radiation is triggered with the entry of a lineage into new adaptive zones which provide new
resources, competition, free environment and trigger temendous burst of evolutionary activities . In the new
zone, individuals multiply rapidly and become diversified at an explosive rate . During this explosive phase,
the organisms acquire more and more specia lisation to the new habitat. The products of adaptive radiation
may form a genus , a family, a subclass or a class. A few examples of those groups which have recent ly
undergone adaptive radiation are spiny-rayed teleost fishes, characin fishes of South Africa, dorosophilid
frogs, geckoni d lizards, finches and mammals.
• Ra diation in mammals occurred immediately after the extinction of dinosaurs, when habitats were
unoccupied.
• Radiation in Anolis lizards on two different islands Hispaniola and Jamaica evolved from two different
ancestral stocks but assumed the same adaptive features to same type of habitats.
• Lizards that started living on broad tree trunks and crown developed long legs and tails for clinging and
balance.
• Lizards that continued to live on the ground also evo lved long legs and tails for running.
• Twig-dw eller and crown-dweller lizards developed relati vely short body, short legs and short tails.
~- Twig or
C. pa
II
'
d
us
cactus spine
I

829. I
I Woodpecker-like I
C.h2es-----..J D ~
G-magnirostris -----..
~~ G. s ca nd e n s C. psittacula----.,.
~
G-fortis _______ C.pauper-----~
~
,..--G. conirostris
G. fuliginosa -----..
C.
p~s
----'----..I
ICactus-feedingl
F
I i n sect ivo rou~ IGround finchesI ; Certhidea
E A J
olivacea
C. crassirostris
I Vegetarianf------J
C
I cam~~~n~hus
I
V-G . d
~
ifficilis
l.-- Pinaroloxias
I
T
ree-finches
I
I
r---'------,
inomata
Geospiza I

831. C
Warbl er-like I
jB
Ir-- --- - -/
I
Ancestral seed-eatingI
groundfinch
FIG . 15.7 : Radiation in the beak structure of finch es A. Beak of ground finch subgenus Geospiza; B. Beak
of warbler finch ; C. Beak of vegetarian tree finch ; D. Beak of woodpecker finch ; E. Beak of insectivorous
tree finch ; F. Beak of cactus ground finch .
Trunk/cro wn Twig Crown Trunk/ground Twig Trunk/ground Crown Trunk/crown
Colonisation of island by ancestral lizard living on trunks and crowns Colonisation of island
~
by ancestral
lizard living on twigs
'--1
-n-H- i-sp-a-nio- Ia-'s'-an-d-'I rl n-J-amaicas'a n---'d:':;1
FIG. 15.8: Adaptive radiation in Anolis lizards.
15.5.2.2 Development of Novel Characters or
Morphological Innovations
Some adaptive radiations or breakthroughs in new adaptive zone were triggered due to the evolution of
some intrinsic factor/factors or the acquisition of some key morp hological innovations . For example,
multicellularity, exoskeleton, shells, pentadactyle limbs and wings, etc., were driving force behind adaptive
radiation which occurred from time to time.
• Evolution of protective exoskeleton, three pairs of legs and wings led to the evo lution of first terrestrial
and aeria l invertebrates.
• The great success of Cichlid fishes is due to the evolution of jaws in their throat. These additional or
second pair of jaws enabled them to crush snail shells and shredding tissue s from other fishes.
• Feathers and wings in some dinosaurs gave them the ability to fly and they evolved into birds .
• Development of large , cleidoic egg s in reptiles provided them to lead a complete land life.
• Transition of terrestrial life from aqua tic life in vertebrates in Devonian Period was made possible when
rhipidistian crossopterygian fishes developed some innovative morphological modifications such as stiff fins
which they could use to walk on the bottom of shallow wate r, lung and nostrils to breathe atmospheric air.
15.5.3 Causes of Adaptive Radiation
The impe lling causes of adaptive radiation are the need for food, safety and for better breeding grounds.
Naturally, the animals migrate to such new habitats which are still unoccupied and with no competition.
15.5.4 Significance
As alread y discussed, adaptive radiation had resulted in the formation of new genera, species, orders ,
subclasses and even classes. The phenomenon of adaptive radiation is, therefore, responsible for
macroevoluti on and megaevolution.
A .lnsects have a distinctive body plan
B.Cichlids have ' throat jaws C.Feathers evolved in dinosaurs that can bite and process food
FIG. 15.9: Some morphological innovations leading to exploitation of new habitat and adaptive divergence.
T he illustrat ions of adaptive radiation or divergen t evolution throw light on how the forms would have
modified and evolved into new forms or in other words how the evolution would have occurre d. Such an
evo lution of a group is known as macroevolution.

832. 15.6 CONVERGENT EVOLUTION OR ADAPTIVE CONVERGENCE OR PARALLEL EVOLUTION
15.6.1 Definition
When unrelated orgnisms with completely different organisation living in the same habitat or same adaptive
zone are found to possess superficial resemblance, this is called convergent or parallel evolution. This
striking similarity in appearance in distantly related organisms is due to natural selection that favours
parallel evolutionary adaptations in similar environment. Similarity due to convergent evolution represents
analogy or homoplasty.
15.6.2 Examples of Adaptive Convergence
1. Adaptive Convergence in the Body Shape and Limbs in Aquatic Vertebrates: Carti laginous and bony
fishes, aquatic repti les (Ichthyosaurs), bird penguin and seals , whales and dolphins belong to different
classes of vertebrates, but have similar fish-like appearance and have fins or flippers for swimming (Fig.
15.10). The similarity in their body shape is so well marked that whale and seal are unders tood as fish by
laymen. Similarity in body shape between animals of distantly related groups represents convergent
evolution and their fin or flippers form analogous or homopl astic or gan s.
2. Adaptive Convergence in the wings of Insects, Flying Reptiles, Birds and Flying Mammals: The most
common examp le of convergent or parallel evolution is the occurrence of wings in insects, flying reptiles,
birds and flying mammals. All of them belong to totally different groups, but have one common feature, the
development of wings for flight.

834. Penguin
Shark
Ichthyosaur
Porpo ise
Convergent evolution for sustained rapid swimm ing in a marine habitat
Ancestral mammal
Ancestral shark
Ancestral fish
FIG . 15.10: Convergent evolution in three marine vertebrates-Shark (cartilaginous fish). Bonyfish.
ichthyosaur (an extinct reptile). Penguin (a bird) and Porpoise (a placental mammal). because similar
adaptations for rapid movement through water were independently selected in each of the lineages.
3. Adaptive Con vergence in the Eyes: The eyes of Octopu s and vertebrates are remarkably similar in
appearance as well as in structure. They seem to be homologous structures but they are analogous. Eyes in
Octopus and Sepia do not have a 'blind spot' and they do not form inverted image as in the case of
vertebrate eyes. This shows that the two type s of eyes are the products of convergent or parallel evolution
alon g two distinct lines .
4. Con vergent Evolution in Marsupial and Placental Mammals: The two groups of mammals, marsupials
and placen tals, have evolved in a very simi lar way independently on separate continents: Marsupials in
Australia and placental eutherian mammals in other continents. As seen in Fig. 16.11 the two groups
present parallel evolution and each placental type is similar to corresponding marsupial variety. For
example, eutherian moles correspond to marsupial mole s or eutherian wolf to Tasmanian wolf and
eutherian bobcat corre sponds to Tasmanian tiger cat.
Austral ia, the home of marsupials separated from the mainland of Asia more than 50 million years ago. At
that time the eutherian mammals were not evolved and land was inhabited by marsupials only. After the
separation of Australian Continent eutherian mammals appeared on mainland, evolved into various forms
and
Placental mammal s
Marsupial mole Numbat Marsupial mouse Spottedcuscus Flying Tasmanian Tasmanian wolf (Anteater)
phalanger tiger cat FIG. 15.11: Convergent evolution in placental mammals and marsupial mammals in
Australia.
comp eted with marsupials. Therefore, marsupial s disappeared from the main land. However, marsupial s
on Australian Continent faced no competition with eutherians and diversified for the next 50 million years.
Th e similarity between different members of marsupials and eutherians can be due to convergent or
parallel evolution because of similar selective pressure in similar environments.
15.6.3 Significance
Th e convergent evolution leads to the formation of analogous similarities among different groups of
organisms which indicates that evolution may lead to superficial resembl ances. N.W. Pirie has pointed out
that a kind of convergence might have played a major role during prebiotic evolution, before the chemical
system accu- mulated characteristics of living organi sms and became dominant or led to organic evolution.
In th e end it could be concluded that in the evolution of life from simple to more compl ex forms both
divergence and convergence have played an important role, but divergence is more frequent at present than
the convergence.

835. ''''-II::&'*J Difference between Divergent and Convergent Evolution
Divergent Evolution
1 . Divergent evolution results in origin of new forms by diversification . So , the new forms are apparent
from the existing forms.
2. Organisms get adapted to different environments or different habitats.
3 . Divergent group splits into a number of groups , each having different appearance.
Convergent Evolution
1. Convergent evolution results in origin of new forms which resemble in appearance to the already existing
form.
2. Organisms ge t adapted to one environ- ment for one habitat.
3. Organisms of different groups assume similar habitat.
372 ~ Evolutionary Biology 15.7 COEVOLUTION
15.7.1 Definition
Coevo lution is a pattern of evolu tion in which two interacting species influence each other's adaptive
changes over the time . It means in co-evo lution the interacting species evo lve together in response to each
other. For example, a change in species A induces a change in species S , which then selects anoth er
change in species A, which in tum introduc es another change in species S and so on. The co-evolution
results in reciprocal adaptations and leads to long-term changes in the interacting populations.
15.7.2 Examples of Coevolution
I . Cheetah-antelope or wolf-d eer interactions are examples of coevolution . Deer are fast and agile in
response to predators, wolves and cougars. The speed and agility of deer promotes agility in wolves to prey
on deer. In tum speed and agility in deer increas es further to escape from the pred ator. This is an example
of coevolution in animals having antagonistic interaction.
2 . Plants with long curved flowers are pollinated by humming birds that have long and curved beaks. Their
beaks are curved to match the curved flowers. This is an exampl e of mutualistic coevolution.
15.7.3 Outcome of Coevolution
In coevolution one organism evolves to its own benefit in respon se to other organi sm. As one changes,
other dependent one also changes. In antagonistic interactions the end point could be comp lete elimination
(i.e.. extinction ). In mutuali sm the end point reaches when there is no further change (i.e.. stability).
• Adaptive convergence
• Anagenesis
• Clade
• Divergence
• Monophyletic
• Plesimorphic
• Synapomorphic

836. KEY TERMS
• Adaptive diverge nce
• Apomorphic
• Co-evolution
• Fussorial
• Paraphylectic
• Polyphyletic
• Adaptive radiation
• Cladogenesis
• Cursolial
• Limb
• Phyletic gradualism
• Punctuated equilibrium
REVIEW QUESTIONS
I . Explai n the phenomenon of adapti ve radiation wi th suitable exampl es. 2. (a) How novel characters or
morpholo gical innovations assist in adaptive radiati on? (b) Support your discussion with two sui table
examples.
3. (a) Give reason why marsupials have survi ved and undergone adaptive radiation in Australia but not on
other continents.
(b) Give differences between convergent and divergent evolution.
4. (a) Describe with two suitable examples con vergent or evolutionary adaptive con vergence?
( b) What is homoplasy?
5. Wh at do you understand from term s phyletic gradualism and punctuated equilibrium?
Give differen ces between the two.
6. Differentiate between:
(a) Sequential and Divergent evolution.
(b) Monophyletic and polyphyletic evolution.
(c) Phyletic gradualism and Punctuated equilibrium.
(d) Anagenesis and c1adogenesis.
(e) Adaptive convergence and adaptive divergence.
7. Write notes on :
(a) Coadaptation
(b) Punctuated equi librium
(c) Phylogenetic tree and its evo lutionary significance
(d) Adaptive radiation in mammalian limb
(e) Conditions responsible for adaptive radiation
8. Define the folIowing term s:
(a) Clade
(c) Co-evolution
(e) Paraphyletic evolution
(g) Synapomorphy
(b) Adapti ve con vergence (d) Sequential evolution (f) Cladogenesis
(h) Homoplasy
(i) Plesiomorphic
FURTHER READINGS

837. I. Ax, P., 1987 . The Phylogenetic System: The Systematisation of Organisms on the Basis of their
Phylogenesis. Wiley, Chichester, UK.
2. Fel senstein, J., 2004 . Inferring Phylogenies, Sinauer Associates, Sunderland, MA .
3. Maynard Smith, J., and E. Szathmary, 1995. The Major Transitions in Evolution. Freeman, New York.
4. Nowak, M.A., 2006. Evolutionary Dynamics. Harvard University Pre ss, Cambridge, MA .
5. Pagel , M . (ed. in chief), 2002 . Encylopedia ofEvolution. 2 Volumes , Oxford University Pres s, New
York .
6. Schulter, D., 2000 . The Ecology of Adapti ve Radiation. Oxford University Press, Oxford, England.
7. Seld on, P., and Nudds, 2004. Evolution of Fossil Ecosystems. Man son Publi shin g Ltd. London.
8. Valentine, J.w., 2004. On the Origin of Phyla . The University of Chicago Press, Chicago.

839. ODD

840. 16
Microevolution and Macroevolution
Biological evolution i s simply a change in the frequency of one or more genes through time within the gene
pool of a population or populations of a species through generations. Although, the changes in gene
frequencies occur gradually, and gene frequencies may remain constant for long periods of time before they
again change in response to changes in the environment. It means the rate of change in gene frequency can
increase or decrease, but the basic process of frequency chang e continues. To differentiate betwe en these
time scale s of evolutionary processes, the phenomena of evolution are divided into microevolution and
macroevolution.
16.1 MICROEVOLUTION
16.1.1 Definition
M icroev olution refers to changes in the allele or gene frequencies in a popu lation over short time sca le.
Microevolution introduces changes at the smallest scale and modifies the genetic equilibrium in the gene
poo l of a Mendelian popu lation from one generation to the next.
M icroe volution oper ates at the population level. It causes the gene pool s of closely related populations of
a species to diverge and contribute to the origin of new species (speciation). It forms the basis for all large
scale evolutionary changes or macroe volution.
16.1.2 Microevolution at Population Level
App arentl y, individual organisms interact with their environment and with other organisms. Each
organism's traits affect its survival and reproductive success in a given environment. It means natural
selection acts on individuals, but the evolutionary impact of natural selection is apparent in the changes
reflected in the gene pool of the population over the time .

842. For example, a population of seed -eating medium sized ground finch, Geospiza fortis, living on Isle
Daphne Major of Galapagos Island was exposed to prolonged drought in 1977. Of the population of 1200
birds, only 180 survived. Most of them had larger and deeper beak and could feed on large, hard seeds,
which were available during drought period. In subsequent generations, the number of finches with large
and deep beak increased manifold over finches which had small beak and fed on small, soft seeds. This
shows that during drought period, the beaks of individual finches did not become large or more deep .
Rather, the average beak depth in the population increased over time, because more deep beak finches
survived and produced more offspring than the shallow-beaked individuals.
Natural selection acts on individuals but evolutionary changes are reflected in the characteristics of a
population and more precisely, as the change of allele frequencies in the gene pool of population. The gene
frequency in the gene pool of population changes because natural selection, under changed conditions
favours new sets of alleles or individuals with new sets of alleles.
16.1.3 Mechanism of Microevolution The basic forces that change allele frequency responsible for
microevolution.
These are:
1. Gene Mutations
2. Recombination and nonrandom mating
3. Gene flow or Gene migration
4. Genetic drift
5. Natural selection
16.1.3.1 Gene Mutations
The primary mechanism for microevolution in the gene pool of a population are
is the formation of new alleles by mutation. Spontaneous errors in the replication of DNA cause permanent
alterations in organism's DNA creating new alleles instantly. Mutation may arise by exposure to physical or
chemical mutagens or ultraviolet light. They also occur constantly at a very low rate even under normal
conditions. Some of these mutations are heritable and are passed on to future generations.
Though , mutations arise at very low rate and very few of them are beneficial or adaptive, they create new
alleles . They are the only means by which new information or character 'comes into being, because the new
proteins produced by mutant alleles can modify the form or capabilities of an organism. When a new allele
proves to be advantageous, the number of organisms with the new allele increase through successive
generations due to successful survival and reproduction.
Since mutations are the only process that creates new alleles, it is the only mechanism that increases genetic
variability in the genome of individuals.

844. 376 ~ Evolutionary Biology
16.1.3.2 Gene Flow or Gene Migration
Allele fr equency in a population can also change with mating that can occur due to interbreeding between
organisms of two different populations. This is called gene flow or gene migration. Gene flow means
movement of alleles from one population to another. Such movements occur by the immigration or
emigration of individuals into or out of a population. When genes flow from one population to another, the
genetic variation in one population increases manifold but vairation between these populations decreases
because of exchange of alleles among their members.
16.1.3.3 Genetic Drift
Genetic drift is the chance alteration of allele frequncies in the gene pool of a poulation. Due to genetic
drift the allele frequencies drift up and down randomly over the time and can lead to the fixation or loss of
alleles without their adaptive value. Genetic drift exert tremendous effects on the gene pool of small
populations, though it occurs in the large population s too. The large populations, statistically defined as
greater than 100 reproducing pairs, are proportionately less affected by isolated random events.
Two types of genetic drift act when a large population modifies into a small population: Fragme nta tion
effect and Pioneer effect.
1. Fragmentation Effect or Bottleneck Effect: Bottleneck effect of genetic drift is seen in the change of allele
frequencies in a population which suffers from chance reduction in its size. Extreme reduction in the size of
a large population may arise by natural happenings , like drought, hurricanes, floods, fire or by man-made
events,such as habitat destruction or overhunting. The small surviving group of few individuals serves as a
'sample' . Their gene pool might have all the alleles present in the parent population or lacking some
alleles. In case, the survivors lack certain alleles, the future population grown from these survivors will
have allele frequency different from that of parent population, but similar to that of survivors.
2. Pioneer Effect or Founder Effec t: Founder effect occurs when a small subpopul ation breaks away from
the large parent population and migrates to a new area and establishes as a new population. The
individuals that colonise a new territory are called foun de r members and their gene pool becomes the
founding gene pool of a new population. The gene pool of new population will become much different over
generations due to genetic drift which results in fixing some alleles and losing some other.
Genet ic drift tends to make the population's gene pool homozygous. Such popualtio ns, sometimes , retain
harmful genes. Moreover, homozygous populations have extinction threat because they may not survive in
the changed environment.
16.1.3.4 Nanrandom Mating or Sex ual Selection
Hardy-Weinberg Principle follows that all males have an equal chance to mate all females with no
preferences. As a result, all alleles have equal chances to combine

846. and to be repre sented equally in the offspring . But in nature a number of factors influence selection of
partners, such as geographical prox imity in case of rooted plants and mate selection based on phenotypes
in birds and mammals. Differentia l mating success may depend on differences in combative abilities. Becau
se of preferential mating certain allele s have better chances of being transferred to the offspring than
others.
16.1.3.5 Natural Selection
N atural selection is the process in which individual organisms with differential adaptations to their
environment are favoured and passed on to the offspring with greater frequen cy. These adaptations are
introduc ed in the gene pool of populations by gene mutation and recombination.
As a result, the offspimg of population are found to be different genetically as well as phenot ypically. This
changed popu lation or descent population is the product of micro evolution.
C hanges produc ed by mutation and recombination may be beneficial or may not be so. But since the
chang ed genotype interact s with the environment, only those changes which increase the rate of
reproduction of the organisms directl y or indirectly becom e more numerous in the population. From this it
could be inferred that variations of adaptive value are preserved and encouraged by natural selection.
16.1.4 Examples of Microevolution
EXAMPLE 1: Size of the Sparrow
House sparrows were introduced in North America in 1852. Sparrow poulations in the north have
developed large-body than sparrow populations in the south. This divergence is due to natural selection
because large-sized birds can survive in cold, the small bodied birds can not. Due to their greater
surface/volume ratio, small birds need more energy to maintain high body temperature.
EXAMPLE 2: Evolution of Resistance
Evolution of resistance in pests to pesticides, weeds to herbicides and pathogens to medicines and bacteria
to antibiotics are examples of microevolution. Natural selection favours organisms whic h possess gene or
allele for resistance and are
able to withstand the environmental stress. A few examples of evolution of resistance are seen as follows:
• Mosquitoes developed resistance to DDT.
• Whiteflies evolved resistance to pesticides.
• Bacteria developed resistance to antibiotics.

848. 16.1.5 Accumulation of Microevolutionary Changes
Microevolutionary change might seem too unimportant to account for such amazing evolutionary
trasnsition s as the origin of dinosaurs or origin of birds/mammals or
378 iii Evolutionary Biology
the radiation of land plants. No doubt, microevolution happens on a small time scale from one generation to
the next, but when such minor changes add up over the course of millions of years, they translate into major
evolutionary changes on a grand scale, in other words , macroevolution.
The four basic evolutionary mechanisms -mutation, migration, genetic drift, and natural selection, can
produce major evolutionary change if given enough time. Life on Earth has been accumulating small
changes for 3.8 billion years, more than enough time for these simple evolutionary processes to produce its
grand history.
16.1.6 Types of Microevolution
Microevolutionary forces operating for a sh orter period produce sequential evolution, whereas when
continued for generations together result in the evolution of new populations from the existing one. The
origin of new populations can occur in two different ways :
I. In a successional manner, and
2. In a divergent manner.
The successional microevolution is the evolution within a single population which results in the
successional replacement of the pre-existing populations by the new ones. This could be seen in successive
strata of palaeontological series. It leads to microevolution or to the formation of clines, when characters of
a population seem to change gradually across its place of distribution. The formation of clines is an
example of gradual changes in response to gradual changes in the climate .
The divergent microevolution results in the splitting of parental population into two or more new
populations with the appearance of genetic divergence. Isolation is the additional factor operating to
establish genetic divergence in the related populations.
16.2 MACROEVOLUTION (ADAPTIVE RADIATION)
16.2.1 Definition
The broad basic pattern of evolution above the spceies level, which results in the production of new
adaptive types through a process of population fragmentation and genetic divergence, is known as
macroevolution.
It operates above the species level and results in the splitting of the population of a species into several
subgroups, each of which exhibits changes in a definite adaptive direction. These changes are known as
adaptive trends and the phenomeon as the adaptive radiation or macroevolution.
Macroevolution includes such evolution ary phenomena as patterns of origination, diversification of higher
taxa above species level and extinction. These patterns have developed over long periods and are usually
revealed by the study of fossils and comparative phylogeny.

850. 16.2.2 Development of the Concept of the Macroevolution
Macroevolut ion operates above species level
and results in the establishment of new
genera, familie s and orders. The changes
in the organisation occur on account of
sudden mutations of larger size, which are
nam ed "macromutations" or "systematic
mutations" by Goldschmidt (1940). IModem study of macroevolution is based on FIG. 16.1: Adaptive radiation in
the
palaeontological work of George Gaylord
Hawaiian honeycreepers.Simp son ( 1947, 1953) who focussed on the
rates and directions of evolution through fossil records, and Bernhard Rensch (1959) who inferred patterns
of evolution from comparative morphology.
16.2.3 Mechanism of Macroevolution
Macroe volution results in the splitting of a single lineage into many new lineages that occup y diverse
habitats and find food in different ways. Thi s is called adaptive radiation. Thi s can also be described as
ecological diversification within a single lineage.
M acro evolution occurs in ecologically isolated populations which have entered new adaptive zones free of
competition. In a new adaptive zone, the number of individu als is far less and the opportunities to ava il
new habitats are more. Therefore, the intraspecific struggle is roughly nil. Moreover, the new zone will be
almost free from the enemies, i.e., there is no interspecific competition. The newly entered popul ation
occupies all the available habitats of the adapti ve zone and its individuals start adapting themselves
according to the conditions and need of each new habitat. It means one population which had acquired the
new zone gets split up into several subpopulations, eac h accumulating mutations. All the subpopulations
evo lve independently but simultaneous ly in different directions. On account of different environmental
conditions, adapti ve modifications occur in different directions. Adapti ve modifications in each
subpopulation have a cumulative effect and are, therefore, directional.
16.2.4 Examples of Macroevolution
EXAMPLE 1: Honeycreeper songbirds evolved different types of beaks after a finch-like ancestor colonized
the Hawaiian Islands. They took to different types of food. Now their beaks are modified for eating insects,
sucking nectar or cracking seeds.
EXAMPLE 2: Hyracotherium
Ev olut io n of M odern Horse : Modern horse , Equus from from Eocene till Recent Epoch evolved as a
result of gradual adaptation for living in grasslands, feeding on grass and running fast to escape from
carnivorous enemies. This is an example of macroevolution by adaptive divergence. The ancestral group
from which horse evolved consisted of small

852. 380 ~ Evolutionary Biology
browse r, named Hyracotherium. It was of the size of a small dog with plantigrade feet, short legs and teeth
adapted to feed on succulent leaves. For fast running, the limbs became long and feet became unguligrade
with hoof. For grazing, their grinding teeth got modified with large and strong crushing surface for
grinding hard grass. The adaptive changes resulted in gradua l evolution of modern horse.
EXAMPLE 3: Ada ptive Radiation in Reptiles: Class the divergent evo lution or adap tive rad iation of
different reptilian or amphibian groups from initia l reptiles and amphibian ancestors are examples of
macroevolution. These earliest reptiles were cotylosaurs. They appeared first in the fossil records during
Cretaceous Period. All the living and fossil groups of reptiles have evolved from them. The evolution of
reptiles, actually represents six major radiating offshoots from the basal anapsid stock. These exhibit furt
her diversification of their populations resulting in the formation of orders and then families and so on. Fig.
16.2 depicts radiation of anapsid stock into six major groups and their diversification.
16.2.5 Essential Features of Macroevolution
Macroevo lution has following essential features:
I. Macroevolution occurs on account of macromutations.
2 . Macroevolution occurs in those populations which have entered or acquired a new adaptive zone.
3. Macroevolution results in evolutionary divergence or adaptive radiation , i.e., the pre-existing population
divide's into severa l diverging descendent popu lations by acquiring special adaptations.
4. Macroevolution produces groups of parallel special adaptations among divergent stocks.
5. Macroevolution leads to speciali sation in a particu lar direction. As a result, forms with special
adaptations become rigidly specialised to narrow adaptive subzones and reach the adaptive peak. This very
often leads to overspecialisation and finally to the extinction because overspeci alised forms are unable to
modify when they enter a new adapti ve zone.
16.2.6 Patterns of Macroevolution
All the c hanges , diversifications , and extinction that happened over the course of evolution of a lineage,
taxa or group are called the patterns of macroevolution. These include :
• Punctuate d equilib rium and stasis
• Directional character changes
• Lineage splitting or evolutionary divergence
• Extinction
16.2.6.1 Punctuated Equilibrium and Stasis
Individual characters as well as complexes of characters never evolve at constant rates. Many features in
fossilized lineages show gradual changes as well as patterns of punctuated equilbrium, i.e., they show long
period of little change, the stasis and then rapid shift from one static phenotype to other.
Punctuated equilibrium is the pattern of rapid evolutionary change in the phenotype of a lineage separated
by long periods of little phenotypic change.
Stephen Jay Gould and Niles Eldredge advocated that most evolutionary changes are rapid bursts of
speciation alternating with long periods in which individual species remain virtually unchanged. The long
intervals of geologic time in which very limited or no significant morphological changes occur represent
'period of stasis' or 'period of equilibria'. The intervals interrupting the period of equilibria are marked by
conspicuous or prominent evolutionary changes. These brief periods of active changes are called

853. 'punctuations'. During these brief periods the lineages actually branch or split. These are called punctuated
equilibria.
Many lineages on the tree of evolution of life exhibit stasis , which means that they are conservative and do
not change for a long time (Fig. 16.2 ). For example, lineages of King crab (Limulus) and Coelacanth fish
have changed so little for a long time after they branched off from the ancestors that they are called living
fossils.
16.2.6.2 Directional Character Changes
Character in lineage may show directional change. This is called directional evolution . For example,
molar and pre- molar teeth during horse evolution gradually changed into grinding teeth , and in the
evolution of carnivores canines grew into long and pointed teeth to tear the flesh. This is an example of
change within a single lineage. Changes in a character can occur across several lineages producing
convergent evolution. For example, trilobites belong to the same clade as modem insects and crustacea.
They lived over 300 million years ago and have changed very little but all the three groups underwent
similar increase in the number of body segments.
16.2.6.3 Lineage Splitting
Lineage splitting is the origin of new species
by parapatric and peripatric speciation
or origin of higher taxa , like genus,
>.
family
,
order
,
etc. The patterns of lineage
"0
o
splitting can be identified by examining the
1Il
phylogenies. The following three patterns
have been identified:
• Frequent Lineage splitting: This gen
erates a bushy tuft of lineage-branches ..--- Body form ------+ on the phylogenetic tree (Fig. A) due
FIG. 16.2:
Two lineages showing stasis
to frequent lineage splitting. for a long time after the bifurcation .
382 1iJ Evolutionary Biology
• Rare lineage splitting: The lineage splitting in some cases is low or rare. The phylogeny tree is
represented by long straigh t branches with very few twigs (Fig. 16.3 A and 16.3 B).
• Burst of lineage splitting: Several lineages may show sudden burst of lineage splitting almost simulta-
neously (Fig. 16.3 C).
16.2.6.4 Gradualism and Saltation
T he persistent accumulation of small changes within a lineage has been termed ph yletic gradualism. It
involves accumulation of small changes over millions of years within one lineage so that the descen dant
population or populations may be recognised as distinct species differing from antecedent populations. The

854. transformation of a lineage over time is termed a anagenesis. For example, in Fig. 16.3 A new species 8
arises by slow and steady transformation of a large antecedent population A.
Ac cording to Gould and Eldredge the phyletic gradualism is very slow and is unable to produce maj or
events of evolution.
Palaeontologist, Otto Schindewolf ( 1950) advocated that differences among higher taxa have arisen
discontinuou sly by saltation (from latin sa ltus, means 'a jump'). Geneticist, Richard Goldschmidt argued
that species and higher taxa arise not from the accumulation of genetic variations that reside within the
gene pool, but from single evolutionary steps as completely new genetic systems.
Howeve r, it is now established that higher taxa do not arise in single steps by macromutational jumps
(saltations), but by multiple changes in the genetically
A B c
Q)
l
E
F

856. Body form
FIG. 16.3: Phylogeny tree showing three types of lineage splitting : A. bushy lineage produced by frequent
lineage splitting ; B. Rare lineage splitting , C. Burst of simultaneous lineage splitting.
independent character s (mosaic evolution).
D
Such characters evol ve gradually through
intermediate s ta ges. However, some
characters do evolve discontinuously as a
result of mutations with large effects.
16.2.6.5 Extinction
Extinction is total disappearance of a lineage
or death of all the members of a species.
About 99.9% of all the species that ever
existed on planet Earth are now extinct. The FIG. 16.4: Anagenesis and Cladogenesis. process of evolution
is basically characterised by continual turnover of specie s, i.e., new species arise and old ones go extinct.
Extiction is extremely important in the history of life. It can be of the following two types:
• Background extinctions
• Mass extinctions
1. Background Extinctions: These occur regularly at low or average rate and
are caused by normal environmental changes, any emerging disease or competition with other specie s.
2. Mass Extinctions: These are rapid extinctions. They occur at mass scale and represent catastrophic
episodes that wipe out huge number of species and lineages in a short time . Five episodes of mass
extinction have been identified to occur on Earth so for.
Probable cause of extinction may be changes in the environment or environmental catastrophes. Localised
distribution and overspecialisation make species vulnerable in the changing environment. Habitat change
and destruction of habitat also lead
intraspecific and interspecific competition may also drive a to extinction. Finally species to extinction.
16.3 MEGAEVOLUTION
16.3.1 Definition
Megaevolution has been described as t he origin or evolution of new types of biological organisation as a
result of genera l adaptation from its predecessor resu lting in the formation of new classes, groups or
phyla.
Megaevolutionary changes are rare and have occurred only a few times in the evolutionary history of living
beings. But the most interesting thing is that all these biological organisations persist without extinction
(with few exceptions). All the phyla and most of the classes of micro-organisms, plants and animals
represent their separate organisation and are produced as a result of megaevolution. The origin of
amphibians from fishes, origin of reptiles from amphibians and the origin of birds and mammals from
reptiles offer best examples of megaevolution.
38 4 ~ Evolutionary Biology
16.3.2 Mechanism of Megaevolution
Du rin g megae volution the organisms of the ancestral stock attempt to enter a new zone, which is

857. uninhabited by these forms and is devoid of competition. The y acquire vari ed modifications in different
directions until one of these is found suitable to the new zon e. It means a group of individuals of the
parental stock develops certain generalised preadaptation s wh ich enable them to enter the new zone.
Therefore, these make a breakthrough into the ada ptive zon e and start radiating into all the available
habitats, thereby developing mo re specialised adaptations which are known as postadaptations.
The mec hanism of megaevolution can be exp lained by taking origin of repti les from amphibians as an
example. Amphibians are amphibious creatures which could live in moist places near some source of water.
16.3.3 Examples of Megaevolution
EXAMPLE 1: Rept iles evo lved as com pletely terrestrial forms which need not depend on aquatic medium
at any stage of their life cycle. At that time, the terrestrial zone was unoccupied, devoid of competition and
accessible. The principal new general preadaptations which evolved in some of the ancestral amphibians to
invade terrestrial zones are:
1. The deve lopment of exoskeleton in the form of scales, plates or scutes which prevented desiccation of the
adults.
2. The appearance of large c1eidoic eggs which enabled the young one s to develop on land.
EXAMPLE 2: Origin of bir ds from some primitive reptiles includes the sudden appearance of wings which
enabled the ancestral form to make the invasion of aerial zone. The fossils of ancestral bird, Archaeopteryx.
exhibit reptilian characteristics together with wings and feathers.
EXAMPLE 3: Evo lution of mammals can be traced back to a series of fossil synapsid reptiles of the group
Therapsida. These developed several mammalian characteristics like the false pa late , tee th differentiated
into incisors, canines, premolars and mo lars , and the limbs became mod ified so that the elbows and knees
were placed under the body. But still these forms were reptiles because teeth were without roots and the
quadrate and articular bones did not form the ossicles. In Therapsid group the main pre adaptation towards
mammalian offshoot was freeing of the quadrate and artic ula r bones from jaw articulation and their
conversion into ear ossicles. The particular change served two purposes:
• it improved heari ng
• direct articulation of mandible or dentary wit h the skull to strengthen the jaws.
16.3.4 Special Features of Megaevolution
Megaevolution exhibits following special feature s:
I. It includes experimentation and exploitation of new zone by the members of the ance stral stock in several
divergent lines . This experimentation involves appearance of new characteristics which may prove suitable
for a new zone .
2. Megae volution operates on individual s which have developed some novel adaptations for the new zone
but are of no use in the environment they are living.
3. The preadapted group of individuals crosses the ecological barriers and makes a breakthrough into the
new zones.
4. The breakthrough and shifts are alway s rapid , otherwise they fail because of extreme negative selection
in unstabl e ecological zone.
5. The new zone is alway s ecologically accessible and is devoid of competitors.
6. The initial shift is alway s followed by adaptive radiation which is actuall y macroevolution.

858. 16.4 TRENDS DURING MACRO AND MEGA-EVOLUTION
In evolution a trend is described as a directional change in the average value of a chara cter in a clad e
over the course of time. Evolutionary trends are seen in individual clades or in a large number of clade s at
the same time. Trends can be active or passive.
1. Active Trends: Activ e trend s are changes within the lineag es in a specific direction, caused by natural
selection. They proce ed in one direction and cause direct ional evolution. For example, increa se in brain
size during human evolution and evolution of one toed limb , develom ent of hoof and increa se in bod y size
during evolution of horse are active trends.
2. Passive Trends: These are changes in the lineages in the clades that occur in both directions with equal
probability. The passive trends are enforced due to function al or developmental genetic constraints.
Both active and passive trends lead to origin and establishment of new lineages.
KEY TERMS
• Clade • Divergent evolution
• Gene flow • Gradualism
• Macroevolution • Microevolution
• Postadaptation

860. • Saltation
• Stasis • Successional evolution
• Extinction
• Lineage
• Preadaptation
• Sexual selection
REVIEW QUESTIONS 1. Describe forces responsible for microevolution to occur.
2 . What is the level of operation of microevolutionary forces and what results are produced by them in
living world?
3. (a) What are lineages and clades?
(b) What type of evolutionary changes lead to lineage splitting?
4. Is lineage splitting a microevolutionary or macroevolutionary event?
5. Illustrate the ro le of macroevolution with suitable examples.
6. Illustrate with suitable examples the mechanism of megaevolution.
7. Differentiate between three types of evo lution. Trace the inter-relationship between them.
8. Differenti ate between the following :
(a) Sequential and divergent evolution.
(b) Micro and macroevolution
(c) Micromutations and macromutations
(d) Convergent and divergent evolution
9. Summarise special features of megae volution.
FURTHER READINGS
I. Futuyma, OJ ., 2009, Evolution, (2n d Ed) , Sinauer Associates, Inc. Publishers, Sunderland,
Massachusetts, U.S.A.
2. Gould, SJ., 2007 . Punctuated Equilibrium, Harvard Univer sity Press, Cambridge , MA.
3. Rench , 8., 1959. Evolution above Species Level, Columbia University Press, New York.
DOD

861. 17
Adaptations
17.1 INTRODUCTION
Adaptation i s one of the two main processes that explain the occurrence of diver se species of organisms.
The other process is speciation or cladogenesis.The organisms are surviving because they are adapted. The
fact becomes more evident when we observe the nature and the living organisms all around . Fishes and all
other aquatic lizards, birds and mammals have markedly different organisation but all of them have one
common feature of possessing fins or paddle-like appendages for swimming. The flying animals all possess
wings or wing-like structures. It means the organisms are adapted to their habitat or mode of life.
Since the environment i s ever changing, different organisms should either change accordingly in order to
survive the rigors of the new environment or be ready for extinction. Therefore, succession of environmental
changes is paralleled by the development of adaptive features, both morphological and physiological
including food and feeding habit, way of living , defensive mechanisms and protection against bad weather.
J ean Baptiste de Lamarck was the first who emphasised that animals modify themselves according to the
changing environment. Darwin believed that animals are preadapted and seek suitable environment. Niles
Eldredge said that adaptation is the heart and soul of evolution.
•17.2 DEFINITION OF ADAPTATIONS
Adaptations may be defined as , 'the modifications in the organisation or physiology of organisms which
enable them to thrive successfully in a particular environment so that they are able to ensure sufficient food
and protective life to sucessfully produce their offspring.'
Adaptation is a heritable, morphological or physiological character which enab les the organism to survive
and reproduce successfully in its habitat.
Adaptations help organisms to:
• Reproduce successfully
• Compete with other living organism s in their niche
• Survive successfully in their ecological niches
An adaptive trait is the aspect of deve lopmental pattern of the organism which enables or enhances the
probabi lity of that organism to survive and reproduce .
17.3 KINDS OF ADAPTATIONS
Adaptations can be classified under follow ing four heads: I. Structural adaptations
2. Physiological adaptations
3. Behavioural adaptations
4. Protective adaptations
5. Animal association adaptations
17.3.1 Structural or Morphological Adaptations
The structual adaptations include changes in the structure of organisms such as body shape and size, body
covering and the internal organisation. These are induced by the physical environment and may be

862. discussed under following heads:
1. Cursorial Adaptations (Adaptations for Fast Running): These include modifications in the animals,
which adapt them for running swiftly on land. The limb bones become elongated and the digits get reduced
in number.
2. Fossorial Adaptations (Adaptation for Burrowing): These are changes in the organisation of burrowing
and cave-d welling animals which are characterised by the possession of a slender unpigmented body with
long attenuated appendages, reduced or functionless eyes, increased sense of smell and touch, changed
feeding habits and ability to burrow, etc. Their limbs are either adapted for digging the burrow or reduced
for efficient borrowing.
3. Scansorial Adaptations (Adaptations for Climbing): These are climbing adaptations found in lizards,
squirre ls, monkeys, apes, sloths, Hyla, etc. These include better and strongly developed shoulder girdle,
much elongated proximal segments of the limbs, prehensile feet with opposable digits, well developed
claws, adhesive pads at the tips of digits or in the sole and the prehensile tail.
4. Aquatic Adaptations (Adaptations in Aquatic Forms): These adaptations include structural modifications
that enable the organisms to live in water. Aquatic animals have stream-lined body, webbed-limbs, reduced
neck, laterally compressed tail and gills as the respiratory organs. The deep sea forms develop
phosphorescent organs. The animals living in fast running hill streams develop suckers or adhesive discs
for attachment to rocks or aquatic plants or they develop power of swimming
Adaptations Ii] 389 against the water current. The pelagic forms develop antisinking devices. The aquatic
insects and their larvae develop long feathery hairs that prevent sinking.
5 . DesertAdaptations: The individuals living in desert have moisture preservation capacity. They develop
hygroscopic skin (lizard) , water cells in the stomach wall (camel) and body covering with thick dermal
scales Or bony scutes (snakes) . The eyes, ears and nostrils are well protected against dust.
6. Aerial or Volant Adaptations (Adaptations to fly in Air): The animals possessing power of flight have
following structural modifications:
• Stream-lined light body
• Pneumatic bones
• Forelimbs modified into wings
• Strongly developed wing muscles
• Reduction in the number of bones and other acces sory structures in the body
• Fusion of certain bones
• Better deve loped humerus and sternum
• Elongated radius-ulna, carpometacarpus, tarsa ls, metatarsals,
• Better-developed sense of smell , touch and sight.
7. Parasitic Adaptations: The parasitic organisms undergo various structural modifications due to parasitic
mode of life. The modifications include simplification in the organisation , elimination of locomotory
organs, loss of sense organs and alimentation and the extraordinary development of reproductive organs .
8. Ecological Adaptations: Ecological conditions induce structural change s in the organisms. A few
examples are as follows:
( a) Anabas, the climbing perch (a fish), is adapted to make excursion on wet paddy fields because it

863. possesses accessory respiratory organs to breathe on land and in muddy water.
(b) Heteropneustis (Saccobranchu s) and Clarias living in ponds with lesser concentration of oxygen have
also developed accessory respiratory organs.
(c) Surface swimmer marine shark s lack air-bladder.
17.3.2 Physiological or Biochemical Adaptations
The physiological adaptations are changes in the physiological or biochemical processes in plants and
animals to adapt themse lves to changed conditions of life. For example, protozoa are adapted to survive
within a temperature range of 2Q-40°C. Elasmobranchs are well adapted to higher concentration of salt in
sea water by retaining urea in their body fluids and, therefore, possess higher osmotic pressure.

865. • Birds and mammal s are able to maintain a constant body temperature.
• Desert animals prevent loss of water by reducing rate of respiration and excreting solid urine or drier
faeces.
390 [i] Evolutionary Biology
• Making venom or secreting slime by animal s as a means of protection against enemies are physiological
adaptations.
• Night p hot o synth esi s (C
4
photosynthesis) in desert cactic is a physiological adaptation to prevent
water loss from stomata by keeping them closed during day time.
17.3.3 BehaviouralAdaptations Behavioural adaptations include
inh erited behaviour activities or ability to learn and act according to the circum stances. For example,
search for food and mate, mating behaviour, remaining hidden in borrows in desert during day time, to
enter the burrow to escape the enem y or predator and to produ ce warning sounds, etc., are all behav
ioural adaptations.
Fig . 17.1: Polar animal-po lar bear
Fig. 17.2: Tree frog and green grasshopper
17.3.4 Protective Adaptations
Adaptation s which protect the animals from their natural enemies are called protective adap ta ti on s.
These are effective only against a particular enemy and minimise the risk of death or injury from natural
enemies. The animal may defend itself either by counterattack or by hiding. Natural enemies hunt their prey
by sight and smell. Therefore, adaptations for protection may be either visual or nonvisual.
The se adaptations comprise the external appearance, attitude, colour, shape and beha viour ; such as
I. Offensive odours .
2. Hard shell s
3. Spines on the body of hedgehog,
Fig. 17.3: Bird's egg
etc.
Fig . 17.4: Colour bands on fish body

867. The various visual adaptations are:
1. Protective Colouration
Protective colouration is the harmony of colour , tone and shade of the body of the animals with their
surrounding, so that they blend with the surroundings and are indistinguishable from the background.
Concealment is achieved by doing away with (I) the outline of the body , (2) the shading that gives a solid
appearance, (3) the spots, stripes and irregular patches of some colour that contrasts with background.
This is known as disruptive colouration. The protective colouration gives rise to uniform tone and make s
the animals invisible to the enemy so long as it is perfectly motionless.
EXAMPLES:
A few examples of concealment and protective colouration are as under:
• Polar anima ls (polar bears , polar foxes polar rabbit) are white in colour and
are invisible agai nst the white background.
• Green colour of tree frogs, snakes, grasshoppers and caterpillars conceals
them with the surroundings.
• Many desert animals have sandy hue of their hide which matches with the
sand .
• The colour of eggs of many birds, which lay eggs in open nests or on open
ground hannonises with the ground and the scatte red pebbles.
• Several species of insects , fish and snakes have bands of dark colour alternating with lighter zones . The
banded pattern breaks the continuous contour of the body and the prey remains undetected from the
surrounding unless it moves . Similarly, zebra is inconspicuous and is detected by lions and leopards only
when it moves.
• Small eye spots are found on the surface of inner wings of moths and on undersurface of wings of
butterflies . These eye spots protect the moth and butterflie s by misdirecting the attack of predator. The
predators tend to attack these eye spots at the apex of wings . The moths and butterflies flyaway with a part
of wing missing and
survive the attack.
• The predator species mimics a
harmless species to attract the
prey. For example, the females
of firefly genus , Photuris ,
mimics the flashing patterns
as mating signals to attract
fireflies of genus Photinus. The
male Photinus are attracted to
these flashes and are caught in
death trap . FIG. 17.5 : Fish with eye spot

869. 2 . Aggressive Colouration
In aggress ive colouration, the predator harmonises with the surrounding and remains undetected by the
prey. For example, many sea bird s like gulls and terns are stee l gray or blue above and white beneath.
This makes them harmonise with the sea when viewed from above, and with the sky when seen from below.
Many fishes, like blue fish and mackerel are also coloured the same way, not to be detected by the aquatic
animals.
A ggressive coloura tion is also meant to frighten the enemy. Many species of moths and butterflies have
large, prominent markings or eye spots on their wings which imitate vertebrate eye. When these animal s
are at rest, the eye spots remain concealed. When disturbed, the wings move exposing the eyespots . The
sudden exposure of spots scares small birds and make them flee.(Fig . 17.3)
EXAMPLE 1: In the fish Chatedon, there is a large eye spot near the tail (Fig. 17.4) and head is crossed by
one or two black vertical bands running through the eye, so as to conceal it. The fish swims very slowly with
tail first. When disturbed, it darts swiftly to a safer place in the opposite direction.
EXAMPLE 2: Turkey cocks, during court ship, spread their feathers and erect the coloured appendages on
the head to threaten their rivals.
FIG . 17.6: Leaf-fish that closely resembles the leaves floating in water : Labotes surinamensis that
resembles leaf of Thespasia populaea plant
A B
FIG. 17.7: A. Disruptive colouration in a tropical reaf fish; B. South American toad, Sufo superciliaris, is
strikingly similar to the leaves lying on the floor.

871. A B
FIG. 17.8: Aggressive colourat ion in moth, Automeris coresus Moth displaying the eye spots on the
hindwings
FIG. 17.9: Chatedon fish with FIG. 17.10: Spider resembling an eye spot near its tail end Orchid flower. 3.
Alluring Colouration
Certain carnivorous forms such a s spiders resembl e in colour with the flowers upon which they rest. The
insects visiting flowers for pollen or honey fall prey to such spiders. A spider resembles an orchid blossom
both in colour and form and allures insects to visit them. (Fig 17.10)
4 . Warning Colouration
Warning colours are conspicuous red and yellow, found in poisonous and unpalatable
animal s. Such bright colours are special mean s of defence and both poisonous as well as nonpo isonous
animals find it advantag eous to adverti se their harmful or dangerous nature.
EXAM PLE 1:
The wasps having powerful sting are coloured yellow, black and red. Most of the conspicuously coloured
spiders found in different parts of the
world are deadly poisonous .
Butterflies which are acrid to taste are conspicuously coloured. Coral snakes, kraits, poisonous lizard
Heloderma punctatum and tiger salamander,
Ambystoma tigrinum , exhibit warning colouration.
Most predators are found to recognise and avoid
the animal s that have warning colours.
5 . Mimetic Colouration
In this case the colour of an animal resemble s with the
colour of some object, animat e or inanimate.
6. Recognition Marks

873. The recognition marks are used by individual s of the
FIG. 17.11: Coral snake,same species to recognis e each other.
TIger Salamander.
EXAMPLES:
• Red and orang e spots on the side of the brook trout.
• Recognition marks are also borne by many insects. In many butterflies and
moths, such marks are visible only when they are flying.
• Sea birds recognise thei r eggs by the mark ing on them . Recognition mark s serv e to harne ss the
gregariou s birds durin g flight.
• A large number of orthopteran insects depend on wing colours during courtship.
• Male s of fiddler crabs use their brightly coloured claws as signa ls, waving them when female s are in
view.
7. Sexual Colouration
I n many cases, sexual dimorphism is displayed by the difference in colouration. In birds and mamm als, the
males are more conspicuously coloured than their females. This is seen in Oriole, Ducks, Domestic fowl s
and Pheasants . This helps to protect the females from predators espec ially when they are nestli ng.
S imi larly, spotted coat of ma le deer, lion and puma is brightly coloured and more con- spic uous to attrac
t their females for courtship.
17.3.5 Significance of Colouration
Co louratio n is of vital importance for organ isms since it enables them to mai ntain themselves
successfully in their environment, and increases chance of surv ival. For example, the striped hide of zebra
spotted skin of panthar are helpful in concealment in its enviro nme nt.
As a matter of fact, zebra becomes invisible at a distance of 6 m in three conditions: ( I) in open plains at
mid-d ay, (2) at close quart er at the time of dusk and (3) on moon nights and under the cover of dense
cover of trees.
C olouration pro vide s camouflaging and helps in protection and successful survival and multiplication of
the race and species . It pro vides them repro ductive success.
FIG. 17.12: Fiddler crab.
FIG. 17.13:
Concealing
colouration as
in panther
17.3.6 Objections to the Adaptive Value of Colouration
I. It is found that anim als which lack conceal ing colou ration fare equally well. The raven wears the same
black plumage from arctic zone to tropics . It is an aggressiv e bird because it has a stro ng beak and
powerful claws, and requ ires no protective colouration.

875. 2. It is believed that keen-sighted enemies are not deceived by cryptic colouration. This objection is
overruled because birds show selection in feeding and they avoid unpalatable insects.
3. Cryptic mimicry depends on colour vision while most animals lack colour vision . But bees and birds
show colour perception of flowers they visit.
4. Experience and memory are required by the enemy and the model both for the dangerous nature of the
model which the harmless forms imitate. Presence of such memory has not been proved. Some animals do
possess association memories.
5. Cryptic colouration can not ensure safety because animals hunt by scent and sound. This is true but
eyesight is important in close range and colour vision is present in some animals.
6. Concept of protective colouration is based upon anthropocentric conception.
17.4 MIMICRY
In mimicry, the animal resembles or imitates other animal , plant or other natural object not only in colour
and markings but also in shape, size, appearance, surface, structure and other details.
17.4.1 Types of Mimicry
Mimicry can be classified into three categories: Protective , Aggressive and Batesian and Mullerian
Mimicry
1. Protective Mimicry
The protective mimicry can be achieved by concealment or by warning.
FIG. 17.14: Flatfish with patchy body colour to blend with the pebbled backgroud.
A. Concealment Mimicry
In concealment , the organism conceals or camouflages with the background either by:
• changing colour to match the

877. background
• changing place to match the background
• mimicking some living or dead object
A B
FIG. 17.15 : A. Kallima (Dead leaf butterfly) showing concealing mimicry, B. Stick insect showing
resemblances to a dry stick.
EXAMPLES:
(i) White crab hannonises with white pebbles on the beach.
(ii) Flatfishes develop patchy skin to match body colour with the background.
(iii) Indian dead leaf bu tt erfly, Kallima, resembles dry leaf and stick insect, Caras ius mimics a dry stick.
The former exhibits surprising resemblance to leaf and latter with a dry stick. Kallima, the dead leaf
butterfly, is a classical example of protective mimicry. Its wings are brilliantly coloured above but have dull
brown undersurface like a dry leaf. When the butterfly settles down and rests , the wings are folded with
their upper surface together. Therefore, the exposed undersurface resembles a dry leaf. Each wing also has
a line resembling the midrib and notches to match those of the leaf. While at rest, butterfly can not be
distinguished from the dead leaf.
(iv) The s tic k insec t or wa lki ng stick exhibits close resemblance with the twig in having slender body,
attenuated limbs, sympathetic colouration and slow movement.
(v) Lea f insect, Phyllium, possesses flattened and expanded body and limbs. It is green in colour and
possesses irregular yellowish spots which simulate the fungus or rust grown upon a leaf.
(vi) The stick caterpillars of geometrid moth, Solenia tetra lunaria, resembles a small twig in colour and
shape . At rest these get stiffen by grasping the branch with their hindlegs and stand out off the branch at
the angle of a twig. Its pointed head resembles the terminal buds and the excrecences on the side resemble
the lateral buds of a twig .
.(vii)Phyllopteryx eques (the Australian sea horse) has leaf-like cutaneous outgrowths over the body which
very much resemble the fronds of sea weeds .
B. Warning Mimicry
I n warning mimicry, the nonpoisonous and harmless organisms mimic the poisonous and harmfu l
organisms and the palatable forms resemble and advertise to be non-palatable. This type of mimicry is
helpful in self defence because the mimics are able to deceive and frighten the enemy and escape.
FIG . 17.16: Walking leaf insect, Phyllium , showing resemblance with a green leaf (an example of
concealing mimicry).
EXAMPLES: (i) Certain brightly coloured nonpoisonous coral snakes of the family Colubridae resemble
poisonous coral snakes of family Elapidae. Several nonpoisonous snakes behave like poisonous snakes by
hissing and striking like cobra. By imitating, they are able to elude the enemy and escape themselves. This
is called cryptic behaviou r.

879. (ii) Palatable and defenseless organisms
resemble in form, colour and action
to inpalatable and distasteful animals
and manage to escape from the enemy.
For example, distasteful and inedible
monarch butterfly, Danais, is imitated
by viceroy butterfly, Lementis, which is
edible and may easily be preyed upon. FIG. 17.17: Phyllopteryx eques
Some spiders that are associated with ants, not only have the same warning colours but also the same
general appearance and hence are mistaken for ants by their enemies. This is also called Batesian mimicry.
(iii) In some dimorphic species, only the female butterflies exhibit resemblance with distasteful or inedible
butterflies . For example, Papilio meriones, is a nonmimetic species found in Medagascar. Its males and
females both possess tail on the hindwing.
(iv) In Papilio dardanus which is found in Africa, the males mimic with P. meriones, whereas females mimic
an unpalatable, Danoid butterfly. (v) Some spiders that live in association with ants, not only have the same
. warning colour pattern but the same appearance mistaken as ants by their enemies.

881. A. Viceroy butterfly
FIG. 17.19: Spider resembling ant
B. Monarch butterfly
FIG . 17.18: Viceroy butterfly and monarch butterfly showing resemblance (an example of warning
mimicry). A. Viceroy butterfly (Lementis), the mimic, B. Monarch butterfly (Danias), the model.
FIG. 17.20: A. South American peacock butterfly 'Automeris memusae' showing eyespots on the hind wings;
B. Eyespots on the shell of Burmese soft turtle, Trionyx hurum.
C. Cryptic Structures
Animals exhibit severa l contrivances to delude the predator and protect themselves.
EXAMPLES:
1. Black Spots or Eye Spots: Butterflies , caterpillars, peacock butterfly, etc., posses s black spots on their
wings or body. These spots may be simple rounded black spots or black spots with light contrast rings or
exactly eye-like. Their sudden display by the animal at the time of danger frightens off insectivorous birds.
Black spots are also displayed by various fishes (Notopterus chi/ala and flatfish ). Eye spots also occur on
the shell of Trinoyx hUnl11I the Burmese soft turtle. Since the predators attack the prey towards the eye or
in their vicinity, they are misdirected by conspicuous eye spots present at the posterior end and the prey is
protected.
2. Dummy Head: In certain anima ls the posterior end of the body is modified into a dummy head. It has eye
spots and antennae-like structures. Their true head is inconspicuous and remains hidden below the body.
The dummy head misleads the predators, which usually attack the head end. The victim suddenly darts in
the wrong direction and the predator simply snaps into the empty spaces .
• The lantern fly from Thailand has conspicuous structures on the posterior end resembling antennae, black
eyes and black beak. These are actually the appendages of the wing tips. On the contrary, the head is small
and remains hidden below the body.
• The butterfly of genus 'Thecla' also exhibits a dummy head at the hind end of the wings by converging
colour stripes on the wings and having antennaelike appendages on the wings tips.

883. FIG. 17.21: Eye spots on the body of Notopterus and Monochrius.
FIG. 17.22: A. Lantern fly showing dummy head at the posterior end of the body;
B. Butterfly, Thecla, showing dummy head; C. Trinidad linus showing dummy head.
• Trinidad linus squarts on ventral surface wit h its head always pointed downwards. Its wings are produced
behind into a pair of processes which are moved like antennae giving it the appearance of head.
• Many snakes, when faced with opponents or prey, hold their head still and move the tip of their tail so as
to divert the attent ion of the animal away from the genuine head, as in Cylindrophis rufus.
2. Aggressive Mimicry
Aggressive mimicry is exhibited by predatory anima ls such as fishes and spiders. These animals either
conceal themse lves so that these are not easily recognised from their surroundings (concealment) or allure
the prey (alluring) . Therefore, aggressive mimicry is of two types:
A. Concealing Mimicry
The animals develop cryptic colour s so as to blend with the surroundings or mimic to its model, so that
they are not differentiated easily. This helps them in getting their prey easily.
EXAMPLES:
• Spiders resemb le in shape and colour to the flowers of Orchids , on which they live. So they are not easily
distinguished from the flowers. By hiding themse lves among the flowers these spiders are able to prey on
bees which visit flowers for nector or pollen.
• The zone-tailed hawk (Butea albonotatus) glides along with vultures in America and has close resemb
lance to them in colour and shape of the wings. Vultures do not prey upon small animals. The hawk exploits
this and dives from the vulture group to take its prey. This was observed by E.G. Peckhammian.
B. Alluring Mimicry
In thi s type of mimicry the animal possesses some lure to attract its prey, whereas it blends itself with the
surroundings. The misleaded animals fall victim and form the prey of the mimic .
EXAMPLES:
• Certain spiders mimic the flowers of orchid and the insects lured to collect nectar are devoured.
• Angler fish, Lophius, lives at the sea-bottom and camouflages with it. The first fin ray of the dorsal fin is
located on the dorsal edge of upper lip some distance in front of the eyes. This ray is known as ilIicium . It
can rotate freely in its ball and socket joint and bears a fleshy-cutaneous appendage , the at its free end.
This is held in front of mouth, swings in all directions and acts as a bait for small fishes.
400 lil Evolutionary Biology
• The females of firefly genus 'Photuris' mimics the flashing patterns to attract fireflies of genus, Photinus as
mating signa ls. The male Photinus are also attracted to these flashes and are caught in death trap.
C. Simulation to Death
Cert ain animals on the approach of danger or prey behave like a dead body. For example, American Oppo
sum , Didelphis virginiana, rolls like a dead body. Some beetles drop like a paddle when these are about to
be caught and lie inert after falling .

884. D. Terrifying Appearance
Som e harm less or defensele ss animal s assume terri fying
appearance and produ ce various noises to frighten the
enemy.
F or example, Grasshopper have green colour but to FIG. 17.23: Spider resem
frighten the enemy spread their wings or
expose red spots
bling orchid flower.
from the underwings so that the enemy is confused and
the grasshopper manages to escape .
E. Warning Colours
Some animals exhibit bright shining warning colours such as red, yellow or black either in patches or
stripes . These advertise the unpleasant taste, toxic nature and foul smell. For examples Danais, a butterfly
and Torpedo, the electri c fish have warning colours.
F. Warning Signals
An imals produce warning signal s in some form or other. Rabbits use their white tail for the same purpose.
Rattle snake s produc e rattling noise while moving on the ground .
FIG. 17.24: Lophius, the anglerfish with illicium
17.5 BATESIAN A ND MULLERIAN MIMICRY
In Bate sian mimicry the palatable and unprotected species mimic the unpalatable or well protected species.
This type of mimicry is called Batesian mimicry. For example, if one species of butterfly whic h is palatable
to birds simu lates the other unpalatable species of butterfly, it gets the advantage of being spared from the
predatory enemies. It means , in Batesian mimicry, the mimic gets the advantage of model.
In Muller ian mimicr y two or more unpalatable species resemble each other. For example, the ctenuchid
moth resem bles a wasp , where both of them are unpa latable.
The Mullerian mimicry is beneficial to both model as well as the mimic because if a bird has learned not to
eat wasp it has automatically also learned not to touch the ctenuchid moth which mimics the wasp.
Similarly, if the predator finds this moth to be unpalatable it will not prey upon the wasp . This sort of
mimicry is advantageous to the moth as well as wasp because it reduces the number of chances of
destruction. Beebe and Kenedy reported that ctenuchid moth which resembles wasp is unpalatable to a
lizard and three species of ants . Wasps are also high ly inedible.
-iti-ll=-iX. Mechanisms of Sympatric Speciation
Batesian Mimicry
1. Discovered by Henry Walter Bates .
2. Palatable species mimics the nonpalatable one.
3. Nonpalatable species is model and palatable species is mimic.
4. Mimic gets the benefit.
Mullerian Mimicry
1. Discovered by Fritz Muller in 1864.
2. Two or more unpalatable forms resemble each other.

885. 3. There is no model ; all species are mimics. They mimic each other.
4. Allthe related species are benefited .
17.5.1 Conditions Necessary for Mimicry
The conditions necessary for the occurrence of protective mimicry have been analysed by Wallace and are
listed below:
1. The mimics of the imitative species occur in the same area which are occupied by the models .
2. Mimics are always or more often defenceless and they mimic the offensive or harmful animals.
3. Mimics are always less in number than the model individuals so that the deceived animals encounter the
mimics less often than the model.
4. The imitators should differ from their allies.
5. Even if the imitation is minute, it shou ld be external and visible.
17.5.2 Significance of Mimicry
The main significance of mimicry appears to be the protection of individuals against enemies. This self
defence is naturally of the survival value and of evolutionary significance. It means natural selection which
is supposed to be one of the most important factors in evolution, favours mimicry, since it assures better
survival and greater success in the multiplication of race.
17.5.3 Objections to Theory of Mimicry
Several objections have been put forward against the theory of mimicry: 1. The most important objection to
theory of mimicry is that birds and other predators do not eat butterflies and not kill them in such a large
number that it may
402 ~ Evolutionary Biology
threaten their existence and may lead to the evolution of mimicry. But Kettlewell (1955-56) conducted
experiments with melanic moths and had shown that birds do eat moth s and that natural selection operates.
Poulten and Carpenter have also shown that butte rflies are devoured by birds . Moreover, marks of damage
done by the beak of birds are often seen on the wings of butterflies, which indicates that birds do prey upon
butterflies. Lastly, Brower ( 1958) has shown that birds do not eat distasteful butterflies. Therefore, this
objec tion is ruled out on the basis of observations.
2 . Uvarov (1932) raised the objection that the ants are devoured by a large number of animals and ,
therefore, it is rather dangerous to mimic ants. Swynnertion ruled out the objection by explaining this obj
ection by explaining that inedibility is a relativ e term depending upon the degre e of hunger in an
insectivorous bird and , therefore, it will definitely reduce the chances of being eaten up.
3. The concept of mimicry suggests that the resemblance between the mimic and the model is due to the
effect of similar environmental factors or due to parallel development of colour pattern to fit into the
similar environments. But often the mimic and model do not live in the same environment. Further, the
resemblance is superficial.
17.5.4 Evolution of Mimicry
There are different views as regards to the origin and evolution of the phenomenon of mimic ry.
1. Chance Similarity: One of the view s believes in chance similarities. In such a vast array of species and
wide range of possible relationship between them and with thei r environment, the poss ibility of chance
resemblances among diverse forms cannot be ruled out. Moreover, occurrence of unintentional
characteristics of functional interference in the evolution of plants and animals supports this view.

886. 2. Natural Selection: Weismann was of the opinion that natural selection is the only factor responsible for
the origin and appearance of mimicry. He suggested that the similarities between white butterflies and
Heliconidae are on account of selection only.
3 . Mutations: According to recent view, mimicry is due to the action of genes . Fisher attributed origin of
mimicry to domin ant gene mutations. Some scientists believe that mim icry arose as a result of single gene
mutation. Fisher and Ford suggested that normally there is only one allelomorph involved, causing the
resemblance between the model and the mimic. The pattern evolved is then perfected by the action of
modifying genes and natural selection. This view is supported by the breeding data obtained from
polymorphic and non-polymorphic species. It has also been shown that mimicry is improved during the
course of time by selection of suitable modifiers.
17.6 CO-ADAPTATION
The recip roca l adaptations of symbiotic or mutually related species to one another are called
coadaptations. The new adaptations which occur in one species are followed
by the appearance of corre sponding adaptations in the other mutually related spec ies. Such relationships
are intrinsically dynamic and continue for millions of years.
C oadaptations are also seen in plants and anim als which have intimate relationship in the form of coex
istence, mimicry, mutualism, symbiosis, predator-prey, host- parasite and for pollination.
EXAMPLES:
• Some predatory fish allow certain smaller fish to enter their mouth and clean food fragments held between
their teeth. This is an example of cleaning symbiosis and benefits both the species. Some fish get their gills
and scales cleaned this way by small fish.
• Adaptations found in a carnivore (like claws and teeth) are adjusted to the type of prey species. The prey
species, in tum , develop adaptations such as speed, protective colouration and chemical secretion to
protect them against the main predator species.
• Coadaptations are found between corals and small fishes. The fish get protection from predators and
corals get food from fish in return .
• Coadaptations between plants and insects for pollination are very common. Honeybees visit flowers of
one selective variety as seen by the colour of pollen in their pollen basket. Most insect- pollinated flowers
secrete nectar to attract insects for pollination. The structure of flower, stamens and location of pistil are
modified to attract insects or bees to ensure pollination as in Salvia.
In a l l th e ca se s of commen sali sm ,
predator-prey relationsip , parasiti sm and
mutuali sm , ea ch or gani sm is coadapted
in specia l ways to adjust or function with
othe r. Suc h modifications ar e known as
coadaptations.
Similarl y, coada ptations ar e found in
respect to the members of the same species
(intraspecific). There could be various sexual
attracting devices, courtship perform ance s,
territ oriality associated beha viours and re- FIG. 17.25: Coevolution of Salvia flower sponses to
overcrowding. for pollination by bee.

888. FIG. 17.26: Different species of Honeybee adapted to collect pollen from different plants.
404 ~ Evo lut ionary Biology
• Humming birds are adapted to feed on nectar from certain kinds of flowers because they can hover around
in the air next to the flowers. They have long beak to reach the hidden nectar in the flowers.
17.7 ANIMA L ASSOCIATION ADAPTATIONS
Animals of different groups and also of the
same group are found living together exhibi ting
different degrees of associations. These include:
I. Social Adaptations: The bees, wasps and
ants, live together forming colonies. These are
differentiated as queens, drones and workers.
The queen is modified to lay eggs and exclu
sively devoted to reproduction. The workers FIG. 17.27: Hummingbird on flower are modified for collecting
nectar, pollen , and
molding them into wax . These possess pollen basket and pollen brush at the hindlegs.
2. Commensal Adaptations: In this association , the parasite is always benefited but the host mayor may not
get the advantage of association. The host and parasite both are modified for the associat ion. An example
of commensal adaptations is seen between Eup aguru s, the hermit crab and sea anemone. Sea anemone
attache s on the back of hermit crab and is carried to distant places to get food and new habitat. Crab is
protected from its enermies due to the unpleasant smell of sea anemore.
3. Symbiotic Adaptations: In these associations, animals are adapted in such a way as to benefit each other
and they become physiologically interdependent. One can not survive without the other. Association
between termites and flagellate Trichony mpha affords a good examp le of symbiotic adaptations.
4. Parasitic Adaptations: In such associations, the animals live together in such a way that one gets not
only the benefit of the other but is totally dependent on it, while the other is harmed by the presence of
former. The former is known as parasite and the latter as host. The parasite modifies itself to that extent that
it affords least resistance to the host.
17.8 BIOTIC ADAPTATIONS AND ORGANISMIC ADAPTATIONS Williams has introduced a new conce pt
about adaptations and has divided adaptations into two categories:
17.8.1 Biotic Adaptations
The adap tations that are beneficial for a group but deleterious for the individuals are known as biotic
adaptations. These adaptation s include parental behaviour and kin selection, social context and
supraindividual structure.
I. Parental Behaviour and Kin Selection: The organisms comp ete not only for food but for producing the
maximum possib le number of successful offspring.
Still the size of brood is limited and moreover to protect the brood , parents exhibit different behaviours
such as nest building, care, feeding the young ones and defense of the young ones by frightening the enemy
or by feigning injury. These behavioural mechanisms evolve as integral parts of the reproductive function.
Further, these mechanisms are altruistic in the sense that one individual aids another. In the primary family
groups the appearance of altruistic behaviours by natural selection is very common and this process is

889. known as kin selection. But these phenomena are the result of behavioural adaptations of organisms and
help in the better survival of the offspring.
2. Social Context: These adaptations include individual sacrifices itself in defence of the group. worker
honeybee frequently dies after stinging, when the sting is ripped out of its abdomen. Thus, it sacrifices its
life in the defence of the hive. Therefore, the unit of selection in this case is hive and not the individual bee.
Other phenomena include warning signals produced by the individuals of any social group of animals at the
approach of a predator. If the warning signal attracts the attention of the predator, the giver of the first
signal would be at a selective disadvantage in respect to other members of the group. Therefore, such
adaptations evolve in the interest of the group or population and are called altruistic adaptations.
3. Supraindividual Structures: These include those adaptive modifications in the individuals which are in
the interest of the group or population, regardless of their impact on the individual itself. For example, the
muskox or other cattle when attacked by a pack of wolves , exhibit a definite behaviour, the bulls tum to face
the predators with their horns lowered, the calves flee and the cows keep themselves near the calves. The
cows also lower their horns towards the wolves and move backward in the direction in which calves had
gone.
The groups which have evolved cooperative behaviour have better chances of survival. Therefore, basic
adaptations are in the interest of the group and are found mostly in cases of social animals.
those behaviours in which an
For example, in honeybees
, a
17.8.2 Organismic Adaptations or Individual Adaptations
The organismic adaptations include adaptations at the individual level, i.e., the adaptations by which an
organism through suitable modification in the physiology, adjusts to environmental stresses. For example,
fair-skinned people, when exposed to sunlight develop tanned skin.
17.9 PREADAPTATIONS AND POSTADAPTATIONS
Sometimes, say per chance, a modification or a bodily change may appear in the organisms which may not
be of particular benefit or may be even neutral for the particular environment in which the organism is
already living. But it may prove beneficial for some other environment or in changed environment. If these
modified organisms happen to enter the new environment for which the change proves beneficial, it means
the organisms are preadapted for that particular environment.
406
~
Evolutionary Biology
For example, r hipidistian crossopterygian fishes had developed stiff fins or lobed fins wh ich could be used
for walk ing on soft ground or in shallow water, fairly rigid back bone s to allow the strong sinusoidal
flexation needed for such type of progression. They had lungs and internal nostrils for aerial breathing. All
these modificat ions naturally preadapt them to make a breakthrough in the terrestrial environment.
Therefore, preadaptation or prospective adaptation is ' the cha nce appearance of a cha nge in a group of
organism s, which enables th em to explo re some new adaptive zone'.
For ex ample, the changes which occurred in the rhipidistian fishes enabling them to live in shallow water,
acted as preadaptation for life on land.
Preadaptation is one of the very important ways in which chance influences evolution. It means that

890. organisms, in question, per chance happen to be in such a geographical or eco logical position that they
could venture a new adaptive zone and explore it successfully by the deve lopment of further adaptive
modification. G.G. Simpson (1953) called such adaptations as the pr osp ective adapta tions. Other
examples of preadaptations are the development of cleidoic eggs that enabled reptiles and birds to
successfully exp loit the dry land and the appearance of wings in birds which enabled them to venture in air.
Actually, any trait might tum out to be preadaptive to some new adaptive zone, if the opportunity for it to do
so presented itself by chance.
The idea of preadaptation
to the organisms could play
is important in showing that how factors intrinsic an important role in the entry of lineages into new adap
tive zones.
F IG. 17.28: A rhipidistian fish with lobed fins, a preadaptation to live on land.
The ad justments made in the structure of organisms to newly occupied environment or the newly occupied
adaptive zone are called po stadaptations. These adaptations are acquired in the organisms during the
period of adaptive radiation and perfect the animal s for living under the prevailing conditions.
17.10 r-AD APTATIONS
Thi s is an adaptation of orga nisms against predation and disease by producing large number of spores,
seeds, eggs, larvae or hatchlings. The number of embryonic forms exceeds the number which can be eaten
by the predators or infected by the disease causing organisms at one time. It is called r selection because it
depends on a rapid , exponential increase in numbers. The rate of increase is close to the organism's
intrinsic rate of increase.
Most o f these species reproduce once a year or even once in many years. The year of reproduction is called
mast year and this phenomenon as mast crop. It is characteristic of many tree species of temp erate zones.
These species do not produce or produc e very little seeds or fruits for several years and then all the trees of

892. species young or old produce seeds in great abundance. For example, some species of Oak in Comanche
county, Oklahoma produce up to 1,00,000 acorns per tree in one year whereas in other years the number
declines greatly. Similarly, bamboos flower only once in 25 years.
The populations of seed-eating animals such as birds, squirrels and insects decline greatly during lean
nonreproductive years, then suddenly increase exponentially. The increase is followed by another lean cycle
of several years.
The populations, high productivity at one time and reduced production in subsequent years has two
significant effects:
I. Increases chances that some offspring will survive to maturity.
2. Discourages predators for specialising on one type of prey since it is briefly available only once a year
or once after several years.
The pioneer species, i.e., the species that are the first to colonise an area also exhibit r-adaptations. The r-
adaptations tends to be selected by an environment with a brief growing season followed by conditions
which cause high degree of mortality independent of population density. These environmental conditions
could be onset of severe aridity in deserts or onset of severe winter in temperate regions .
17.11 k-ADAPTATIONS
k-adaptations are found in organisms which produce only one or only a few offspring during one
reproductive cycle. Under relatively constant year round conditions in which the environmental carrying
capacity for a particular species is reduced and competition for limited resources among species with
overlapping need is very high. In such circumstances species exhibit K-adaptation or K-selection. For
example, larger, longer lived organisms like whales, emperor penguin, elephant, etc., exhibit K-adaptations,
because a longer life span permits repeated production called iteratoparity which compensates for raising
only one offspring at a time.
Adaptations are of great significance from the evolutionary point of view . The phenomenon of
preadaptation enables organisms to explore new and unoccupied habitats or zones and provide them better
opportunity of evolution and survival. Adaptations enable organisms an upper hand during struggle for
existence whereas organisms become perfectly adapted to a given environment by the appearance of
postadaptive modifications.
• Adaptations
• Behavioural adaptations
• Concealment mimicry
• Fossorial
• Protective colouration
• Social adaptation
KEY TERMS
• Aggress ive mimicry
• Biotic adaptations
• Coadaptations
• Mimic
• Preadaptations
• Warning mimicry
• Alluring mimicry

893. • Cursorial
• Cryptic behaviour
• Mimicry
• Postadaptations

895. 408 iii Evolutiona ry Biology
REVIEW QUESTIONS
I. Define adaptations. How adaptations differ from mimicry ?
2. Discuss significance of colour and colour pattern s in the evolution of living forms.
3. Discuss evolutionary significance of adaptations.
4. Differentiate between the followin g:
(a) Adaptive divergence and adapti ve convergence
(b) Biotic and organic adapt ation s
(c) Pre- and Post-adaptations
5. Write short notes on:
(a) Adaptive divergence/adaptive radiation
(b) Co-adaptation s
(c) Adaptations and evolution
6. Define mimicry. Discuss its evolution and significance.
7. Differentiate between:
(a) Batesian and Mullerian mimicry
(b) Protective and agressive mimicry
(c) Warning and concealing mimicry
(d) Mimicry and protective colouration.
8. Write short notes on:
(a) Conscious mimicry
(b) Alluring mimicry
(c) Mimicry and evolution.
between protective colouration and mimicry. Discuss significance of 9. Differentiate
co louration . 10. Give reasons why:
(a) Males in birds and mammals are more brightly coloured.
(b) Animal s found in desert have dull and sandy gray hide.
(c) Certain animals have banded colour pattern .
(d) Some animal s show change in colour with the change in season. II. What is the role of preadaptations
and postadaptations ?
FURTHER READINGS
I. Abrams, P.A., 2000a. The Evolution of Predator-prey Interactions: Theory and Evidence. Annu. Rev.
Ecot. Syst., 31: 79-108.
2. Abrams, P.A., 2000b . Character Shifts of Prey Specie s that Share Predators . Ann . Nat., 156 (Suppl.)
545-561.
3. Avise, J.e., 2000. Phytogeograph y. Harvard University Press, Cambridge, MA.
4. Axclirod, R. and W.O. Hamilton, 1981. The Evolution of Cooperation. Science, 211: 1390-1396 .
5. Barrett, R.D.H ., and D. Schluter, 2008. Adaptation from Standing Genetic Variation. Trends Ecol. Evot.,
23: 38-44 .

897. 6 . Bennett , A.F., R.E. Lenski , and lE. Mittler, 1992. Evolutionary Adaptation to Temperature . I. Fitness
Respon ses of E. coli to changes in its Thermal Environment. Evolution, 46: 16-30.
7 . Brodie , E.D., Jr. BJ. Ridenhour and E.D. Brodie 111, 2002 . The Evolutionary Response of Predator s to
Dangerous Prey: Hotspots and Cold spots in the Geographic Mosaic of Coevolution between Garter Snakes
and Newt s. Evolution. 56: 2067-2082 .
8 . Currie, C.R . and others, 2003 . Ancient Tripartite Coevolution in the attine ant-microb e Symbio sis.
Science. 299: 386-388.
9. Fehr, E. and U. Fischbacher. 785-791.
10. Labandeira, c.c., 2002. The In C.M. Herrera and O. Pellmyr (eds) , Plant-Animal Interactions: An
Evolutionary Appro ach, pp. 26-74. Blackw ell Science , London.
11. Moran , N.A., 2007 . Symbio sis as an Adaptative Process and Source of Phenotypic Complexity. Pro.
Nat. Sci., US A, 104: 8627-8633 .
12. Perfeito, L., L. Fernande s, C. Mota , and I. Gordo , 2007 . Adaptiv e Mutations in Bacteria. High Rate
and Small Effects. Science, 317: 8 13-8 15.
13. Reznick , D., and l Travis, 2002 . Adapt ation . In c.w. Fox, D.A. Roff, and OJ. Fairbairn (cds.),
Evolutionary Ecology : Concepts and Case Studies, pp. 44-57. Oxford University Press, New York.
2003. The Nature of Human Altruism. Nature, 425:
History of Association s between Plants and Animals. DOD

900. UNIT V
Fossils and History of Life on Earth
Chapter 18. Origin of Life on Earth
Chapter 19. History of Life on Earth
Chapter 20. Fossils and Fossil Records
Chapter 21. Origin and Evolution of Horse Chapter
22~
Origin and Evolution of Man

903. 18
Origin of Life on Earth
18.1 ORIGIN OF LIFE (BIOPOIESIS)
Life originated on the Earth millions of years ago, and since then innumerable varietie s of living being s
have evolved. Human beings are the most recent and most highly evol ved organi sms.
Little is known about how , when and where life originated on Earth, because it is impossible to go back in
time and observe life's beginning. Also there is hardly any evidence except some fossils in the ancient rocks
to give some glimpses of ancient life. But the fossil records are either incomplete or not easily read . Scienti
sts have proposed a number of presumptions from time to time to explain origin of life.
18.2 ANCIENT AND MEDIEVAL BELIEFS
18.2.1 Theory of Special Creation
Hypothesis of special creation is the oldest hypothesis and is based on the mythological belief that divine
God created life. Its essential feature s are :
• All living plants and animals existing now were created by some supernatural power, the God or Creator.
• These forms were designed according to the surroundings.
• They have existed unchanged from the time they were formed.
According to Christianity, the chapter of 'Genesis' in Bible states that the world was created by the Creator
in six days from materia prima. The heaven and Earth were created on the first day, sky and sea on the
second day, dry land and plants on third day, Sun, Moon and stars on fourth day, fish and fowl on fifth day
and animal s including man and beast s on sixth day. First man, the Adam , formed from clay while the first
woman, Eve, was created from his 12th rib. According to Hindu mythology, God of Creation, Lord Brahma,
created different forms of life from variou s parts of his body.

905. 4 14
~
Evolutionary Biology
18.2.2 Theory of Catastrophism
According to this theory, the Earth has undergone several catastrophes or destruction. After every
catastrophe, God created a new living world . This theory is almost sim ilar to the theory of special
creation.
18.2.3 Theory of Panspermia or Extraterrestrial Origin or Cosmozoic Theory
Ea rly Greek thinkers believed that life is distributed throughout the cosmos in the form of resistant spores
of living forms , the cosm ozoa. These reached the Earth accidentally from some other planet either through
meteors or cosmic dust. On getting favourable conditions for life they developed into organisms and
evolved into existing forms. This theory was revived by Richter (1865) and was supported by Arrhenius
(1908) and others.
Objections: Living matter cannot survive the extreme cold and dryness, and the intense radiation of interp
lanetary space. Moreover, this theory does not explain how life originated and from where and how spores
reached Earth.
Support: The evid ence for the existence of life on other planets of cosmos came from:
• discovery of fossils of microorganisms in meteorites in 1961
• discovery of fossils in rocks from Mars
• discovery of liquid water under the surface of Europa , the ice-shrouded moon of Jupiter.
• space research has shown that bacteria can withstand the rigors of space travel and can survive in airless
and waterless environment.
18.2.4 Theory of Directed Panspermia
The Nobel Laureate, Francis Crick and Leslie Orgel, have recently revived the theory of Panspermia. They
assumed that there are civilisations at advanced stages
In support of theory of Panspermia, Crick and Orgel cited two biological anomalies i.e., genetic code and
role of molybdenum in biological system. 1. Genetic Code: There is only one code for all forms of life on
Earth because
life has originated and diversified from a single 'seed' or germ. 2. Molybdenum: The metal molybdenum
plays an important role in biological
systems. It forms a base for many enzyme systems . Although , so important , it
constitutes just 0.02% of the total metal composition of the Earth. The metals
like chromium and nickel, very much similar to molybdenum in their properties
and constituting 0.02 and 3.16% respectively, have no biological importance.
Crick and Orgel contend that the biological composition of the inhabitants of
Earth should reflect its composition and when it is not so it means life must
have been exported to this planet from some other planet where molybdenum
is present in large proportions .
on other planets in our Galaxy and that the inhabitants of one such planet decided some thousands of
million years ago to experiment whether life can develop in new environment in their neighbouring planets.
So they infected some planets on our Galaxy, including Earth with some microorganisms. This was called
directed panspermia.

906. 18.2.5 Theory of Spontaneous Generation or
Abiogenesis or Autogenesis
Until seventeenth century people believed in abiogenesis (Gr, a, not; bios, life; genesis , origin) or
spontaneous generation of living organisms from nonliving inanimate matter. Empedocles, Anaximander,
Xenophanes, and Aristotle were the main propounders of this theory.
• According to Epicurus (342-271 B.C.) worms and numerous other animals were generated from the soil or
manure by the action of moisture and warmth of the sun and air.
• According to Aristotle (384-322 B.C.), the common worms, bee larvae, wasps, ticks, glow worms and
various other insects are born from dew, rotten slime, manure , dry wood, sweat and meat, etc. Eels develop
from sea mud, frogs and salamanders from coagulated slime and butterflies from cheese.
• Two thousand years later, Van Helmont (1577-1644) described that mice arise from wheat barn and
sweaty shirt kept in a pot for 21 days in dark room.
• Similarly, a variety of bird, the 'barnacle goose' was presumed to be derived from some worm barnacle
living in sea, or from goose tree.
Theory of abiogenesis was discredited by Francisco Redi (1626-1698), Spallanzani (1729-1799) and
Pasteur (1822-1895).
Fly
Flies
Parchment paper
Muslin cloth
Eggs and larvae
(Maggots)
of fly

908. Meat pieces
A B c
FIG. 18.1: Redi's experiment to disapprove the theory of spontaneous generation: A. Uncovered jar; B. Jar
covered with muslin cloth; C. Jar covered with parchment paper. for months together even when flask was
left open; C. But when its neck was broken microbes appeared immediately.
416 [i] Evolutionary Biology 18.2.6 Theory of Biogenesis (Life from Life)
According to biogenesis, 'Life can arise from pre-existing life' and not from the nonliving matter by
abiogenesis. This theory is supported by the following evidences: I. Redi's Experiment: Italian physician,
Francisco Redi (1621-1697), demonstrated that maggots could not be created from meat but the smell of
meat attracted flies which laid eggs on the flesh. The maggots appeared when eggs hatched. Redi placed
lumps of boiled meat in three jars A, Band C. Jar A was left uncovered, Jar B was covered with fine gauze
or mus lin cloth and Jar C with parchment paper. The meat decayed in all the jars but maggots appe ared
only in the uncovered jar A where flies could lay their eggs on meat. Plenty of flies were seen sitting and
laying eggs on muslin cloth and parchment paper, but no maggots appeared on the meat in Jar Band C. (F
ig. 18.1).
2 . Spallanzani's Experiment: Italian scientist, Lazzro Spallanzani (1729-1799) poured hay infusion in eight
bottles and boiled all of them. Four of them were just corked and other four were made airtight. After a few
days, he found that there was thick growth of microorganisms in all the corked bottles but no organisms in
the airtight bottles. He argued that air contains microorganisms and was the source of contamination.
3. Pasteur's Experiment: The remaining doubts were cleared by Louis Pasteur (1822-1895). He kept a
mixture of sugar and yeast powder in a flask and filled about half of it with water. He, then , softened the
neck of the flask and drew it out in the shape of'S'. The contents of the flask were boiled till strong current of
steam rushed out from the curved neck , boiling killed the microbes in the flask and made the contents
sterile. The flask was cooled and left undisturbed. It was noted that the contents of the flask remained
unchanged even after 18 months. But, when neck of the flask was broken the solution of the flask came in
contact with air and microbial growth started. It means air contains microorganisms, which could not reach
the solution in swan-necked flask and settled in curved neck. (Fig. 18.2).
Pasteur 's experiment revealed that even the minute organisms arise from pre-existing organisms of their
kind. This supports the concept that life cannot arise spontaneously under conditions that exist on Earth
today.
Conditions on the primaeval Earth billio ns of years ago were assuredly different from present conditions.
The first form of life appeared as simple self-duplication particles that might have arisen spontaneously
from non living chemical substances.
18.3 MODERN HYPOTHESIS OF ORIGIN OF LIFE OR BIOCHEMICAL ORIGIN OF LIFE
Th e modern hypothesis of origin of life was formulated by Haeckel, who considered that the most primitive
organisms would have generated spontaneously at some time from some inorganic matter, as a result of
formative action of some special external physical forces as electric charges, ultra-voilet light and corpuscu
lar radiations of radioactive elements.
Swan-necked
flask
Microorganisms in atmosphere . ..
.

909. .---..... Broken neck
Sugar solution
Bacteria present
A B c
FIG. 18.2: Pasteur 's experiment with swan-necked flask to discredit theory of spontaneous
generation . A-Hay infusion was boiled nicely till the steam rushed out of the neck of the flask ; B-
Microorganisms did not develop
The geniu s work of Haldane, A.1. Oparin, Stanley Miller, H. Urey and others provided evidences in favour
of this hypothesis. Russian biochemist, A.1. Oparin (1922) , knitted the intricate biochemical theory of
origin of life. The present biochemica l theory has separated the entire process of origin of life on Earth in
the following steps:
18.3.1 Origin of Universe (Big Bang Hypothesis)
Th e big bang hypo thesis was proposed by Abbe Lemaitre ( 193 I) and supported by Gamow ( 1948) and
Dicke (1964) . According to this theory, about 20 billion years ago, the universe was a single piece of
highly condensed, red hot, rotating, gaseo us cloud of cosm ic dust. This was called pr imaeval matter,
cosmic matter or yelm. It was formed of particles (like neutrons, protons and electrons) and antiparticles .
The universe expanded and its temp erature came dow n. The particles and antiparticles collided and
formed hydrogen atoms. These then fused into progre ssively heavier atom s of different elements.
T his was the beginning of cosmic evolution. The cosmic matter exp loded by big bang or radionuclear
explosion into numerous small and large masses of gaseous hydrogen, called nebulae.
A nebu la was a cold and spinn ing clo ud of cosmic dust. Due to spinning, it separated into numerous
galaxies. A galaxy is a family of numerous stars held together
by mutu al gravitational attractions.
The huge star famil y to which our
Sun belongs to is called m ilky way.
It is ab out 13-20 thousand million
ye ar s ol d and has around 150-200 I
Interstellar dustbill ion stars.
18.3.2 Origin of Solar System
and the Earth
A ccordi ng to Nebular hypot h esis,
proposed by Kant (1755) and Laplace
(17 96), our solar system arose 5 to 6
billion years ago from a huge, red hot ,
spinning cloud of cosmic dust and gas
containing mi llions ofatoms of differ
ent types. The central mass of nebula
condensed to form the primitive Sun.
It gradually became hotter and brighter
due to the conversion of gravitational
energy into heat. Thermonuclear reac
Primith/~ lO~r1h
tion s started due to high temperature FIG. 18.3: Origin of solar system and of Earth. and the Sun then

910. started emitting solar radiations.
Becau se of rotational movement of nebula, the peripheral masses condensed into plan ets-Mercury, Venus ,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Initially, our Earth wa s a fiery spinning ball of hot
gases and vapours of various elements. Through hundreds of millions yea rs, the gases gradually condensed
into a molten mass and different element s got stratified according to their density. Heav y elements like iron
and nickel sank to the centre and formed the core of the Earth, ligh ter elements such as silicon and alum
inium rose to the surface and solidified to form Earth crust. The part of the Earth between core and crust
formed mantle. Th e ligh test ones like helium, hydrogen , oxygen, nitrogen and carbon flowed out

912. A
Atmosphere (Gaseous envelope around earth ) Mantle (Pyrosphere) Core
(Baryosphere)
('\-=;2+
Crust (Lithosphere)
Mantle
B
c FIG. 18.4: Condensation of hot gases and vapours of various elements to form primitive Earth.
Hydrosphere
The Earth
Our Earth lies at a distance of 148.7 million km from Sun. Its diameter is about 12,735 km and a mass of 6x
10
12
tonnes. It is divisible into three parts: baryosphere, pyrosphere and lithosphere. (Fig 18.4).
• Baryosphere is the core of Earth. It is formed mainly of nickel and iron. It
is believed to be molten and is divisible into an inner core region with a radius of about 1280 km and an
outer core region having a radius of about 2200 km.
• Pyrosphere is the mantle part of Earth having a radius of about 2800 km. It is formed of elements like
silica, aluminium, magnesium and manganese, which are lighter than iron and nickel.
• Lithosphere is the thin crust or surface layer of Earth. It is 20-25 km in thickness and is formed of rock
basalt under the sea and granite in the continents. It comprises of igneous, sedimentary and metamorphic
rocks forming strata according to their age.
• The gaseous envelope around the Earth is called atmosphere. It is about 1600 km thick.
• The watery part on the Earth surface is called hydrosphere. It covers 75-80% of Earth's surface.
of the surface and formed the gaseous atmosphere which was much different from what it is today.
18.3.2.1 Atmosphere of Primitive Earth
Earth's first atmosphere was formed of hydrogen and helium . It did not last long. The extremely powerful
winds from the early Sun, the relatively low gravitational force of Earth and heat generated by Sun and
Earth quickly dissipated the light gases of first atmosphere.
The secondary atmosphere of Earth arose around 4.2 -3.8 billion years ago (Bya) from the volcanic
activities. It was formed of carbon dioxide, water vapour, carbon monoxide, nitrogen, hydrogen, ammonia,
methane, hydrochloric acid and hydrogen sulphide. Other compounds in the primitive atmosphere were
dicarbon and cyanogen. All these compounds existed in gaseous form while water formed superheated
steam. Such an atmosphere is called reducing atmosphere.
Evidence of existence of reducing atmosphere on primitive Earth exists in the thick river deposits, around 3
Bya old in South Africa and other places. Such deposits include sand grains of sulphides of iron (FeS), lead
(PbS) and zinc (ZnS). These compounds are highly unstable in the presence of oxygen. Their existence in
ancient deposits indicates that free oxygen was absent in the primitive atmosphere.
High Temperature: The original temperature of the Earth is estimated to be 5,OOO-6,OOO°C. At such a
high temperature, elements like hydrogen, oxygen, carbon and nitrogen coul d not exist in free state . These
combined either amo ng themselves or with metals forming oxides, carbides and nitrites . As a result carbon
was found as dicarbon, cyanogen, methane and as metal carbides . Nitrogen existed in combination with
metals to form nitrides; oxygen formed oxides, and hydrogen combined with ox ygen, nitrogen and carbon
forming water, ammonia and methane, respectively and carbon with nitrogen formed cyanides.

913. The temperature of Earth remained high fro m 4 .6 to 3.8 billion years ago , keep ing the Earth surface
molten . Between 3.8 and 3.5 billion years ago Earth temperature cooled and first step in the origin of life
began .
Some scientists have suggested that methane in the atmosphere was produced by methanogen bacteria.
Deposits of methane and methanogens are found in 3.5 Mya old rocks .
18.3.2.2 Energy Sources on Primitive Earth
The energy sources on primitive Earth were:

915. to lightning provided energy for the formation
of simple m olecules in the atmosphere of
• Solar radiations: There was no primitive Earth. radiation belt around the Earth. Therefore, solar
radiations in the form of UV rays or infra-red rays reached the Earth along with visible sunlight.
• Electric discharge: Violent electric discharges were produced due to lightning and thunders across the sky
of primi tive Earth.
•Volcanic eruptions: Frequent volcanic eruptions emitted energy
In
the form of heat.
• High temperature of primitive Earth also contributed energy for chemical reac tions.
• Cosmic rays from the space (cosmos) also formed an energy source. FIG. 18.5: Presumed primitive
atmosphere of the Earth.
FIG. 18.6: Electric discharge produced due
Origin of Life on Earth [i] 421
• First step in the origin of life took place about 3.5 billion years ago.
• When first life appeared on Earth, the atmosphere was hot and reducing type. It was devoid of free
oxygen. These conditions fueled chemical reactions that led to the origin of life.
18.4 BIOCHEMICAL OR CHEMOSYNTHETIC ORIGIN OF LIFE
This theory is also called natu ralistic theory or Oparin-Haldane th eory . It finds the widest acceptance. A
vague idea that life could have originated on the primordial Earth in some warm little ponds, was innovated
by Charles Darwin in his letter to distinguished botanist, Sir Joseph Hooker. A detailed theory based on this
idea was proposed by Russian biochemist, A.I. Oparin in 1923 and was supported by J.8.S. Haldane in
1928.
A.I. Oparin (1923) and Ha ldane (1928) stated that life originated about 3.5 billion years ago from some
nonliving organic compounds in the oceans of primitive Earth through a series of chemical reactions.
According to them :
• Spontaneous generation of life (abiogenesis) under present conditions is not possible but could have
occurred under the conditions prevailing in the primitive atmosphere of primitive Earth.
• Earth's initial atmosphere was of reducing type, i.e., rich in hydrogen and very low in oxygen, much
different from present day oxidising atmosphere.
• High temperature, lightning and solar radiation provided energy required for chemical reactions.
• First living beings arose from simple inorgan ic and organic compounds as a result of progressive series
of chemical reactions (abiogenesis) by polymerisation.
• The process of transformation of the nonliving chemicals into living matter extended over a billion years .
Lederberg considered three stages in the origin of life, namely che mogeny, biogeny and cog noge ny.
18.4.1 Chemogeny or Chemical Origin of Life
About 4 billion years ago, Eart h's atmosphere had ammonia, methane and water va- pours. There was no
free oxygen. Formation of various simple and complex organic molecules from these gases is called
prebiotic evolution. It invo lves following steps:
Step 1. Formation of Simple Organic Compounds
The primitive inorganic molecules interacted and combined to form simple organic compounds like

916. alcohols, aldehydes, glycerol, fatty acids , amino acids, sugars and nitrogenous bases .
I. Formation of Hydrocarbons (Micromolecules): When the temperature of Earth surface coo led down to
I,OOODe or even lower, a variety of simple saturated and unsaturated hydrocarbons were forme d
presumably by the following methods:
FIG. 18.7: Formation of simple and complex organic molecules which accumulated in primitive oceans.
Atmosphere
CH. NH, HCN fonn ed by

917. In the lava
~
Metal Carbidesvolcanic
Methane Ammonia
Hydrogen
eruption cyanide metals

919. 0 ,
Unsa turated Hy droca rb ons
0 ,
e.g. Ethylene : Acetylene
Alcoho ls, Aldehyde s Energy of
Amino acids ultraviolet rays
Ketones : Organic acids
and electric
e.g. Glycerol. Fonnaldeh yde discharge
FIG. 18.8: Summary of steps involved in chemical origin of life.
(a) The combination of highly reactive free radicals CH and C H
2
CH + CH ---. HC = CH Acetylene CH
2
+ CH
2
---. H
2C
= CH
2
Ethylene
CH
2
+ CH
2
---. CH
4
+ C Methane 2CH
4
---. HC = CH +3H
2
Acetylene (b) The metal carbides reacted with
steam CaC
2
+ H
20
---. HC = CH + CaO 3Fe
4CS
+ 16H
20
---. C
1sH32
+ 4Fe
304
2 . Formation of Oxy and Hydroxy Derivatives of Hydrocarbons: Both unsaturated as well as saturated
hydrocarbon s reacted with superheated steam and formed oxy and hydro xy-derivatives such as aldehydes
(formaldehyde , and acetaldehyde), ketones and acids. For example:
HC = CH +
--.
CH
3CHO
Acetylene Acetaldehyde
Carbon monoxide reacted with water forming form ic acid. Due to its reactivity CO is considered to have
particip ated in the format ion of prebiot ic compounds. CO + H
20
Carbon monoxide
CH3CHO + CH3CHO Acetaldehyde
HCOOH
Formic acid
Condensation
--------.
CH
3CHO
.HCH
2CHO Aldol
2CH3CHO + H20 2CH3CHO + H20
Polymerisation
CH
3CHO
.HCH
2CHO Carbon monoxide
2CH
3CHO
+ H
20 Acetaldehyde
Oxidation reduction
--------.
CH
3COOH
+ CH
3CH20H Acetic acid Ethyl alcohol
2HCHO Formaldehyde Polymerisation
+ CH
3CH20H Ethyl alcohol
2CH
20HCHO Glycol aldehyde + H
20
Oxidation reduction
+ CH
3CH20H Ethyl alcohol
+ CH3CH20H
Ethyl alcohol
CH20HCOOH + NH3 Glycolic acid
--------. CH2NH2COOH + H20 Glycine
3 . Formation of Carbohydrates: Small chain compounds of C, H and 0 were also formed from hydro xy
derivatives. These first formed compounds must have been glucose and fructose . Their condensation
resulted in the formation of disaccharides, and polysacc harides (i.e.. sugars and starch).

921. 424
~
Evolutionary Biology
4 . Formation of Fatty Acids and Glycerol: Conden sation and polymeri sation of the aldehydes and keton
es and their oxidation resulted in the formation of fatty acids. Such compounds had lesser percentage of
oxygen than long stra ight chain s of carbon . In primitive oceans , glycerol and fatty acids combined
resulting in the formation of fats.
5. Formation of Amino Acids: Combination of hydrocarbons, ammonia and water under the influence of
freel y available energy reacted to form amino compounds, commonly known as amino aci ds.
18 .4.1.1 Stanley Miller and Harold Urey's Experiment (Proof for Prebiotic Synthesis of Organic
Molecules)
In 1953 , Stanley Miller and Harold Urey recreated the probabl e conditions of primitive atmosphere of
early Earth and demonstrated that simple organi c compounds like amino acids , hydroxy acids , aliphatic
acids, sugars and urea can be synthesised in the laboratory from a
FIG. 18.9: Stanley Miller, The Origin-at-Lite Researcher recreated conditions of primitive atmosphere that
surrounded the young Earth, and obtained some of the key building blocks of life.
mi xture of methane, hydrogen, water vapour and ammonia. They supplied energy by heating the chamber
containing the above mixture up to 800
DC
and discharges by electric sparks. After about one week , the
liquid was found to conta in a number of amino acids such as glycine, alanine and aspartic acid (Fig.
18.10).
• The abiotic synthesi s of organic molecules has been repeated many time s by a number of scientists who
have obtained most of the amino acids present today in protein molecules and nitrogenous bases of nucleic
acids (Adenine was formed from hydrogen cyanide). Bahadur (1954) obtained all possible amino acids by
subjecting a mixture of ammonia, ferric chloride and paraformaldehyde to strong sunlight.
• Melvin-Calvin obtained amino acids and sugars by treating a mixture of hydrogen, water vapours,
ammonia and methane. It appears that the essential building blocks of macromolecules (the amino acids
and nucleotides) of living organisms could have been formed on the primitive Earth.
• Simple organic compounds could be formed in nature even today by lightning discharge s or ultraviolet
radiations, but organic compounds so formed undergo spontaneous oxidation or are taken up and degraded
by the present day living organisms. The presence of Oxygen and living organisms on the present day Earth
prevents the abiotic formation of organic molecules.
• Analysis of meteorites also revealed the presence of similar compounds in the ir content. This indicates
that sim ilar processes are also occurring in space on other planets.

923. i
F
ormic
aci
d
Formic acid Formic acid Formic acid
Formic acid
iormic acid
ormic acid
Water
vapour
i
+-- Water in ormic acid (Cold) ormic acid Cooling
-.W
aterout (Hot)
Condensed
water containing complex organic compounds
Stopcock
Organic
molecules Alanine
Aspartic acid
Glycine Urea
--- Lactic acid
--- Acetic acid
---
FIG. 18.10: Apparatus set-up by Miller and Urey to demonstrate that simple organ ic compounds can be
synthesised under conditions simulating the primitive atmosphere.
Molecules that are building blocks of organ ic compounds of living organisms are formed spontaneously
under conditions designed by Miller-Urey to simulate primitive Earth's atmosphere.
18.4.1.2 Sea as the Hot Dilute Soup
The primitive oceans were frothy places with plenty of simple organic molecules. These were being bomb
arded by ultraviolet and other ionising radiation s. In these

925. 426 Ii] Evolutionary Biology
oceans synthesis of carbohydrates, fats, amino acids and other complex organic substances probably
occurred by condensation and polymerisation inside the bubbles. The sea water containing molecules of
these organic substances in abundance was described by Haldane as the hot dilute soup or prebiotic soup.
18 .4.1.3 Mud and Clay
Accordi ng to the rules of thermodynamics, such organic molecules cannot accumulate
in soluti on in large numbers because they break up as fast as they are built up. Scientists presume that
these organic macromolecules accumulated on the charged surfaces of mud or clay or minerals in the
underwater rocks. The clay and mineral molecul es brought the organic molecules together and also
provided a catalytic surface for continuous chemical reactions. These reactions were comparatively faster
than reactions occurring in the hot dilute soup.
18 .4.1.4 Factors fo r Accumulation of Organic Molecules in Hot Dilute Soup
Organic molecules, thus formed could accumulate in the primitive ocean forming
hot dilute soup because of prebiotic cond itions, such as:
• Absence of Oxygen: Due to absence of free oxygen on the primitive Earth, the organic molecules could
remain unspoiled and reacted to produce new and more complex organic compounds. Thus, reducing
atmosphere of primitive Earth played important role in preserving these compounds.
• Absence of Living Beings: The organic compounds formed on primitive Earth accumulated because there
were no living forms to consume them. Such organic molecules could not accumulate today because
enormous number
of microorganisms use and degrade them .
Step 2. Formation of Complex Organic Compounds or Macromolecules
In the ho t organic soup, simple organic molecules underwent conde nsation, polymerisation and chance
chemica l reactions forming new and more and more complex organic macromolecules, such as
polysaccharides, fats, proteins, nitrogenous bases, nucleosides and nucleotides. Sugar molecules combined
to form starch, cellulose and glycogen. The fats were formed by the condensation of fatty acids with
glycerol. Amino acid molecules formed long polyp eptide chains.
In liv ing organisms, biological reactions are speeded up by enzymes, but in primitive sea these enzymes
were absent. So such reactions occurred far more slowly. Therefore, nature could synthesise all the
compounds known to exist in the present day living beings but at a very slow pace .
Step 3. Formation of Molecular Aggregates or
Prebiotic Molecules
Oparin and Sydney Fox held that complex organic molecules synthesised abiotically in the ocean on
primitive Earth formed large spherical colloidal aggregates due to

927. Moveme nt of
selected molecules
Phosphol ipid ./ membrane
Origin of Life on Earth [i 427 Higher concen tration
of selected molecules Coacervate
•
•
•
••••
• •
• •
Colloidal particle
•
FIG. 18.11: Formation of coacervates from organic polymers in aqueous medium.
intermolecular attraction. These colloidal particles remained suspended or floated on the surface as
droplets or bubbles. Such colloidal aggregates of macromolecules were called coacervates, proteinoid
microspheres, protocells, micelles or liposomes depending on their composition (lipid or proteins) and how
they formed . Lipoid bubbles are called coacervates and proteinoid aggregates as proteinoid microspheres.
Inside the colloidal droplets or bubbles, there was interaction between the molecules and macromolecules
due to hydrogen bonding, ionisation, solubility and surface tension , etc. This resulted in further increase in
the size of macromolecules and in their different orientation. For example, coacervates were forrned when
polar molecules of phospholipid oriented on water surface forming double layered phsopholipid membrane,
somewhat similar to present day plasma membrane.
1. Coacerv ates: Colloidal bubbles bounded by lipid bilayer were the simple coacervates. Complex
coacervates had more than one type of macromolecules, like gluco se, amino acids , simple proteins, nucleic
acids , etc. These membrane bound bubbles or vesicles had following advantages over the colloidal
particles lying unenclosed in hot soup:
• Selective permeability of membrane surrounding the droplets selectively permitted the entry and exit of
compounds to and from the droplets.
• Selective premeability allowed concentration of particular compounds within the vesicles. This allowed
certain reactions to occur within the droplets .
• Presence of basic proteins entrapped nucleic acids into these droplets . This resulted in high concentration
of proteins and nucleic acids within such vesicles.
• Small size of vesicles or membranous droplets allowed a chain of reactions to occur in isolated chambers.
As a result building up and breakdown reactions occurred more rapidly.
• With the help of such proteins that could act as enzymes, coacervates could carry out such functions as
synthesis and hydrolysis of starch and breakdown of glucose to release energy.
• They were able to preserve their organisation and use energy and matter entering them .

929. 428 ~ Evolution ary Biology
• By accumulating matter from the surroundings, such droplets or coacervates were able to multiply by
budding and increase in number more rapidl y. It is presumed that coacervates with
p roteins acting as enzymes and ATP
as source of energy were the first
abiotic structures at the margin of
nonliving and living. Although, they
were not living structures, they had
many characteristics clo se to the
cells.
How ever, coacervates pre sent poor
model s for protocells becau se:
• The coacervates are formed from the
mixture of contemporary bioproteins
(not the ancient or primitiv e proteins Coacervate drops that existed in past).
Bud
A
• Coacervates are unstable and dis
integrate with time.
2 . Proteinoid Microspheres: Oparin
called the colloidal bubbles as proteinoid
microsphe res or protobionts. He obtained
s uch bubbles by heating a mixture of B
proteins and polysaccharides and shaking
th e so lut ion. Sydn ey Fox obta ined
FIG. 18.12: A . Coacervates in a drop;
proteinoid microspheres by heating a
B.
Budding in a coacervate
mixture of dry amino acids up to l30-1 80°C and then cooling in water.
Some microspheres showed buddin g or binary fission. Microspheres had following characte ristics :
• Microspheres are easily formed when water is added to thermal proteinoids .
• These exhibit great uniformity in size and shape.
• These resemble coccoid bacteria and tend to form chains of vary- ing length similar to streptococci
bacteria.
• Microspheres are considerably stable.
• Electron micrographs of proteinoid microspheres revea l a double layered cover, corresponding to cell
membrane.

931. -_· ··h.<;1~:::~
A B FIG. 18.13: Diagram showing A. Binary fission in microsphere with double layered membrene;
B. Buds on proteinoid microspheres.
• Both Gram positive and Gram negative microspheres have been produced.
• Microspheres exhibit nonrandom motility. This is enhanced by the addition of ATP. These can be seen
either attracting or repelling each other. Sometimes these may join together.
• Information cou ld communicate within an aggregate of microspheres.
• On being transferred in hypotonic solution, the microspheres swell up and in hypertonic solution, these
shrink. This behaviour simulates osmosis without any lipid.
• Microspheres retain enzyme-like activities of proteinoids.
• Microspheres tend to divide either by binary fission or by budding.
On this basis, it can be concluded that microsphere-like aggregates could have been the forerunners of first
living organisms. These provide exce llent mode l for protocells because:
(a) Microspheres originated from proteinoids prod uced under conditions existing on the primitive Earth.
(b) They exhibit structural and functional attributes of contemporary cells.
When solutions of oppositely charged colloids (e .g., gum arabic and gelatin) are mixed, coacervate
droplets are formed. These have much higher concentration (sometimes hundred folds) of organic polymers
than in aqueous phase. These droplets represent open systems that keep accumulating materials from their
environment.
If an enzyme along with the monomer substrate is added to the reaction mixture, polymers are formed and
are retained within the droplet. Oparin demonstrated that the artificial coacervate system formed with gum
arabic , histone and enzyme phsophorylase can synthesise starch from simple phosphorylated glucose
molecules or hydrolyse starch into glucose.
18 .4.1.5 Sydney F. Fox's Experiment
Sydney F. Fox from Florida University in 1957 heated a mixture of about 18 amino acids in anhydrous
condition to about 16D-200 °C. On cooling, the amino acids
had linked together to form long chains, which resemb led the polypep tide chains of proteins. When these
were heated and cooled again, these formed some cell-like structures. Fox called them microspheres.
18.4.1.6 Current Bubble Hypothesis of Chemogeny
Geophysicist, Louis Lerman (1986) proposed that chemical processes leading to the chemical origin of life
from abiotic material took place within the bubbles on the surface of primitive oceans. The various events
leading to the chemical evolution of protocells or protobionts were:
• Volcanic erruptions under the oceans released variety of gases . These got enclosed in the bubbles.
• Gases inside the bubbles got concentrated and reacted to produce simple organic molecules.

933. 430 ~ Evolutionary Biology
•Bubbl es that persisted for a long per iod under the water, popp ed up the surface and released their
contents into the air.
•By obtaining energy from Sun 's ultraviolet radiation , lightning and other energy sources, the simple
organic molecules reacted and formed complex organic molecules .
•These comp lex organic molecules fell back into the sea water in raindrops.
([JJ)

934. •
FIG. 18.14: Microspheres of Sydney F. Fox that resemble the earliest cells.
In ocea n they aggregated and enclosed in bubbles again .
Step 4. Formation of Nucleic Acids (The Naked Genes)
Th e next step was formation of nucleic acids by the polymerisation of nucleotides. These self-replicating
polynucleotide s (nucleic acids) got establi shed in the primordial Earth about 3.5 billion years ago.
Because of errors during replication different varieties of nucleic acids were formed. These molecule s with
different nucleotide sequences competed for the availabl e nucleotide precursors for self duplication.
Certain RNA polynucleotide molecules developed the quality of directing synthe sis of polyp eptide s. Such
RNA-en zyme s are found to occur even today and are called
3 . When bubbles
persisted and rose to
the surface . they
poppped . releasing
their conte nts to the air.
2. The gases ,

936. 5. The more complex
concentrated inside the organic molecules fell bubbles, reacted to back into the sea in produce simple organ
ic raindrops . There . they molecules . could again be enclosed
in bubbles and begin the process again .
FIG. 18.15: The current bubble hypothesis.
ri bozymes. RNA-guided protein synthesis in present day organisms, suggest s that RNA was the first carrier
of genetic information. This shows that the genetic code and translation of nucleotide sequences into amino
acid sequences was established at a very early stage of organic evolution. It means life originated as ' RNA
wor ld' , which consisted solely of self-replicating RNA molecules.
It has been proposed that RNA evolved first as the hereditary material. DNA developed as a more stable
hereditary material much later. Next, coding system developed and RNA became specia lised to carry out
three different functions in protein synthesis and three types of RNAs evolved . Later on, DNA developed
ability to mutate, resulting in the diversification of living cells .
The nucleic acids were like naked genes . Their formation was the first step to enter the vaguely defined
frontie rs between life and non-life .
Step 5. Formation of Nucleoproteins or Protobionts
Due to aggrega tion, giant molecules of nucleoproteins were formed by the union of nucleic acid and basic
protein molecules.
1 . Protoribosomes: These nucleoproteinoid particles had fibrous or globular appearance. The globular
nucleopro teinoid microparticles might have been a model for early ribosomes and are named ' p rotor
ibosomes' .
2 . Protoviruses: Some giant nucleoproteinoid molecules had certain characteristics of a ' free living gen e'.
These could be compared to present day viruses having nucleic acid core and protein covering. They were
called protoviruses or protobionts by Oparin.
18.4.2 Biogeny or Biological Evolution Step 6. Formation of First Cells (Eobionts or Protobionts) The
following events could have been responsi ble for the formation of protobionts and first cells from
coacervates:
• Polarised films of phospholipids became organised to form surface membrane, somew hat similar to
plasma membrane around the coacerva tes.
• RNA molecules got incorporated in the membrane bound coacervates with amino acids or proteins.
• Aggrega tion of self-rep licating RNA and their associatio n with proteins to form nucleoproteins changed
the bubble-like structures into first living system . These were called protocells, eobionts or protobionts. In
these protobionts , RNA acted as hereditary materia l as well as enzymes (called ribozymes ) to control and
speed up biochemical reactions.
The first cell probably arose from these protobionts, and were anaerobic prokaryotes. They gave rise to two
groups of cells Monerans and Protistans, which acted as ancestor to present day prokaryotes and
eukaryotes.
432 ~ Evolutionary Biology
• Microfossils

937. The earliest evidence of life appear as microfossils in ancient rocks that were 3.5-3.2 billion years old.
These microfossils were single-celled and 1-2 mm in diameter. They resembled present-day bacteria.
• Archaebacteria
In 1966, fossils of Archaebacteria were found in 3.2 billion years (3,200 million years) old rocks from
Africa. They were cyanobacteria-like organisms and are named Archaeospheroides barbetonensis.
P alaeobotanists of Birbal Sahni Institute of Palaeobotany, Lucknow have obtained fossils of cyanobacteria
from India which were estimated to be 2,900 million years old. Fossils of filamentous and colonial forms of
cyanobacteria have also been obtained from Western Australia.
S ome surviving respresentatives of these Archaebacteria are still present on Earth in oxygen-free depths of
the Black Sea, boiling waters of hot springs (thermophiles'heat-Iovers') and deep sea vents (halophiles-'salt
lovers'). Methanogens are among the most primitive bacteria that exist today. They produce methane (CH
4)
gas using CO
2
and H
2.
1. Monerans (i.e., the cells without well defined nucleus) evolved into prokaryotes , like bacteria and
cyanobacteria. These lacked the nuclear membrane. Therefore, their nuclear material (DNA) was not
isolated from the surrounding cell cytopla sm.
2. Protistan s (the cells with distinct nucleus) had a nuclear membrane around their genetic material. This
membrane isolated genetic material from cell cytoplasm. From protistans arose eukaryotes which evolved
into Protozoa, Metazoa and Metaphyta.
Step 7. Evolution of Modes of Nutritions
I. Chemoheterotrophs: The first living organisms or cells were chemoh eterotrophs. They presumably
obtained energy by the fermentation of prebiotic organic substances available from the sea broth. They
multiplied rapid ly in an environment with a cupious supply of disso lved nutrients.
With rapid increase in the num ber of chemoheterotrophs, the nutrients from sea water began to disappear
and gradually exhausted. This led to the evolution of several other modes of nutrit ion, such as:
(i) Parasiti sm: Some forms started living within the bodies of living cells and obtained their food from
them. This method of existence is known as parasitism and these individua ls as parasites. Monerans,
viruses and few protista are parasitic.
(ii) Sa prop hytis m: Some organisms started drawing their nourishment from the bodies of dead and
decaying cells. Many bacterial groups adopted this method .
(iii) Predation or animalism: This was animallike way of eating in which one organism eats another in
whole or in part and obtains its food in this manner.
(iv) Chemosynthesisers or chemoautotrophs: Protocells with enzymes of metabo1- ic pathways could use
less complex nutrients and synthesised more complex molecules.
T he che moa utotrophs obtained energy by a relatively simple and inefficient process of fermentation.
These early fermentative or anaerobic chemoautotrophs were simi lar to our present day anaerobic bacteria
and yeast. They released large amount of carbon dioxide in the atmosphere that paved way for the utilisatio
n of CO
2
in synthesising organic compounds by trapping solar energy. They were the foreru nners of
photosy nthetic cells.

938. Fermentation
Enzymes
2 . Anaerobic Photoautotrophs: Evolution of chloro phyll molecul e enabled certa in protocells to utilize
light energy and synthesise carbohydrates. They were the photosynthetic cells.
T he first photosynthetic cells (photoautotrophs) were anaerobic. They neither used water nor release d
oxyge n. Similar to present day sulphur bacteria, they cleave d hydrogen sulphide into hydrogen and
sulphur. Hydrogen was used in the synthesis of organic compounds (food) and sulphur was released as a
waste product similar to the release of oxyge n during photosynthesis in aerobic autotrophs.
6CO
Fermentation
• C H
12
0
6
+ 6H 0 + 125
2
+ 12H 5
2
Chemical energhy
6 2
This was the beginning of autotrophism . Because, in these forms energy was released by anaerobic
respiration or fermentation, this method is called chemosynthesis and such forms were called
chemosynthesisers. Sulphur bacteria, iron bacteria and nitrifying bacteria are present day
chemosynthesisers, which occur in sulphur springs and oxyge n-free mud .
3. Ae robic Photoautotroph s: It is presum ed that acc umulation of CO
2
in atmosphere and formation of
chlorophyll molecules resulted in the evo lution of autotrophic forms. The first aerobic photoautotroph s
were cyanobacteria-like forms. They used water as hydrogen source and carbon dioxide as source of
carbon in photosynthesis. They released free oxyge n in the atmosphere and are also called oxygen
producing photosynthesisers. They appeared about 3,300 to 3,500 million
(3.5 billion) years ago.
° + 602

940. Chemical energhy
• C H12 66C02 + 6H20 Fermentation
.
6
Chemoauto trophs
The chemoautrotrophs formed the beginning of autotrophism.
The first ph otoautotrophs were anaerobi c and anoxygenic as they di d not use water as raw material as in
present day photoautotrophs. They used hydrogen sulphide, thiosulphate or hydrogen instead of water. They
evolved about 3,500-3,800 million years ago.
18 .4.2.1 Impact of Photosynthesis
Cyanobacteria synthesised their organic food from carbon dioxide, and water using solar energy which was
converted into chemica l energy entrapped in the organic compound, glucose, and released oxygen as
byproduct. The aerobi c forms started using free atmo spheri c oxygen
in the evolution of respiratory
in oxidising the organic compounds. This resulted
mechan ism. The carbon dioxide evolved during respiration was utilised in photo synthesis. This ensured :
I. Regular inflow of energ y in the biosphere.
2. Changed the atmosphere from reducing to oxidising.
3. Evolved method of utilising carbon dioxide and converting it to sugar.
C
6
H
1206
+ 60
2
----+ 6C0
2
+ 6H
2
0 + Energy
18.4.2.2 Effect of Oxygen on Evolution (Oxygen-reduction) or Oxygen Revolution
1. Formation of Oxidising Atmosphere: With the increase in the number of photosynthesising organisms,
oxygen was liberated. It started accumulating in the atmosphere and the reducing atmosphere of primiti ve
Earth changed into an oxidising atmosphere.
• Aerobic autotrophs becam e more abundant.
• Free oxygen started reactin g with methane and ammonia and oxidised them into carbon dioxide and N
2
respectively. As a result, methan e and ammonia from the primiti ve atmosphere of the Earth disappeared.
CH
4
+ 20
2
----+ CO
2
+ 2H
20
4NH
3
+ 30
2
----+ 2N
2
+ 6H
2
0
• The Earth 's atmosphere changed from primitive reducing atmo sphere to the modern oxidi sing
atmosphere.
2. Formation of Ozone Layer (Ozonosphere): An ozono sphere developed by the reaction of oxygen under
the influence of UV radiations.
O
2
+ 20
2
----+ 20
3
(Ozone)
Accum ulation of ozone in the atmosp here forme d a protective layer that acts as a radiation belt. It
protects Earth and life from high energy UV radiations.
3. Ox ygen Revolution and Formation of Present Day Atmosphere: The evolution of oxidising atmosphere
resulted in oxygen revolution and put an end to abiotic synthesis because the organisms consumed and
degraded intermediate products formed abiot ically.
4. Impact of Free Oxygen: Free oxygen in the atmosphere finally led to the evolution of areobic mode of
respiration which yielded more energy on oxidation of foodstuff as compared to anaerobic respiration.
The green autotrophs became the source of food for nongreen heterotrophs and photosynthesis and

941. respiration developed as nature 's main balancing mechanisms.
18.4.3 Cognogeny Evolution of Mechanism of Perception, Expression and Communication
Step 8. Evolution of Eukaryotes
Th e protocell s were prokaryotic and archaebacteria-like and the eukaryoti c cells evolved from the archaic
prokaryotic cells. There are two views regarding their mode of origin of eukaryotic cells:
l. Endosymbiotic Origin: According to Lynn Margulis of Berkeley University, some anaerobic predator host
cells engulfed primitive aerobic bacteria but did not digest them. The oxygen respiring bacteria established
them selve s permanently inside host cells, and developed mutal association. Such predator host cells
became the first eukaryotic cells.
Original prokaryotic cell
~---'~cp
DNA Chromosomes
Aerobic bacteria
Multiple invaginations
of cell membrane
Symbiotic bacteria Protoeukaryotic compound organism
+
Aerobic bacteria
Aerobic bacteria
The bacteria
become mitochondria
Endoplasmic reticulum and nuclear membrane formed from the cell
~embrane invaginations
Chromosomes
Eukaryotic

942. ®
Eukaryotic cell of plant cell animals. fungi and some protists
FIG. 18.16: Formation of chloroplast and origin of a eukaryotic cell by the invagination of surface
membrane.
The pr edators that captured both aerobic bacteria as well as photosynthetic bluegreen algal cells evolved
into eukaryotic plant cells. The aerobic bacteria established them selv es as mitochondria, and blue green
algae as chloroplasts.
Eviden ce that Endosyonbiosis Changed Life on Earth
In 1966, microbiologist Kwang Jeon found that his culture of amoebae was struck of infection with X-
bacteria. Because of this infection amoebae became dangerously sick and only a few amoebae survived the
epidemic . Several months later, the surviving amoebae and their descendents were found to be
unexpectedly healthy. The infection of X-bacteria no longer made amoebae sick. Antibiotics used to kill
bacteria inside amoebae caused death of amoebae also. It means host amoebae could no longer survive
without their invading X-bacteria . Jeon discovered that the bacteria evolved symbiotic relation with
amoebae.The protein they made was needed by amoebae to survive .
Jeon concluded that similar endosymbiosis resulted in the origin of eukaryotes.
2. Origin by Invagination: According to this view organelles of eukaryotic cell s mig ht have evolved by
invagination of the surface membrane of prim itive prokaryotic cells .
Cell membrane
Mitochondrion
A B c D E FIG. 18.17: Formation of mitochondria by invaginatio n of surface membrane.
18.5 THE EARLIEST CELLS
It is presumed that the first living organi sms were heterotrophs and presumably obtained energy by the
fermentation of organic sub stances which were dissolved in the sea water in which these organisms were
residing. In course of time , the sugar and amino acid mo lecu les on which these first living forms were
dependent gradually exhausted, and CO
2
in the atmosphere became much more abundant. Presumably, it is
at this time that the chloroph yll molecules arose, and the heterotrophs evolved into autotrophs, which cou
ld manufacture their food from carbon dioxide and water. After the origin of auto troph ic forms, the
atmosphere got free oxygen that plants liberated during the process of photosynthesis. So, the earlier forms
differentiated into green, and non-green ce lls, which have evolved into plant and animal kingdoms
respectively.
Sin ce many simpler and lower animals are aquatic and marine, and the cells and body fluids of all animals
contain salts, it is inferred that life began in the ocean. Many biologists believe that life origin ated in the '
prebiotic soup' on the floor of deep sea at a temperature of I05 °e. Th is mineral-rich fluid gushed out from
the hot water vents. Such hot water vents are still found on the floor of Atlantic Oc ean. Similar hot-water
springs are pre sent in Yellowstone Nation al Park, U.S.A.
Su ch a n en vironment c o u ld have been life-inducing but not life suppo rting because of ve ry high
temperature.
Acco rding to ano ther group of sc ientists, life originated in ancient ocean beach es whe re pr efer ential

943. accumulation of organic material could occur due to tides and shore-line rocks. Eve n at present , wa ter in
the tidal zone is rich in oxygen, CO
2,
light and minerals and is most suitable for plant and anim al growt h.
The earliest animal rem ain s are all in rocks of marine
FIG. 18.18: Mineral-rich fluid gushing out from a hot-water vent on the floor of Atlantic Ocean (able to
support bacteria and archaea).
FIG. 18.19: Inhospitable hot-water spring in Yellow-Stone National Park (blue colour is because of heat-
tolerant cyanobacteria).
origin. Organisms invaded the freshwater and land later.

945. FIG. 18.20: The oldest frozen Isua rocks of Greenland .
438 IjJ Evolutionary Biology 18.7 EARLIEST EVIDENCE OF EXISTENCE OF LIFE ON EARTH
Th e rocks at Isua, Greenland are believed to be the oldest rocks in the world. These were formed about 3.8
billion years ago. These contain a type of carbon produced only by living things. Scientists believe that
these carbon remains represent a kind of chemical signature of life left in the barren, frozen rocks and we
can date life from this point.
A bout 3.4 bill ion years old rock deposits from Australia contain fossils in the form of stromatolites. These
are composed of calcium carbonate secreted by cyanobacteria. The internal patterns
FIG. 18.21: Traces of ancient organisms in the form of stromatolite in the rock deposits in Strelley Pool
Chart of Western Australia .
o f layers of CaC0
3
in these stromatolites are so complex that they could only be created by living
organisms. These are believed to be the fossils of either bacteria or Archaea. The oldest eukaryotic fossils
were found in 2 billion years old rocks.
O nce the life was established, the archiac ancestoral forms multiplied and diversified into a variety of
forms and established present day tree of life. It comprises of three major categories: the Domains of life-
Bacteria, Archaea and Eukarya. The Domain Eukarya is differentiated into four kingdoms: Protista,
Plantae, Animalia and Fungi. Domains Bacteria and Archaea are made up of single-celled microbes,
Domain Eukarya
A
Doma in Doma in Bacteria Archaea

947. Methane producers
~
• loversa K
-:'\.
• Hotacid lovers
Domain Doma in Bacteria Archaea
KIngdom Kingdom KIngdom KIngdom ProUata Plantae Anlmalla Fungi
Vertebra tes Mushrooms
e ~
Dinoflagellates
~
Diatoms
_ - - - - - - - -DomalnEukarya- - - - - - - _ (Protists, Plants. Animals. Fungi)
FIG.18.22: Evolution of life on Earth as it exists today.
while Domain Eukarya has both unicellular Protista and multicellular forms belonging to kingdoms
Plantae, Animalia and Fungi .
18.8 EVOLUTION OF EUKARYOTIC ORGANELLES
18.8.1 Origin of Mitochon dria and Chloroplasts
About th e origin of mitochondria and chloroplasts in eukaryotic cells, endosymbiotic theory was proposed
first by Max Taylor and later by Stanier Marguli s and others. According to thi s theo ry, some prokaryotic
cells with flexibl e surface became active pred ators, and fed on bacteria-like aerobic heterotrophs, capable
of oxidative phosphorylation , and cyanobacteria capable of photo synthe sis. Some such ingested aerobic
or photo synthetic prokaryotes establi shed symbiotic relationship with their hosts, and evolved into
mitochondria and chloroplasts respecti vely.
Evidences for the origin of mitochondria and chlorop lasts from prokaryotic endosymbio sis include:
• Mitochondria and chloroplasts bear strong resemblance to bacteria.
• Both organelles (mitochondria and chloroplasts) have their own genetic material.
• Both organelles have uniparental inheritance through cytoplasm of female parent.
• Ribosomes of both these organelles are prokaryotic in size, biochemistry and in sensitivity to antibiotics.
• Sequence of nucleotide s in the DNA and ribosomal RNA of these organelles is closely related to some
free-living species of bacteria.
• At the time of division of eukaryotic cells, the divi sion proc ess of these organelles is remini scent of
bacterial cell division .
• Some organi sms living today are similar to hypoth etical ance stral condition. For example, Pelomyxa pa
lustris (amoeba) lacks mitochondria but has a perman ent population of aerobic bacteria. One species of
Param ecium harbour algae.
18.8.2 Origin of Nucleus
E ukaryotic nucleus is also presumed to have endo symbiotic origin. According to Lake and Rivera (1994),
nuc lear membranes were derived from some captured prokaryoti c cells which also contributed to the
genetic material of Eukaryotic cell. Gupta and coworkers (1994) also supported the above view. According
to them, the initial eukaryotic cells were formed by the fusion of an arch aebacterium and a Gram negat ive
eubacterium. Some microbiologists have describ ed the presence of both 'e ubacteria- like' and 'archaea-

948. like' genes in eukaryotic nucleus.
18.9 LIFE ON OTHER PLANETS Scientists presume the possibility of origin and presenc e of life on some
other planets of universe, where conditions similar to those responsible for the origin of life on

950. prokaryotes . prokaryotes.
prokaryotes. prokaryotes.

952. prokaryotes. prokaryotes
.
Infolding of plasma membrane
Endoplasmic
Endoplasmic Endoplasmic Endoplasmic
Endoplasmic
Endoplasmic Endoplasmic Endoplasmic
prokaryotes.
Endoplasmic
prokaryotes.
reticulum
prokaryotes.
Nuclear
prokaryotes.
envelope
prokaryotes
.
Endoplasmic
Endoplasmic Endoplasmic
Endoplasmic Endoplasmic
Endoplasmic
Endoplasmic
Endoplasmic
Endoplasmic
Endoplasmic Endoplasmic
Endoplasmic
FIG. 18.23: The origin of eukaryotes through serial endosymbiosis. The proposed ancestors of
mitochondria were aerobic , heterotrophic prokaryotes and proposed ancestors of chloroplasts were
photosynthetic prokaryotes.
Earth are still existing. The total number of stars in the universe is estimated to be 10
23
.
It is presumed that
at least 5% of these stars may have condition s suitable for life. But most of the planets of solar system have
temperature either too high or too low to support life, and some of the planets do not have an atmosphere.
Origin of Life: At a Glance
1 . With the cooling of Earth, free atoms in the atmosphere came together and formed inorganic molecules.
2. Inorganic molecules interacted and produced simple organic compounds like sugars, fatty acids.
glycerol, amino acids and nitrogenous bases.
3. These molecules by further interaction gave rise to large complex organic compounds like
polysaccharides, proteins, lipids, nucleotides and nucleic acids.
4. Large organic molecules aggregated into coacervates. These were able to absorb and release materials,
grow and divide.
5. Nucleic acids took control of the activities of coacervates and acted as ribozymes which became the first

953. primitive living system (eobionts).
6. Nucleic acids and proteins combined, forming self-reproducing systems (free genes).
7. Free genes diversified and joined , producing primitive heterotrophs resembling viruses.
8. Evolution of lipid-protein membrane which separated the eobionts from surrounding environment, gave
rise to first living cells.
9. Some primitive heterotrophs evolved into first autotrophs by acquiring the ability to form organic food
for themselves.
10. Further evolution resulted in the diverse forms of existing organisms.
Moon lack s both water and atmosphere. Therefore, there is no possibility of the existence of life on moon .
Me rcury is extremely hot because of its nearnes s to the sun and it also lacks atmosphere. Therefore, it must
also be barren and lifeless.
Where Life Originated?
Following facts prove that life has originated in the sea:
1. Many simple and lower animals live in the sea.
2. Inorganic salts of protoplasm occur as major constituents in sea water. 3. Earliest animal remains are all
found in the rocks at sea bed.
From the sea water , many organisms later invaded freshwater and land. Some land animals such as
crocodiles, sea snakes, whales and seals have gone back to the sea.
Time of Origin of Life
The Earth probably came into existence about 4-6 billion years ago. Fossils of certain prokaryotic cells
have been found from the rocks 3.5 billion years old. It is thus presumed that life appeared about 3.7 billion
years back. There was well formed atmosphere at that time, but it contained no free oxygen.

955. Condition s on Venus are not properly understood, because its atmosphere is filled with dense white clouds,
probably made up of something other than water vapours. The knowledge about it acquired so far suggests
that it is extremely hot and dry. But Jupiter, Saturn, Uranus and Neptune have dense atmosphere made up of
thick clouds of hydrogen, helium, methane and ammonia which are similar to primitive atmosphere of early
Earth. Therefore, it suggests all the possibilities of presence of life on these planets . But the planets are
supposed to be too cold (temperature ranging from 2,000° to --4,OOO°F) so that water remains ever frozen.
Since liquid water is very essential for life, the presen ce of life is improbable on these planets. Condition s
of Pluto have not been explored out but most probably it is also very cold.
18.9.1 Ancient Bacteria on Mars
Mars is the only other planet where life could be supported . The temperature on its surface ranges from
30°-60°C during day time. Its atmosphere is thin and composed of nitrogen, carbon dioxide and water
vapours. Even scattered clouds have been observed in the atmosphere. Some astronomers have observed
seasonal changes in the colour of Mars from bluish green to yellow or brown. Moreover, the pictures sent
by Mariner-9 of America have shown the possibility of presence of life on Mars. But pictures sent by Viking
in 1976 have made the possibility of life on Mars doubtful. NASA has predicted to establish human
settlements on Mars by 2020 A.D or so.
There are possibilities of coexistence of life in the universe on many other celestial bodies outside the
planets of our solar system. Among the billions of visible stars, there could be hundreds of other suns which
have their own planetary systems and there could be millions of planets with conditions suitable to support
to life. This will be something very amazing to find life on planets other than Earth.
EXOBIOLOGY
(Recent Reports from Mars and other Planets)
I. In August 1996, scientists from National Aeronautics and Space Administratio n (NASA) conducted
experiments on meteorite, called Allan HiIIs-84,OOl. This is thought to have knocked off from Martian
surface about 15 million years ago and landed on Atlantic ice sheet about 13,000 years ago.
2 . The chunk of rocks (collected from Antarctica in 1984) and fragments of Murchison meteorite which
landed on the township of Murchison in Australia in 1969 and studied in 1984, are found to contain water
and organic molecules. These rocks are estimated to be about 4.5 billion years old. Analysis of gases
trapped within the pockets of these rocks indicate that the meteorites are from Mars. The presence of
ancient bacteria in these meteorite was concluded on the basis of following informations:
• Biotic Combinations of Minerals: The meteorite rocks have iron oxide
(magnetite) and iron sulphide (pyrrhotite) associated with microfossil-like

957. elements. These two mineral s do not precipitate together under acidic conditions exce pt by living forms.
• Organic Residues: Meteorite rocks are found to contain some large complex organic molecules called
polyc yclic aromatic hydrocarbons (PAHs). These molecu les resemb le the PAHs which bacteria produce
on being subjected to rock formation. Therefore, presence of PAHs indicates presence oflife on early Mars.
Presumably, bacteria-sized simple organisms existed on Mars 3.6 billion years ago.
• Bacteria-like Structures: Microfossils of the size of 20- 100 nanometers were seen in carbonate patches
within the meteorite.
3. NASA spaceship, Sojurner Rover, landed on Mars surface on July 4, 1997 on a boat-shaped rock , called
Yogi rock named after the famous cartoon character, Yogi Bear. The photographs sent by its space craft ,
Path Finder, showed that Martian surface had rugged terrain suggesting presence of water and occurrence
of floods. Scientists believe that Mars once had plenty of water but it has mysteriously disappeared.
Presence of big hole in ozone layer over Martian atmosphere seems to have been the sole reason for deserte
d surface and absence of life on Mars.
4. Recent reports sent by Galileo Space Probe (August, 1996) indica te presence of water on one of Jupiter
's moon, Europa. It sugges ts possibility of the presence of some kind of primiti ve life on Europa.
5 . In December 1997, Galileo mission to Jupiter reported the presence of salts (magnesium sulphate) on
frozen surface of Jupiter 's moon , Europa.
6. A recent report on Venus has emphasised the effect of green house gases on Venus, turning it to be
excessively hot. Unfortunately, our Earth is also heading for the same fate because of excess ive enviro
nmental pollution, deforestation, increasing concentration of carbon diox ide and depletio n of ozone layer.
7. A new planet, orbiting another star, Rho Coronae Borealis was observed in 1997 by Robert Nayes of
Harvard Smithsonian Astrophysical Observatory. It is of the size of Jupiter. It is the first planet discovered
outside our solar system. Its discovery supports the presence of many more solar systems and planets in our
universe .
8. Europa, the large ice covere d moon of Jup iter is also considered to provide condi tions for the origin of
life.
KEY TERMS
• Abiogenesis
• Biogenesis
• Catastroph ism
• Eobionts
• Microspheres
• Prokaryota
• Stromatolites
• Archaea
• Biogeny
• Chemogeny
• Eukarya
• Monera
• Protista

959. • Archaeba cteria
• Biopoiesis
• Coacervates
• Eukaryotes
• Panspermia
• Protobio nts
• Autogenes is
• Bubble hypothesis
• Cognogeny
• Microfossils
• Proteinoids
• Ribozymes
REVIEW QUESTIONS
I . Summarise Oparin concept of origin of life on Earth.
2. What is modem concept of origin of life? Enumerate its essential features.
3. Describe briefly the contributions of Stanley Miller and Louis Pasteur.
4. Write what you know about the history of Earth and origin of life.
5. Discuss various theories of origin of life on Earth. Which one seems to be most plausible and why?
6. What are coacervates? Discuss their importance in the origin of life.
7. Mention differences between the primitive Earth atmosphere and the present atmosphere of the Earth.
How the present atmosph ere has evolved?
8. Enumerate the energy sources in the atmosphere of primitive Earth which led to the synthesis of various
organic molecules .
9. Describe Stanley Miller's experiment and explain how does it prove the biochemical theory of origin of
life?
10. Explain with reasons:
(a) The primitive atmosphere of Earth was without oxygen.
(b) All organic compounds formed on primitive Earth had hydrogen and carbon. (c) The sea on primitive
Earth was both of numerous organic compounds but it was not exposed to any disintegrating activ ities.
II . Describe two different methods of orig in of first cell. How it looks like? 12. Enumerate different steps in
the chemical evolution leading to the synthesis of self-duplicating organic molecules.
13. Explain with reasons whether plants came first or the animals.
14. At what point in the evolution of contemporary cells did life begin? 15. In what respects are proteinoid
microp heres similar to contemporary cells? 16. According to most authorities, what was the condition of
the prebiotic Earth with respect to lithosphere (crest) , hydrosphere (water) and atmosphere (air)? 17. Write
a note on the role of water in the origin of life.
18. Differentiate between:
(a) Coacervates and microspheres
(b) Chemosynthesisers and photoautotrophs
(c) Primitive atmos phere and modem atmosp here of Earth.
19. Define the following: (a) Archaebacteria (c) Congogeny
(e) Abiogenesis
( b) Protobionts (d) Biogenesis (f) Microspheres
20. Describe significance of virus in suppo rt of biochemical theory of origin of life.
21. Justify the statement that 'life originated in water'.

960. 22. How would Earth be different had there been no origin of photoautotrophs?
23. Can chemical evolution of organic compounds as occurred on primitive land occur under present
conditions on Earth?

962. 24 . Why is it accepted that RNA was formed first as hered itary material and life evolved as RNA world?
25. What are coacervates and what are their life-like properties?
26. Explain "Evolution of lipid membrane around droplets in primitive ocean hastened the proce ss of
formation of first cells".
27. What is endosy mbiosis?
28. What role did endosymbiosis play in the evolution of eukaryotes?
29. (a) What molecules are thought to have been present in the atmosphere of early Earth?
(b) Which molecule that was notably absent in the prim itive atmo sphere is now a major component of the
present day atmosphere?
(c) What proces s was respon sible for increasing its concentration in the atmosphere?
FURTHER READINGS
I. Allwood, A.C., M.R. Walter, B.S. Kamber, P. Marshall, and t.w. Burch, 2006 . Stromatolite Reef from the
Early Archaean Era of Australia. Nature. 441 , 714-718.
2. Cavalier-Smith, T., 2003 . The Phagotrophic Origin of Eukaryotes and Phylogenetic Classification of
Protozoa . Int. 1. Syst. Evol. Microbiol., 52, 297- 354 .
3. Chela-Flores, J., G. Lemarchand, and J. Or6 (eds), 2002. Astrobiology : Origins f rom the Big-Bang to
Civilisation. Kluwer Academic Press, Dordrecht, The Netherlands.
4. Knoll, A.H ., 2003. Life on a Young Planet: The First Three Billion Years of Evolution on Earth.
Princeton University Press, Princeton , NJ.
5. Miller, S.L., 1992. The Prebiotic Synth esis of Organ ic Compounds as a Step toward s the Origin of Life.
ln Maj or Events in the History of Life. J.w. Schopf (ed.) Jones and Bartl ett, Boston , pp. 1- 28.
6. Morowitz, H.J., 2002. The Emergence ofEverything: How The World Became Complex. Oxford
University Press, New York.
7. Pagel, M. (ed. in chief) , 2002. Encyclopedia ofEvolution. 2 Volumes, Oxford University Press, New York.
8. Mona stersky, R., March pp. 54-81.
1998. "The Rise of Life on Earth", National Geographic,
DOD

964. 19
History of Life on Earth
The univ erse is estimated to be formed about 13.7 billion years ago. The planet Earth which is home to all
the life, came into being abo ut 4.6 billion years ago and life existed on Earth for about 3.8 billion years.
Geologists have attempted in various ways to estima te the age of the layers of rocks. The entire lifespan of
Earth is called the geological time . Plants and animals have changed gradually during the passage of
geological time and present a chronological sequence of events, which led to the evolution of more and
more complex forms from the simple ones. The se evolution ary changes are read in the form of fossils that
are found in succeeding rock beds or strata.
A n English geologist William Smith ( 1769- 1839) realised that in a series of undisturbed rock layers, the
bottom layer must be the oldest and the fossils of plants and animals found in lower layers must have
existed at an early date than those found above them or in the new rocks. Also a given rock mass is
distinguished from the earlier and later rock masses by the type of fossils it contains. It postulated that rocks
having fossils of simpler plants and animals represent older rocks and those having more complex forms are
comparatively recent.
19.1 GEOLOGICAL TIME SCALE
B y studying the fossils present in the rocks, the geologists have divided the geological time into intervals,
which are characterised by significant changes that occurred in the organisation of organisms from time to
time. These divisions are of different durat ions and of different categories. The major divisions of
geological time are known as eras. These in tum are divided into periods , which are further subdivided into
epoc hs. The se eras, period s and epochs are arranged on the time scale in an order of their age, and this
arran gement is called 'geologica l tim e sca le'.
19.1.1 Divisions of Geological Time Scale
The first geological time scale was developed by GIovanni Arduin a in 1760. He proposed that the rocks of
the Earth can be divided into primary (oldest), secondary
History of Life on Earth iii 447 (intermediate) and tertiary (youngest) groups. The geological time scale
within its limits gives us a pretty good idea of the sequence of events in the history of Earth.
19.1.2 Eons
The 4.6 billion years long period of Earth's geological time scale is divided by geologists into two major
divisions, caned Eons. An Eon is the largest division of geological time scale, consisting of two or more
eras. The two Eons are named Cryptozoic Eon and Phanerozoic Eon.
Cryptozoic Eon also caned Precambrian Eon is the period of rarity of life and absence of fossils. It
includes the Azoic and Archeozoic Era, characterised by absence of life or presence of simple living forms
whose fossils are not available.

966. Fossils become
abundant
FIG. 19.1: Geological time scale .
Phanerozoic Eon (Gk. phanero, visible + zoon, life) is the period of visible lifeforms and abundance of their
fossils. It is divided into three major eras: Palaeozoic, Mesozoic and Coenozoic Era.
The n ames of the units or divisions or subdivisions have their own significance. These are either based
upon the name of the place, where particular type of rocks were found in abundance (Permain, after the
name of city Perm of Soviet Union) or where these rocks were first noted, or after the name of a particular
group of people who were residing there where the rocks of that period were first recognised (Ordovician
after the name of Ordovician, an ancient tribe that lived in that part of Wales where Ordovician rocks were
discovered). Otherwise, the name designates some particular characteristic of the period. Carboniferous or
coal bearing period is named so because a great deal of coal was formed during this period and Palaeozoic
Era of ancient life represents the fossils of primitive forms in abundance.
The g eological time scale has been broadly divided into six major divisions which are called eras. There
are evidences that each era was ended by widespread geological disturbances called revolutions. Each
revolution includes changes like upheaval of the Earth surface in certain regions (to form mountains) and
lowering of land in other regions, which resulted in the formation or elimination of inland seas. These
revolutions changed the distribution of sea and land and also of organisms living there in. They wiped out
many of the previous forms causing major extinctions. So far five major extinctions have occurred and each
of them defines the end of an era. The most famous is Cretaceous extinction. During each of the eras, there
were lesser events, which were less widespread and less far reaching in their effects than the revolutions
responsible for the separation of eras. These events are called disturbances in the Earth crust. These
disturbances also marked definite breaks in the geological and fossil records and divide an era into periods
and epochs.
19.1.3 The Eras
The eras are divisions of geological time that stand between the eon and the period. These indicate the
characteristic stages of development of fossils borne by them and the degree of evolutionary advancement
of life. The various eras are:
I. Azoic
II. Archeozoic
III. Proterozoic
IV. Palaeozoic
V. Mesozoic
VI. Coenozoic
Time of no life
Time of initi al or beginning of life
Time of earliest life
Time of ancient life
Time of middle life
Time of recent life
19.1.4 The Periods A period is the subdivision of an era of geological time distinguished by a particular
system of rocks and fossils. The Coenozoic, Mesozoic and Palaeozoic eras have
"+':I!::tiQII Geological Time Scale with Notes on Events in The Evolution of Life and Environmen

967. t Epoch Duration
..
"
in
W
'l:
Millions tll of Years
a..
0
- - 2,000
'0
N
0
Ql
s:
Time from Beginning of Period to Present
Millions of Years 3,600
Geolog ical Plant Life An imal Li fe and Climatic
Conditions
Great volcanic activities , no recognisable fossils, indirect evidence of living things from some sedimentary
deposits of organic material in rocks.
0
~ First Great Revolution (Considerable Loss of Fossils)
0
- 500 2,000 Cool climate, '
0
-
e
volcanic eruptions, repeated
2
glaciations.
Primitive aquatic plants, algae, fungi
e
o,
and bacteria . Shelled protozoans, coelenterates, flatworms, and primitive
annelids.
Second Great Revolution (Cons iderable Loss of Fossils)
- 100 600 Warm
C
'
co climate, great
C
~E
submergence
co
of land.
o
Land plants probably first appeared, marine algae abundant. First indication of fishes ; corals and
trilobites
abundant; diversified molluscs.

968. Appalachian Revolution (Some Loss of Fossils)
C
- 75 500 Climate
co
became
'0
N .~ progressively Algae, fungi, and, first land plants.
"E warmer.
0
- 20 425 Slight climate
C
cooling;
0
'
co
C
extensive
'
0
N
i:ii
co
"'.a
0
continental
Ql
seas.
ro
c,
- 60 405 Broad distribution of uniform C climates,
co
'2 increased
"'" 0>
temperature.
Ql
0
First known land plants
-clubmosses; algae dorni- nant.
First forests, first gymnosperms and first known liverworts,
horse -tails and ferns.
- 50 280 Climate
cli)
uniform; warm
.s e and humid at
o.Ql
first, cooler
to 0._
'Ci) •..!.
later as land
.-'" c
.~ -e rose ; spread
::2 co of tropical~ inland seas.
Mosses and seed ferns
dominant;
gymno sperms increasingly widespread. (Early coal
forests).
Abundant marine invertebrates;
appearence of first vertebrates (Ostracoderm).
Wide expansion of invertebrates; first insects, rise of fishes.
Diversification
in fishes;
Bryozoans &

969. Corals sharks
and lungfishes abundant;
evolution of
amphibians.
Rise of insects; sea lilies at peak; spread of ancient sharks; Radiation of amphibians

971. Contd ...
Epoch Duration "
I n ...
.;:
Millions
W Gl
of Years
a.
c:
- 40
·
III
c
III
CD
>
>,
~ '"
~ a.C
CD
·0
c:
- 25
-c
'0
CD
Cl
Cl
0
c:
·0
N
,... .§
CD
III CD
(ij
a..a..
T ime from Beginning of Period to Present
Millions of Years 320
Geolog ical and Climatic
Conditions
Plant life Animal Life
Uniform
climate
throughout world; warm and moist
345 Rise of conti
nents building of Appalachi an moutain
ranges climate became arid and varied;
glaciation
in Southern
Hemisphere . Great forests of seed -ferns and gymnosperms (Great tropical coal forests) .

972. Dwindling of ancient plants, decline of
lycopods and horse tails;
great conifer forests
Appalachian Revolution (Some Lo ss of Fossils)
- 50 230 Continents
elevated;
world-wide
0
sub-tropical
·iii
'" climate cool Gymnosperms dominant, dec1ining towards end; extinction of seed ferns.
~ and dry; and appearance of deserts
Cil
- 45 180 Culmination
a
o f world-wide
0
cool than
CD
a:: ·iii
mild climates.
'0 '"
ll>
..,
::::l
Building of
Cl
Sierra Navada
Navada
0 mountains.
·0
N
0
'"
- 72 135 Development
::E of climatic
diversity,
spread of
'"
::::l
inland seas
ll>
0
and swamps;
~
building of
o Himalayas,
Andes and Rockies, Cycads and conifers
common ;
appearance of first known flowering
plants.
Rise of flowering plants especially
monocotyle dons, decline of gymno
sperm.
Amphibians
dominant on
land; insects
common;
appearance
of first reptiles. Extinction of
ammonites
and trilobites;
abundance of
primitive reptiles; appearance of mammal-like

973. reptiles; decline of amphibia.
Transition of
reptiles to
mammals ; rise of progressive reptiles and egg lying mammals; extinction
of primitive
amphibians .
Dom inance
of dinosaurs;
appearance
of first toothed birds; spread of reptiles; rise of insectivorous
marsupials.
Dinosaurs
reached peak
became
extinct; toothed birds became
extinct; beginning of teleost fishes and modem
birds; archaic
mammals

975. common.
Contd ...
Epoch Duration
In
'0
l!
0
Millions
w ;:
Gl
of Years
D.
TIme from Beginning of Period to Present
Millions of Years
Geological Plant life and Climatic
Conditions
Rocky Mo
untain Revolution (little Destruction of Fossils)
Animal life
Palaeo5 63
Development cene of climat ic belts.
Eocene 22 58 Zoned climatic belts well established. Modemisation of angio
sperms .
Extension of angiosperms.
Oligo 11 36 Lands lower,
-cene climate warmer.
Miocene 13
25 Cooling of fij' climate .
(;j (ijE tE ~:::E'"
Pliocene 11 12 Cool and tem-
'0
Q)
perate climate
Ol
away from
s
~ equator; con-w
tinuous rise of
0
'0
mountains in
N
0
Western North
c:
Q)
0 America .
o
Ple isto 1 1 Periodic
-cene continental (2) glaciat ion in
north . Recent 0.025 0.025 End of last ice (1) age; climate
e
warmer; '"
climat ic zones
E
distinct
2

976. '"
a
World wide
tropical
forests , rise of monocots and flowering plants .
Development of grasses ;
reduction of forests .
Decline of
forests ; spread of herbs and grassland .
Increase of herbs ; great decrease of woody plants .
Dominance of herbs.
Evolutionary
explosion of
mammals .
Placental
mammals
diversified and specialised ;
hoofed mammal s and carnivores established .
Archaic
mammals extinct ; appearance
of modem
mammals .
Mammals
at height of
evolution ; first man-like apes.
Abundant
mammals ,
man evolving ; elephant , horses and camels
almost like
modem forms.
Age of man ;
extinction of
many large
mammals .
A ge of man,
development of human cultures .
been subdivided into smaller units of time called periods. The names of most of the periods have been
derived from the name s of the areas in which the rocks were first studied and described. The geological
time scale includes 12 periods which are as follow s:
• Periods of Palaeozoic Era
I. Ca mbrian: L. Camboria, meaning Wales.

977. 2 . Ordovicia n: From an ancient tribe that lived near the ancient Wales.
3. Silur ian: From the Silures , an ancient tribe of Wales.
4. Devonia n: From Devonshire, England.
5. Mississippian: From the upper Miss issippi valley.
6. Pennsylvanian: From the states of Penn sylvania called the Carboniferous.
7. Permian: From the province of Perm in Ural Mountains of Russia.
• Periods of Mesozoic Era
8 . Triassic: L. trias, mean ing three , refers to the natura l three fold division of these rocks in Germ any.
9. Jurassic: From the mountains between France and Switzerland. 10. Cretaceous: L. Creta, meaning
chalk, refers to chalky limestone.
• Periods of Coenozoic Era
II . Tertiary
12. Quaternary
19.1.5 The Epochs
Th e subdivisions of the periods are called epochs. The rocks deposited during an epoch are referred as
series. Epochs usually represent the upper, middle and lower parts of a period. The epochs of Coenozoic
Era are:
I . Palaeocen e
2. Eocene
3. Oligocene lTertiary period Tertiary period
4. Miocene
5. Pliocene
6. Pleistocene: Most recent
}Quaternary period7. Recent: present Priod
The above epochs are based on the relationship between the past and present forms of life.
19.2 AZOIC ERA This can be described as the earlie st time in the history of Earth which is characterised
by the complete absence of living organisms. During this period Earth was formed ,
cooled and underwent many changes which created conditions favourable for the appearance and
preponderance of living organisms. The rocks of this period are only of igneous type and are devoid of
fossils.
19.3 ARCHEOZOIC ERA Beginning: About 3,600 million years ago.
Duration: About 2,000 million years, including a discontinuity from Proterozoic
Era. This discontinuity stayed for 200 million years involving major fossil loss . Geological Conditions:
Great volcanic activities and a time of storms and extensive
erosion . The rocks of this era are very deeply buried and are exposed only at the
bottom of the Grand Canyon in Arkansas and along the shores of Lake Superior.
On account of excessive heat and pressure along with the catastrophic activity, most
of the fossils were destroyed. However, the occurrence of inorganic limestone and
graphite (pure carbon) indicates the presence of plants and animals. Life: The early organisms represented
in these rocks might have been simple,
unicellular organisms having soft body. Bacteria and alga-like fossil materials have
been described by Barghoom and Schope in the rocks from frozen lsua rocks of

978. Greenland. These earliest forms occur in the form of chemical signature of life in
the barren frozen rocks. For perhaps 1.8 billion years, these simple, single-celled
creatures had the Earth to themselves.
19.4 PROTEROZOIC ERA (The Era of Former Life)
(Gk. Proteros =former + Zoe =life)
Beginning: About 1,600 million years ago.
Duration: About 500 million years .

980. Geological Conditions: Deposition of large amount of sediment to form
sedimentary rocks
. Due to numerous volcanic eruptions lava flowed. There was at least one period of glaciation.
Life: The fossils found from the rocks of Proterozoic Era indicate that not only the life was present but had
reached great complexity. Algal filaments, fungi, sponge spicules, radiolarians, jellyfishes, branchiopods
and worms are represented by fossils. The first organisms were soft-bodied with few or no hard parts that
could be converted into fossils. Hence, fossils are poorly represented.
From the scanty fossil records of Proterozoic Era it could be inferred that this era was occupied by
bacteria, blue green algae and fungi, shelled protozoans, coelenterates (like jellyfishes and corals) ,
sponges, annelid worms , brachiopods and few molluscs also existed during this era.
The Proterozoic Era ended by the Killarney Revolution which resulted in the worldwide continental uplift,
mountain building activity and erosion . On account of these changes the fossil contents of the rocks
between the strata of late Proterozoic and early Palaeozoic exhibit a discontinuity.
The Ar cheozoic and Proterozoi c Eras have also been described collective ly under a common heading the
"Cryptozoic Eon " or the 'Precambrian Time' (lime before the Cambrian period).
The fossils became abundant in the rocks deposited about 600 million years ago in the Cambrian Period of
Palaeozoic Era. So, the period prior to that is charactersied by scanty fossils of the primitive marine
invertebrates. The lifespan of 600 million years of Earth's crust has been isolated from the rest of the life-
span and is designated as Precambrian time. It constitutes about two-third to three-fourth of the total life of
Earth.
Palaeozoic, Mesozoic and Coenozoic era together form Phanerozoic Eon .
19.5 PA LA EOZOIC ERA (Gk. Palaios =ancient; zoe =life)
Th e Palaeozoic Era which is also called ' the cradle of ancie nt life' had a duration of about 370 million
years starting about 600 million years ago and ending by 230 million years ago. The fossil records in this
era are very extensive and reflect the preponde rance of different plants and animals both in sea and on
land. Almost all the major invertebrate phyla are represented even in the early Palaeozoic Era. The fossils
of first vertebrates appeared late in the era and the reason for it may be that the early chordates were soft
bodied and were not preserved in the form of fossils. This era has been divided into seven periods. These
are as follows:
19.5.1 Cambrian Period
(Named after Cambria. the Latin name for City of Wales)
Th e earliest subdivision of Palaeozoic Era was named as 'Cambrian' by Adam Sedgwick in 1835 because
the rocks of this period were first discovered in the City of Wales which is called Cambria in Roman
language.
B eginnin g: About 600 million years ago.
Dur ation: About 100 million years.
Climate: The Cambrian Period started with the melting of glaciers, slow rise of
s ea level and warm climate. The oceans flooded inland and as a result what rep- resents North America
now was under sea in Cambrian Period. In other continents also the major land areas were under sea
water.

981. F lora an d Fa una: The Cambrian fossils are in abundance but are represented only by marine plants and
animals. There is no direct evidence of existence of land plants and vertebrates, though primit ive spore
bearing land plants are supposed to have occurred sporadically. Fossils of marine algae are widely
distributed . The fossils of bacteria , fungi and other prim itive plants are not found, though were present
then.
All th e present day invertebrate phyla are represented in the beds of Cambrian rocks. Fossils of protozoa,
sponges, coelenterates, annelids, molluscs, brachiopods, echinoderms and the trilobites have all been
recorded. But only those invertebrates got fossilised during
this period which had hard parts. Most of them were brachiopods and the trilobites, the most primitive
arthropods. These constitute over fifty percent of the known fauna of that time.
Further evolution since Cambrian included the ramification of the body patterns which were established
during Cambrian
.
The period ended by Green Mountain Disturbances or Vermontion Disturbances in North-Eastern part of
North America, Canada; and mountain formation activities in European Continent.
19.5.2 Ordovician Period
(Named after Ordovices, a tribe natives of ancient Wales)
The name Ordovician was proposed by Charles Lapworth in 1876 to commemorate the Ordovices, an
ancient tribe that inhabited the City of Wales.
Beginning: About 500 million years ago.
Duration: About 75 million years.
Climate: The climate in Ordovician Period was uniformly warm with glacial activities in certain regions.
Continents were flooded with shallow sea and volcanic activities were abundant in eastern North America.
The period ended by Tectonic disturbances in eastern North America leading to the formation of Tectonic
Mountains.
Flora and Fauna: Ordovician Period marks the appearance of first vertebrates in the form of armoured
jawless fishes, called Ostracoderms, which were dwelling in the freshwater rivers. Their fossils are in the
form of bony scales. Life still flourished in oceans and fossils of land animals are not recorded.
The first corals appeared in Ordovician Period and started their reef building activities. Trilobites reached
their climax, Brachiopods and Graptolites became abundant. Snails, clams and giant cephalopods like
squids and Nautilus, etc., also made their appearance. Phylum Echinodermata came into prominence.
Crinoids became dominant and even starfish appeared.
19.5.3 Silurian Period
(Named after an ancient tribe, Silures which inhabited Britain)
The term 'Silurian' was proposed by Murchison in 1835 to designate rocks that were exposed in the borders
of Wales and England, the territory which was formerly inhabited by the Silures.
Beginning: 425 million years ago .
Duration: Roughly 20 million years.
Climate: During Silurian Period the climate was mild and some areas were arid .
The sea alternately advanced and retarded during this period and volcanic activities took place. The period

982. ended by Caledonian disturbance in Europe and resulted in the rise of Caledonian Mountains in British
Isles, Scandinavia, North Africa and East Central Asia.
456 IiJ Evolutionary Biology
Flora and Fauna: The Silurian Period is characterised by the occurrence of two important events. During
this period the first land plants and the air breathing animals made their appearance. The first land plants
were represented by ferns. The first terrestrial animals or air breathers were large scorpion-like arthropods
(eurypterids), millipedes and wingless insects . Eurypterids were the ancestors of modem scorpions. Marine
invertebrates still dominated the scene . The trilobites and graptolites gradually declined. The corals made
huge coral reefs and evolved new species. Sea lilies became more numerous. The ostracoderms were
present and ancestral forms of cartilaginous and freshwater fishes (Placoderms) also made their
appearance.
19.5.4 Devonian Period
(Period of 'Devon' in England)
The name 'Devonian' was proposed b y Sedgw ick and Murchiso in 1839 after Devon or Devonshire, a
county of South western England, where rock formations of this period were first found.
Beginning: About 405 million years ago.
Duration: About 60 million years.
Climate: The climate in Devonian Period was mild with local dryness. Certain
areas were very dry. The period was ma rked by great volcanic activities and the formation of coal, oil and
gas. The close of Devonian is marked by a localised disturbance, the Acadian Disturbance or Acadian
Orogeny, which centered in and adjacent to New England and Eastern Canada (Acadian Region) but its
effects were felt southwards as far as Cape Hatters.
Flora and Fauna: The land plants which made their appearance in Silurian Period became widespread in
Devonian Period. The Devonian flora included jungles of seed ferns, horse tails, scouring rushes and
lycopods. The trees in late Devonian were very tall measuring 30 to 40 feet in height. The occurrence of
seed plants indicates that during Devonian Period occurred the transition of water dwelling plants into
terrestrial forms. Fossil forests of Devonian Period have been uncovered near Gilobia, New York, where
numerous tree stumps with a diameter up to 3- 3 Y:z feet have been obtained.
The animal life still thrived abundantly in the sea only. The brachiopods reached the peak of their
development. The coral s continued their reef-building activities. Crinoids and starfish occurred in profusi
on. The ammonites, a special group of cephalopods, first appeared in this
period . Their shells vaguely resembled
ram's horns, that is why they owe their
name ammonite (Egyptian God Ammon
is represented by ram's horns). The only
group which exhibited decline during
this period was Trilobita.
The Devonian Period is frequently

984. known as the'Age of Fishes' because FIG. 19.2: An Ostracoderm (a fish - like form).
Fin Air sac
(swim bladder)
Lateral line
Operculum
Gills Tail fin
Twochambered
heart
A B

986. C
FIG . 19.3: A. Crossopterygian fish; B. A lobe finned fish with air bladder connected to pharynx , C.
Resemblance of fin of Crossopterygian fish and amphibian limb. H =Humerus , R =Radius and U =Ulna.
for the first time, fossil records revealed the existence of numerous and varied fish forms indicating their
abundance and diversification in this period. The ostracode r ms which appeared in Ordovician became
much more abundant into a variety of fish forms. The first to be evolved were the pla coderms or spiny-
skinned sha r ks. They were small, armoured freshwater forms , which had variab le number of paired fins.
Some had just two pairs of fins corresponding to the paired appendages (arms and legs) of higher forms ,
whereas some had as many as five pairs of additional appendages between these two. The placoderms were
the ancestors of Condrichthyes or cartilaginous fishes and Osteichthyes or bony fishes and became extinct
by the end of this period .
True sharks (Cho ndricht hyes) appeared in freshwater during Devonian Period but migrated to the ocean.
Offshoot of the sharks , called a rt hrodires, attained large size. They were very interesting, firstly because
they had jointed neck that permitted the skull to move up and down and secondly they were the largest
animal s of this period . The most abundant arthrodire was Dinichthys which was about 30 feet long.
458 ~ Evolutionary Biology
Th e ancestors of bony fishes also appeared in freshwater streams. By the midd le of Devonian Period they
di verged into three main for ms: lungfish, lobe-finned fish and the ray-finned fish.
T he Dev on ian ancestors of modem lung-fish C ho anicht hyes, were ab le to survi ve dry periods by
burrowing in the moist beds of streams or lakes because they could take air through their nostril s and their
air bladde r was modified into lung.
T he lobe-finned fish, crossopterygians, are characterised by the possession of lobed or fleshy round ed
bases of the paired fins. They were on the direct line of evolutio n from fishes to land vertebrates. (F ig.
19.4)
T hey became extinct by the end of Palaeozoic Era except the solitary living genera , Latimeria, the living
fossil.
The r ay-finned fish ramified in Mesozoic Era and gave rise to modem living genera of bony fishes.
FIG. 19.4: Lobe-finned crossopteryg ian, Latimeria.
FIG. 19.5: Transition of crossopteryg ian lobe-finned fish into an early amphibian . A. A bony fish; B. A
lobe crosspterygian fish; C. An amphibian
T he late Devon ian Period is charac
terised by the appearance of the first land
vertebrates. These animals were amphib
ians and were called Stegocep ha lia ns
(roof-headed) becaus e their skulls were
covered by bony armour. They coul d
crawl on small weak legs.
The earliest known stegocephalian is
named Ichthyostega. It was about 2 feet
long and exhibited a curious combina

987. tion of newly acqui red characters and
the characteristics of its fish ancestors.

989. St egoce phatians forms evolved from the FIG. 19.6: Ichthyostega.
lobed-fin lung fishes which succeeded in
leaving the water and establi shing themselves on land. The lobed fin with its axis of internal bones was
converted into a walking leg which of course was too weak to support the body weight on land.
What could be the reason which compelled the crossopterygians to invade the land is not known. Most
probably the competition in water became very severe due to overcrowding. The land offered food and
protection to many animals that could live out of water. It seems logical that they made their first approach
to land across the moist and sandy beaches, where food was left by the tides.
19.5.5 Mississipian Period or Lower Carboniferous
(Period of Mississippi)
The name "Mississipian" was proposed by Alexander Winchell in 1869 to designate the area in Eastern
Mississippi basin, where excellent rock formations of this period were uncovered. It has also been called
Lower Carboniferous because this period and Pennsylvanian Period were characterised with major coal
deposits of the world .
Beginning: About 345 million years ago.
Duration: About 25 million years.
Climate: The climate was warm and dry with desert conditions in a number
of areas and huge swamps in others . Conditions varied widely from one area to another. In Eastern North
America, the Appalachian area was somewhat disturbed. Pocono mountain formation occurred by the
deposition of coarse debris and deltaic accumulation which later on formed the Alps in Europe. Formation
of coal, oil, gas, lead, zinc, gold, silver, gypsum and rock-salts also occurred during this period.
Flora and Fauna: The Earth was thickly covered with plants related to ferns, club mosses , scouring
rushes, spore-bearing trees, primitive seed plants and seed-bearing ferns. In various swampy areas, the
vegetation developed into dense tropical forests.
In the dense forests the animal life was now abundant. The graptolites disappeared in Mississipian Period .
The echinoderms became greatly reduced. Crinoids reached the peak of their development, and
brachiopods with long radiating spines also became abundant.
Fossils of terrestrial life of this period are relatively few but fossils of salamanderlike amphibians have been
uncovered from swamps. The sharks were most abundant and a particular group of 'shell-crushing sharks'
reached the highest stage of development. Their teeth were adapted for crushing the shells of molluscs and
arthropods.
19.5.6 Pennsylvanian Period or Upper Carboniferous
(Period of Pennsylvania)
Begi nning: About 320 million years ago.
Duratio n: About 40 million years.

991. The term 'Pennsylvanian' was first used by H .S. Williams in 1819 and refers to
State Pennsylvania. The rocks of this period are well exposed and contain abundant coal.
Climate: The climate was uniformly mild and damp but became cold with local glaciations and appearance
of deserts. The swamps got buried by the repeated advances
of sea-water on land. Numerous mountain disturb ances occurred all over the world, which were suggestive
of Appalachian Revo lution. Ancestral Rockies appeared as a result of great folding of rocks accompanied
by outpouring of volcanoes. The climate was most suitable for the spread of swamp vegetation which
resulted in the formation of extensi ve coal beds . For this reason , Mississipian and Pennsyl vanian Periods
are often combined together and are collectively designated as Carboniferous Period (coal forming).
Flora and Fauna : The land flourished with luxuriant plant growth. In addition to horse tails, scouring
rushes , club mosses , the woody plants appeared and attained the height of over 100 ft. The most common
trees were scale trees. The insect life was most abundant. Dragon flies, cockroaches, etc., attained
maximum size. A dragon fly with a wing span of 20 inches has been unearthed from Belgium.
Amphibians expanded and became more abundant. Labyrinthodonts having somewhat lizard-like
appearance came into existence. The first reptiles, called stem reptiles, also appeared in Pennsylvanian
Period . Seymouria from Texas is considered to be the amphibian ancestor of reptiles. It is very much lizard-
like in appearance but also exhibits similarities with salamander.
19.5.7 Permian Period ("Period of Perm" a Russian Province)
Begi nn ing: About 280 million years ago.
Duration: About 50 million years.
The name Permian was introduced by Murchison in 1841, because fossil rocks
of this period were found in the Russian province of Perm .
iij1=J!:WPfl
Geologic al Time Scale (in million of years) since the beginning of each period and epoch Era
Coenozoic
Mesozoic
Palaeo zoic Periods and Epoch
7 . Recent
6. Pleistocene
5. Pliocene
4. Miocene
3. Oligocene
2. Eocene
1. Palaeocene
3. Cretaceous
2. Jurassic
1. Triassic
7. Permian
6. Pennsylvanian

992. 5. Carboniferous
4. Devonian
3. Silurian
2. Ordovician
1. Cambrian
Duration Million years
0
0 Till date
1-0.025
12-1
25-12
36-25
58-36
63-58
135-B3
180-135
230-180
280-230
320-280
345-320
405-345
425-405
50~25
600-500

994. Main fossils
First fossils of man
F
irst bird fossils
First mammal fossils
First reptile fossils First amphibian fossils
First vertebrate fossils
Climate: The Permian Period marks the end of Palaeozoic Era. It was characterised by great changes in
the climate as well as the topography. The sea retreated and continents uplifted all over the world .
Therefore, the shallow seas which covered the region from Nebraska to Taxas at the beginning of this
period, gradually receded off, leaving the dry land. At the end of Permian Period general uplifting of the
great mountain chain from Nova Scotia to Alabama took place which marked the Appalachian Revolution.
Widespread glaciation occurred in Southern Hemisphere that extended from Antarctic to the Equator in
Brazil and Africa. The climate, in general, became colder and drier.
Flora and Fauna: The swamp dwelling plants were replaced by more hard and woody plants. The true
conifers and cycads became most abundant and trees somewhat similar to date-palms appeared.
The marine animals were similar to those of the two preceding periods. The trilobites disappeared
completely and crinoids became rare. Spiny-shelled brachiopods declined by the end of Permian Period.
Ammonites became still more numerous and diversified into several new forms.
Lobe-finned Amphibians fish
Mamma ls
Birds
Reptiles
Present
o
·0
N 0 c ell U
100
o
·
0
N 0 l/J ell
::E
200
0
Ol
to
l!!
to
ell
>

995. 300
'0
l/J
C
~
~
400
o
· 0 N 0 ell (ij o,
Mammary glands H
air
500

997. 600
Ancest ral
verte brates Invertebrate ancestors
FIG. 19.7: A tree of life showing diversification of vertebrates.
Life o n land exhibit ed marked variations . The Permian insects were similar in size. New genera of
Mayflies, beetles , dragon flies were added. Fishes were still abundant in sea but freshwater fishes became
more prominent. Labyrinthodont amphibians still thrived in swamp s. The reptiles which appeared in
Pennsylvanian exhibited progressi ve development. The first reptile s, the cotylosaurs, appeared in Permian
Period. A group of mammal-like reptiles, the therap sids also arose in Permian Period.
Many of the Palaeozoic forms perished and became extinct, because they could not adapt themselves to
such drastic changes in the climate. Even many of the marine forms also disappeared owing to the cooling
of the water and decrease in the sea area.
19.6. MESOZOIC ERA (Era of Intermediate Life)
_____{Gk. Meso=middle + zoe =Iife}_ . . _
The Mes ozoic Era began about 230 million years ago and lasted some 167 million years. The outstanding
features of Mesozoic Era were: (I) Disappearance of ancient amphibi ans and their replacement by reptiles,
and (2) the differentiation and final extinction of numerous varieties of reptiles called dinosaurs, which
thrived in sea, in air and on land. For this reason, Mesozoic Era has been popularly called 'Age of
Reptiles'.
Flora
Me sozoic Era represents a time of transition from aquatic to terrestrial life and diversification in both
plants and animals for various terrestrial habitats. Plants such as seed ferns, lycopods and horsetail which
were common in Permian and early Mesozoi c, periods underwent mass extinction and became insignificant
in Mesoic Era. These were replaced by genera of medium-sized ferns, cycad-like plants and coniferous
plants which still thrive on land. The fossil plants of late Mesozoic Era exhibit marked similarities with the
present day vegetation. Gymnosperms and Angiops perms also appeared in the late Mesozoic.
Fauna
The Mesozoic animal life can be discussed under followings heads:
I. Culmination of cephalopods
2. Evolution of reptiles
3. Origin of birds
4. Origin of mammals

999. 1. Culmination of Cephalopods: Ammonites formed the dominant invertebrate fauna of Mesozoic oceans.
More than 6,000 species of ammonites have been described from Mesozoic deposits. Some of them attained
large size and others develope d a variety of knobs , spines and ridges on their shells . Ammonites saw the
culmination in Jurassic Period, but dimini shed in number during Cretaceous Period
and were completely exterminated by the end of Mesozoic Era. The modem squids and Octopus were
represented by Belemnites in Mesozoic Era.
Other Invertebrates: In addition to cephalopods, other invertebrates were equally abundant in Mesozoic
Era. Protozoans and Bryozoans, although, became rare in Triassic Period (early Mesozoic Era), increased
during Jurassic Period because the early Triassic seas were relatively cold and unhospitable for certain
types of life. Different groups of molluscs such as Gastropoda, Pelecypoda and Cephalopoda becam e more
diversified. Even freshwater clams and gastropods were plentiful and air breathing snails became equally
numerous. Arthropods attained much diversity. The trilobites were replaced by shrimps, crabs, crayfish and
lobsters in Triassic Period. Barnacles and true crabs appeared in Jurassic Era. Starfish, seaurchins,
crinoids and sea cucumbers were also represented in Mesozoic Era.
2. Evolution of Reptiles: Reptiles saw their culmination in Mesozoic Era. They attained supremacy and
were called rulers of Earth. For this very reason this period in geological history is correctly known as
"Age of Reptiles". A few paragraphs that can be devoted here to the story of most diversified reptil es
obviously cannot do justice to the dramatic spread and remarkable achie vements of this group . About a
dozen different orders of reptiles evolved and attained dominant position not only on Earth but in the air
and also in the sea. Some walked on all the four legs, others used a bipedal gait with the body supported by
hindle gs and balanced with a long tail. Some were herbi vorous, other were carnivorous. They ranged from
small size to over 100 feet. in length . The six major evolutionary lines have been presented by reptile s. The
most primitive one compri ses of ancient stem reptiles and turtles which originated in Permian Period . The
stem reptile s, Cotylosaurs, became extinct in Triassic Period and turtles (Anapsida) evolved complicated
dermal armour and have survived up to the present time with little or no change .
19.6.1 Origin and Evolution of Reptiles
Reptiles are the first truly terrestrial vertebrates which have become totally independent of aquatic medium,
since amphibians were never fully adapted for land life and had to return to water to. lay eggs. This
problem was overcome in the reptiles by laying hard shelled yolky eggs and the development of certain
embryonic envelopes known as the embryonic membranes, amnion and allantois. The amnion is a two
layered membrane growing out of the ventral wall of embryo and completely surrounding it. The space
between the empryo and amnion is filled with amniotic fluid which forms an artificial pool around the
embryo. It guards the embryo against mechanical injuries and also serves to resist sudden changes of
temperature in the environment. The allantois helps in aerial respiration.
19.6.1.1 Time of Origin
Reptiles most probably evolved from their ancient stegocephalian ancestors in Carboniferous Period and
before the close of the Permian Period, many of the principle lines of repti lian evolution had become estab
lished. The evidence for this belief is partly direct through the Permian palaeontological records and partly
464 ~ Evolutionary Biology
indirect, based upon the appearance of such groups in Triassic Period which must have had a long
antecedent evolution. Of the total fifteen orders of reptiles about 7 had evolved during Permian and other

1000. six or seven also appeared in Permian Period only. These are Chelonia (turtles), Sauroptery gia,
Ichthyosauria (fishlizards), Squamata (lizards), Rhynchocephalia (beaked reptiles), T hecodontia (crocodile
like) and the Dinosaurs.
19.6.1.2 Adaptive Radiation in Reptiles
Mesozoic Era is the age of reptiles. During this era, reptiles diversified to occupy all the possible available
habitats which the wide world displays . These became adapted to live in different habitats and exhibited
morphological or structural modifications.
The reptilian group, from which th is radiation has occurred, is represented by a short-legged crawling
cotylosaur, Limnoscelis. It was a primitive swamp dwelling and slow moving form, from which all other
reptilian groups have evolved. These are:
1. Arboreal Reptiles: Though the possibility of existence of arboreal reptiles cannot be denied but their
fossils are not availab le. But, today a large number of living reptiles are arboreal. For example, geckoes
and lizards with adhesive padded feet or chameleons which have grasping hands and feet and a prehensile
tail are arboreal reptiles.
2 . Aerial Reptile : The flying reptiles are represented by pterodactyles with winged fingers. These vary in
size from the size of a house sparrow to the largest of the nature's flying mechanism. They had leathery
wings and naked body. The wing membrane was supported by limbs and the fourth finger of the forelimbs.
Other digits were left free. They had a wing span of 27 feet or so.
3 . Amphibious Reptiles: These are partially aquatic forms and are represented by turtles, crocodiles ,
ancestra l plesiosaurs, parasuchia and many dinosaurs .
4. Aquatic Reptile : Truly aquatic reptiles were plesiosau rs, ichthyosaurs, mesosaurs (sea lizards) and sea
crocodiles. Ichthyosaurs were long, sleek creatures, resembling dolphins. They had flipper-like limbs and
were excellent swimmers . Plesiosaurs resembled lizards.
5. Fossorial Reptile : The fossils of fossorial reptiles are not obtained except for few fossil forms from coty
losaurs and pelycosaurs which had powerful muscles of the limbs adapted for digging. The present day
living fossorial forms are snakes and limbless lizards.
19.6.2 Dinosaurs
Dinosaur s are the terrible lizards which lived on this Earth 200 million years ago, long before the first man
appeared on this Earth. They mastered the land and ruled the Earth majestically for more than one hundred
million years during the Mesozoic Era, after which they showed a sudden decline because of reasons best
known to them.
o
o~
Q) U
Rhynchocephalians
)
Mammals
o
o~
Q)
::E
o

1001. °iii
Ul
l'1
...,
o
"iii
Ul
<1l
°C
I

1003. Labyrinthodont Amphibians
FIG. 19.8: Adaptive radiation or macroevolution in reptiles.
19.6.2.1 Nearest Relatives of Dinosaurs
Of the present day living forms, crocodiles on one hand and birds on the other hand are the closest allies of
dinosaurs.
19 .6.2.2 Ancestral Stock
According to Friedrich Von Huene dinosaurs have arisen from cotylosaurian stock which arose in
Carboniferous Period .
Early Dinosaurs: Early dino saurs were small creatures not larger than the fox terrier. They were cold-
blooded. They could run fast on four legs and could scamper along on their hind legs. Their front legs had
claw-like hands. They balanced themselves by means of a long skinny, lizard-like tail. They were
carnivorous using their front legs for grasping and tearing their prey.
Habit and Habitat: Dinosaurs exhibited two distinct groups on the basis of their feeding habits : The first
group includes carnivorous forms which could run swiftly on their hindlegs and used their forelegs for
grasping and tearing the prey, and the second of herbivorous forms.
Physical Conditions: When dinosaurs appeared, the climate was very warm and moist. There was no winter.
The warm sea flooded over much of the land. The land was marshy and swampy with green plants and
swamp forests . During their existence and dive rgence the climate grew semi-arid, which was the most
compelling cause of dinosaurs divergence.
Size: The smallest dinosaur, Comp sognathus, was about two and a half feet with the size of a domestic cat.
The other extreme was exhibited by Gigantosaurus of East Africa who was about 120 feet or more in length
and weighed about 40 tonnes .
19.6.2.3 Classification
Dinosaurs diverged into following different groups:
1. Saurischia: (Dinosaurs with Reptilian Pelvic Girdle): They were carnivorous two-legged dinosaurs
which probably preyed upon archaic mammals and lizards .
During Jurassic and Cretaceous periods some of the saurischians retained bipedal and carn ivorous habit
(T he ropoda) but attained gigantism. Tyrannosaurus was the largest land carnivore that ever existed. It
was about 50 feet long, 18 to 20 ft tall and 8 to 10 tonnes in weight. The other well known forms included in
this group are Allosaurus and Gorgosaurus. Allosaurus was
~bout
35 feet long . It had strong, heavy
hindlegs on which it could run . Its hindfeet had 3-clawed toes . Forelimbs were short with curved claws. Its
jaws were like that of a crocodile and teeth were big sharp blades.
Other Saurischia, beg inning in the late Triassic Period changed to a plant diet, reverted to four-legged
stance and increased in size giving rise to gigantic, longtailed, long-necked herbivorous sa uropods. Among
well known sauropods are Plateosaurus , Diplodocus, Brontosaurus and Brachiosaurus. They were the
largest four-foot ed animals that ever lived on land. Brontosaurus (thunder lizard) reached a length of about
70 feet (21 metres) and weighed about 30 tonnes. It had a heavy tail. Its body was of the size of a small
blimp (nonrigid) and legs appeared as treetrunk. The neck was as long as the tail but the head was quite
small. It might have been busy munching plants nearly every minute with its small mouth . It was living near
water and using its long tail like a whip and strike it against the head of the enemy chasing him .
Bra chiosaurus (ann lizard) was the biggest and heaviest of all the dino saurs. It was so tall that its little
head could easily have looked over a three-storey building. It weighed about 50 tonnes. The bones of its

1004. forelegs were unusually large. It spent most of its time under water with its head j ust above the surface for
breathing. It cou ld not move around much on land.
Diplodo cus (double beam) was about 87 feet long, the longe st of all the dino saur s, but lighter and
thinner. Its head was very small, borne on a long neck so that it cou ld breathe air, while resting or moving
in water.
2 . Ornithischia (Dinosaurs with Avian Pelvic Girdle): They were beaked herbivores. They walked on all the
four legs. The Omithischia (wit h bird like pelvic girdle) were poorly represented in Trias sic Period and
saw their cu lmination in Juras sic Period. They were herbivorous from the very beginning of their
evolution. Although, some of them wa lked upright, the majority had a four legged gait. Some of them
developed leaf-shaped teeth with serrated edges, whereas others lost the ir front teeth and deve loped a
stout horny bird like beak and webbed feet. They were commonly called ' duck-billed dinosaurs' (order
Ornichopoda). Examp les are Camptosaurus, Iguanodon, Trachodon , Nanosaurus and Anatosaurus. The
other group (order Stegosauria) with four legge d stance deve loped thick armour in the form of spikes,
plates and bony protuberances. Stego saurus and Ankylosaurus are the well-known plated dinosaurs.
Another gro up deve loped horns on their head. They are Triceratops, Protoceratops and Torosaurus.
Campto saurus (the flexible lizard) was not very large but it had horny beak at the front of its mouth for
nipping off plants. It cou ld wa lk on its two large hind legs but cou ld bend down to 'move on all the four
legs. Iguanodon (Iguana lizard) was the first giant reptile to be described. Trachodon (rough-toothed
dinosaur) had two
thousand teeth in his large mouth which resembled the beak of a duck . Its feet Brontosaurus Diplodocus
(The Heaviest Dinosaur) (Longest Dinosaur)
Stegosaurus1-2 meters (Plated Dinosaur)

1006. Tyrannosaurus rex colored (Longest Flesh-eden Dinosaur)
Triceratops Iguanodon (Horned Dinosaur)
FIG. 19.9: Some large size d dinosaurs.
FIG. 19.11: Ankylosaurus with FIG. 19.10: Podycosaur (Dimetrodon) bony plates like a turtle shell.
were webbed . They are regarded as great grand children of Camptosaurus. They spent most of their time in
water. Their heavy tail and webbed feet helped them in swimming. They were herbivorous using their beak
like a shovel for digging plants out of the mud.
Stegos aurus (the covered lizard) was covered with two rows of heavy bony plates arranged on its back. The
tail had four sharp , strong spikes . It walked on all the four legs but front legs were comparativel y very
short . With head close to the ground , it could easily nibble at low plants . Its brain was very small, of the
size ofa walnut, large enough to work itsjaws and four legs. Near its hips was the second brain, twenty
times larger than its first brain. It worked its tail and hindlegs. Ankylosaurus (curved lizard) is named so
because its ribs were curved with bony plates like a turtle shell. A large number of long spikes stuck out
from the sides of its body to protect its short legs.
Tri ceratops (three-horned face) was a 30 feet long herbivorous dinosaur. It had three sharp horns and bony
shell that protected its head , neck and shoulders. Protoceratops is a large and hom-faced dinosaur, 5-6 feet
in length .
19 .6.2.4 Aquatic Reptiles
While dinosaurs were jumping around on the land, some frightful reptile cousins entered the sea . These
creatures were sometimes 50 feet long. They had long neck and tail. Their limbs were modified into padd
les. The jaws bear spiked teeth. They belonged to three different categories:
I . Some of them appeared like a huge fish with limbs modified into flipper s and tail with a caudal fin. They
had a shark-like fin on their back . Their jaws were drawn out into long pointed beak with sharp teeth.
These are called ichthyosaurs (e.g. Ichthyosaurus), Ophthalmosaurus and Tylosaurus.
FIG. 19.12: Protoceratops, a large and horn-faced dinosaur 5-6 feet in length.
History of Life on Earth ~ 469
EJasmou~l50 ft _1
AI'c:heIon1 12 ft_1 Tyto,uuvs (20 ft ) ISMIiUJrclI
(Tl.WtIe)

(kNhyos.aurs)
FIG. 19.13: Aquatic reptiles.
2 . Turtles form another group of sea reptiles. They were over 12 feet long and some of them weighed 6,000
pounds. The turtle family is still living today but the present day turtles are much sma ller. Their body is
encased in bony plates. Archelon is an example of turtles.
3. The third gro up includes sea serpents or sea lizards. They are ca lled mosasaurs, e.g., Elamosaurus.
19.6.2.5 Aerial Reptiles
The aerial reptiles resembl ed bats to some exte nt. The patagium consisted of two
l ateral folds of ski n supported by limbs and the last digit of each foreleg. The larges t of these reptiles was
'flying dragon' Rhamph orhyn chus. It was about 27 feet with wing-s pread. It hunt ed on small anima ls.
Causes of Extinction of Dionsaurs
Afte r being masters of the land , sea and air for 120 million yea rs, most of the rep til es di ed out rath er
sudde nly. The only possible explanation for their sudden demise was the gradual coo ling and drying of the

1007. atmos phere. The dino saurs bein g cold-blooded could not live comfortably in cold wea ther. The plant
eaters could not eat the new kind s of plant s which had tough er stems and leaves. So scarcity of food and
unsuit able clim ate were the main reasons for the extermination of these giant forms , Some sma ll and
simple form s of reptil es, however, were able to get along with these changes. They survived and evo lved
further.
Ca nadian palaeontologist, Dale Russell ( 1982) is of the opinion that ' had the dinosaurs surv ived, they wo
uld possibly have evo lved into very inte lligent bipeds and had becom e the masters of the world' . A fossi l
of Stenomychasaurus at Russell's Nationa l Museum in Canada shows that in these form s the ratio of brain
we ight to body weight is similar to that which exists in some of the lower mammals at present. Thi s
indicates a trend of increase in ence phalisation.
FIG. 19.14: Rhamphorhynchus (a flying reptile, the largest flying dragon had a wing span of 27 feet).
470 lil Evolutionary Biology 19.6.3 Origin of Birds
Alt hough reptiles do min at ed the sce ne in Mesozoic Era, many other important groups of organisms
appeared during this era. Birds evolved from the same bipedal thecodont s. The first fossil birds found in the
rocks of Jura ssic Period belonged to genera Archaeopteryx and Archaeorn is. Archaeopteryx was about
the size of a crow and possessed feather s and wings but had a long reptilian tail very much unlike the mode
rn birds and a toothed beak . Fossils of Hesperornis, an aquatic diving bird , and Ichthy ornis, a powerful
flying bird, have been found from Cretaceous Period .
O strom pointed out that the skeleton of Archaeop teryx is very similar to those of several small thecodont
dinos aurs that were without feathers . Archaeopteryx was apparently a member of this group that happened
to evolve a gene complex that made its scales grow into feathe rs. It used its feathers for insulation and as a
net for catching insects . The gliding or flight developed in its descendents. Thecodont dinosaurs, Long
isquama, also had long, unusual scales but its descendents are missing. However, descendents
ofArchaeopteryx underwent adapti ve radiation evolving into a large and distinct group of birds.
FIG. 19 .15: A. Skeleton of Archaeopteryx; B. Restoration of Archaeopleryx
19.6.4 Origin and Evolution of Mammals
Wh en great dinosaurs died out, the little mammals had the world for themselves. Mammal s were warm-
blooded animals whose body is clothed with hair. They had larger brain in smaller body. They gave birth to
young ones and the female mammals had milk glands or mammary glands to suckle their young ones.
19.6.4.1 Origin of Mammals
1. Time of Origin: The most probable time for the origin of mammals is middle Permian when most of the
land in the Southern Hemisphere was subjected to severe

1009. glac iation. From the geological records it is concluded that first mammals or mammallike forms appeared
not later than Middl e Triassic or not earlier than Lower Permian.
2 . Place of Origin : Mammals are presumed to have evolved in the plains of South America and Africa.
3. Causes of Origin : Broom has visualised the following cause s for the origin of mamm als:
• increasing aridity of climate which led to the evolution of speed
• extensive glaciation espec ially in the South ern Hemisphere which required the retention of body heat
(poikilothermy)
This resulted in the acquisition of warm blood and heat retentive clothing in the form of hair.
4 . Stock: There had been two different views regarding the ancestors of mammals. Huxley (1880)
advocated their origin from Amphibia . But this view does not hold good.
At present, Cyn odo ntia, a group of reptiles from South Africa whose fossi ls are found in Triassic rocks , is
considered to be ancestor of mammals. The cynodonts had dog-like muzz le and heterodont teeth (clearly
div isible into incisors, can ines ,
FIG. 19.16: Cynodontiathe ancestral to mammal. premolars and molars); possessed two occipital condyles
and a prominent dentary
in the lower ramus . Structurally, cynodonts bridge the gap between reptiles and
mammals but are low in reptilian scale .
19.6.4 .2 Mesozoic Mammals
Mammals appeared in early Mesozoic (Triassic Period) when dinosaurs were abun-
dant and were flourishing exponentially . The mammalian fossils have been obtained from colder regions,
i.e., in Germany, England and North America indicating that they were warm-blooded or had some
protective mechanism whic h could restore body heat.
The mesozoic mammals were small in size averaging that of rat a lthough some attained the size of Fox-
Terrier. They were adapted to varie d diet, feeding on insects, worms, young birds and repti les, whi le some
mesozoic mammals remained scanty and unchanged, most probably because the ruling reptiles did not
permit them amp le opportunity to surv ive and diversify.
19.6.4.3 Archaic Mammals
The archaic mammals appeared in ear ly Tertiary time. They were very primitive with extremely small brain
, simple triangular teeth and pentadactyl plantigrade limbs (i.e., with 5 digits in the hands and feet). They
underwent specialisation and divergence throughout Ecocene Period . Three distinct groups can be
identified in the archa ic mammals. These are:
1. Creodonta: This group included the families ofArchaic carnivores. The creodonts had carnivorous
dentition but otherwise resembled herbivorous Condylarthra. Their terminal phalanges (unguals) were
more claw-like. But they had a very small brain case. The examples are bear-like Arctocyon, dog like
Dromocyon , otter like Oxya ena, cat-like Dissacus, hyaena like Hyaenodon and some like minks , Sinopa .
Some of them from Mongolia attained giant size about one yard in length.
2 . Condylarthra (Animals with Knuckle Joint): It is a group of primitive ungulates which paralleled
Creodonta. They had a long heavy tail and with more or less cursorial light limbs. They survived through
Palaeocene and Lower Eocene .

1010. 472 [j] Evolutionary Biology
O ne of them, Phenocodus, ranged in size from a fox to a small sheep. Its canines were tusk-like, the
grinding teeth were low crowned and simple, suited for succulent herbage. The skull was long and low with
a sagittal crest. The feet were five toed, semiplantigrade.
3 . Am by lopod a: They were archaic heavy-limbed slow moving ungu lates occurring in swamps. The feet
were short and hoofed. They attained elephantine size. Pan tolambda, Coryphodon and Unitatherium
belong to this group. Unitatherium are giant homed ungulates.
19 .6.4.4 Adaptive Radiation in Modern Mammals
The modem mammals include practically all the existing mammal s along with those which have become
extinct. They evolved in lower Cretaceous and diverged out into the following groups in Coenozoic Era.
These are:
1. Ca rn ivor a (Fissipedia and Pinnipedia)
2. Rodentia (Gnawing mammals)
3. Artiodacty la (Eventoed ungulates)
4. Perissodactyla (Odd-toed ungulates)
5. Proboscidia (Elephants and Mastodons)
6. Insectivora (Insect eaters)
7. Cetacea and Sire nia (Whales, Sea-cows)
8. Primates (Lemur, Monkey and Man)
19.6.5 Divisions of Mesozoic Era
The Mesozoic Era has been separated into three periods viz. Triassic, Jurassic and Cretaceous.
19 .6.5.1 Triassic Period
(Period of three fold division)
Beginning: About 230 million years ago.
Duratioon: About 50 million years.
Word 'Triassic' was coined by a German geologi st, F. Von Alberti for the striking
three fold division of contrasting rocks of Germany.
C limate : The clim ate durin g Triassic was hars h and dry, and ranged from
mild subtropical with seasonal changes. It was the period of contrasting Pali sad e
Disturbances in eastern North America which resulted in uplift of sediments of Newark group along
Atlantic Coast.
F lora and Fa una: The forests were formed of highly developed gymnosp erms represented chiefly by
conifers and cycads. Seed ferns became extinct. Ferns and scouring bushes abounded in the undergrowth.
The primitive amphibians (Stegocep ha lian s) became extinct. The dino saur s appeared as very small
lizard like forms but were to grow enormously both in

1012. size and number and in the coming 150 million years were the rulers of Earth. Some of the reptilian groups
returned to sea life (Ichthyosaurs and Pleisosaurs) and some invaded air (Pterosaurs). Another important
event which took place late in Triassic Period was the first appearance of mammals which retained egg-
laying habit (Monotremes).
19.6.5.2 Jurassic Period
(Period of 'Jura Mountain s ' in France)
Beginning: About 180 million years ago.
Duration: About 45 million years .
Jurassic Period gets its name from mountains which lie between France and
Switzerland. The name was first used in 1799 by A. Von Humboldt. Climate: The climate was comparatively
mild than found in Triassic Period . It
was warm and humid and with plently of rainfall. Shallow seas swept over much of
the continental area and mountains were laid in Western United States. The period is
characterised by Morrison formation leading to the formation of Rocky Mountain
Region and Nevadian Disturbance leading to the formation of Sierra Nevada ranges ,
extending from Mexico to Alaska.
Flora and Fauna: Conifers, cycads and ferns were widespread. Angiosperms,
the flowering plants , appeared for the first time .
While the strange gigantic dinosaurs roamed and ruled the land, small and
insignificant mammalian group also flourished humbly and the ancestral Archaeopteryx
made its first incipient flight. Other animals which appeared for the first time in this
period were lizards and crocodiles.
19.6.5.3 Cretaceous Period (Period of Chalk)
Beginning: About 135 million years.
Duration: About 72 million years .
The Cretaceous Period gets its name from the Latin word creta, meaning chalk and was coined in 1882 by
J. J. d' Omalus d' Halloy. It signifies presence of calcareous rocks all over the world .

1014. Climate: The Climate was warm and somewhat uniform over most of the Earth surface , but became cooler
later on. The seas spread widely over the continents in the earlier parts of this period. Australia still had
some ice and there was some desert land. The rocks of this period were mainly formed of algae and shells.
The period ended by Laramide Revolution.
Flora and Fauna: The flowering plants spread all over the world replacing many earlier types of
vegetation. Deciduous trees became abundant and dominated the landscape .
In the sea , sharks and bony fishes much like the modem forms flourished. Among the still abundant
dinosaurs were a large number of vegetarians and a few giant carnivores dinosaurs. The largest flesh-eater
of the period was Tyrannosaurus
rex 'turant Lizard King' . The pterodactyles and pleisosaurs attained largest size. Mesosaurs (fierce sea
lizards) also becam e most abundant. Snakes first appeared in this period . New variet ies of birds
appeared. Fossils of Hesperornis and Ichthyornis are found from Cretaceous rocks. Mammals were still
small and less numerous. Prototheri an s or archaic mam mals were represented by forms having long
heavy tail, short pentadactyl limbs. They reproduced by laying eggs. The marsupials (pouched mammal s)
diverged from these archaic forms and so also eutherians mammal s (the placental ma mm als). The first
placental mammals were small and subsisted on insects and worms . They were the progen itors of
insectivorous mammals .
T he La r a mide Revolution brought the end of Cretaceous Period and the Mesozoic Era. It included
colossal, world wide mountain-building activities accompanied by dropping temperatures and
disappearance of many swampy tracts . As the climate grew colder and drier by the end of Cretaceous , a
number of plants which served as food for the herbivorous reptiles disappeared. Herbivorous reptiles, in the
absence of food, could not survive and due to disappearance of herbivorous anima ls even the large-size d
carnivorous reptiles also became extinct.
19.7 COENOZO
IC E
RA (Era of Recent Life)
(Gk. Kainos =new + zoe =life)
Coenozoic Era , up to the present, has an estimated duration of about 63 million years . It derives its name
from Greek word kainos meaning recent and zoikos or zoe ' pertaining to life' . Cocno zoic Era is called
'Age of Ma mma ls' because of radiation of different mammalian groups . This era is also marked by great
adaptive radiation in birds and insects and the culm ination of angiosperms. The era has been divided into
two periods: Tertia ry and Q uarterna ry. Both these periods have been further divided into epochs.
19.7.1 Tertiary Period (Third Period)
B eginning: 63 million years ago.
Duration : 51 million years ago.
The Tertiary Period gets its name from Giovanni Arduina (1960). It signifies all
relatively recent and more or less unconsolidated material that contai ned fossils resembling those animals
which are still in existence or else are represented by forms closely allied to them.
Climate: In the b eginning the climat e was fairly warm but gradually it became colder and the climate belts
developed. The sea covered the parts of continents so as to assume present form. The period ended by
Cascadian Revolution , which included widespread mountain building activities so that by the end of

1015. Tertiary Period , the mountain ranges attained their present extent and shape .
Flora and Fauna: Practicall y all the ha r d wood trees and conifers of today were represented by the
beginning of Tertiary Period. Grasslands appeared in this period .

1017. T he Tertiary Period is characterise d by the spectacular rise of mamma ls. The mammals of early Tertiary
Period were small, short- legged and flat footed; their jaws were relatively longer than at present; their
brain capacity was limited . During the span of Tertiary period there was a general increase in the size in
vario us groups; teeth became specialised for varied feeding habits; the brain capacity increased and
mammals became more and more specialised. The modem orders of class Mammalia originated. The
advanced modem placental mammals, includ ing man, (which are distinctive in bringing forth their young
ones ready to live an independent existence) also made their appearance.
The Tertiary Period has been separated into five epoc hs, name ly Paleocene,
Eocene , Oligocene, Miocene and Pliocene.
1. Paleocene Epoch
Beginning: Abou t 63 million years ago.
Duration: About 5 million years.
Climate: Ideal for mammals, climate mild.
Life: Brachiopods had passed their peak of development in this epoch but made
up a very small portion of coenozoic life. Mammals of this epoch were primitive. Carnivores were first
represented by an archaic group called Creodonts (Gr. Kreas, flesh; odontos, tooth) . They ranged from the
size of a wease l to that of a large bear. Their claws were sharp and well developed but their teeth were not
specialised like modem carnivores. They became extinct by the end of Eocene.
2. Eocene Epoch
Beginning: About 58 million years ago .
Duration: About 22 million years.
Climate: Climate started becoming slightly cooler, mountains eroded and no
continental seas.
Flora and Fauna: The term Eocene has been derived from two Greek words meaning " Dawn of modern
life" . The flowering plants (Angiosperms) became most abundant and successful. The fossi ls of Eocene
indicate that many of the modem genera of plants like maples, dogwoods, walnuts, elms and peeches were
in existe nce. Archaic mammals (Monotremes) were existing. But the most significant event was
diversification of placental mamm als. By the end of Eocene many groups of modem mammals had come
into existence. Progenitors of camel, pig, horse, rat and monkey were present in Eocene. By the end of this
epoch two groups of mammals, whales and sea-cows (Cetacea and Sirenia) returned to sea and one group,
of bats (C hiropter a) conq uered air. Most of the modem orders of birds were also established.
3 . Oligocene Epoch
Beginning: About 36 million years ago.
Duration: About I I million years.
Geolo gical Conditions and Climate: Erosion of land continued and climate was
still slightly cooler.

1019. The O ligocene takes its name from Greek word meaning "a little modem". During this epoch climate was
pleasant , although parts of the continents began to have cooler weather. Great movements of the Earth' s
crust occurred between America, Europe and Asia which were previously separated, became continents
during this epoch.
In gen eral, the plants and animals were much like those of Eocene. Monocots increased and flowering
plants flourished. Archaic mammals became extinct. Eutherian mammals became still more numerous. Odd-
toed ungulates began to radiate. The horse of Oligocene Epoch had three toes and rhinoceros attained
larger size. Balcuchitherium , the giant rhinoceros was probably the largest land mammal that ever existed.
Some of the archaic carnivores like creodont s continued to exist, whereas others were replaced by true
carnivores. One family Miacidae included the ancestors of modem cats, dogs and bears. Anthropoid apes
also made their appearance in Oligocene. In addition turtles, crocodile s and alligators attained their
greatest development.
4. Miocene Epoch
Beginning: About 25 million years ago.
Duration: About 12-13 million years.
Geolo gical Conditions and Climate: In this epoch, the Alps Mountain Ranges
re ached their height and Himalayas were pushed up. Some regions became dry and arid, while others
cooler and wetter. There was a general lowering of temperature. Flora and Fauna: As the climate became
cold, deciduous trees became more and more abundan t. The northern flora began to retreat. The
development of modem mammals continued apace. The orders of carnivores, ungulates, rodents, elephants,
bats, sirenia, whales and dolphins etc., were well spelt out. The primates already in existence in Eocene
radiated during Miocene. In Miocene appeared the Proconsul, which is presumed to be the ancestor of both
man and apes. Three toed horse, Meryc hipp us, also evolved in this period.
5. Pliocene Epoch
Beginning: About 13 million years ago.
Duration: About 12 million years.
Geolo gical Conditions and Climate: In this epoch there was cool and temperate
climate away from Equator. There was continuous rise of mountains of western North America .
Flora and Fauna: Man y plants such as tulips and magnolia trees which were previously widespread in the
temperate regions, became exterminated from Europe. Grassland became more widespread. Placental
mammals closely approximated the modem species. Elephants were most numerous. Wild horses were
abundant and migrated to Old World. Horses, giraffes and gazelles spread westward from Asia towards
Europe through India. Fossils of Rhinoceros, Mastodon s, Hippopotamus, Camels and Tapirs are found
from Pliocene. Deers and Antelop es were abundant. Horses had evolved into one-toed form. The man-like
forms appeared in Pliocene. Austra lopithecus, the first ape man has been discovered from rocks of
Pliocene.

1021. History of Life on Earth Ij] 477 19.7.2 Quarternary Period (Fourth Period)
Duration: About one million years .
Climate: The Quarternary Period which spans perhaps final one million or million and a half years of the
Earth's history is a relatively short geologic period. The climate is varied in Quarternary and presents
climatic zones. The period is characterised by four periods of glaciation, between which the sheets of ice
retreated (interglaciation). These have been described as four epochs of the Ice Age . The last of the ice age
Epochs came to an end only about 10,000 years ago . The intervals between glaciations represent
interglacial periods, which are marked by warm climate. Until a few years ago, it was widely believed that
the Ice Age had come to an end and represents the Pleistocene epoch. The post-glacial time constitutes the
Recent Epoch. But most geologists of today believe that the Ice Age is not yet over and we are merely in an
interglacial stage . Life continued to flourish throughout the Ice Age, because the Ice never covered more
than about a quarter of the land area. But various forms became extinct because they could not endure
changing conditions.
All the modem forms of life are represented in Quarternary Period. The great game animals roamed over
much of the North America and Europe. The members of elephant tribe, the mastodons, mammoths and
woolly rhinoceroses reached their peak of development and became extinct. During Quarternary Period
buffaloes became abundant and Bison latifrons became the giant. Modem carnivores such as wolves , foxes,
lynxes, racoons, otters , weasels and others are well represented throughout the period. The most important
event in Quarternary Period is the r ise of man who has become the master of Earth. His success has been
due to the erect posture which freed his arms and hands; and large brain with which he could manipulate
tools. Because of this, Quarternary Period is described as the 'Age of man'.
The Quarternary Period has been separted into two epochs: Pleistocene and Recent.
1. Pleistocene Epoch
The Pleistocene Epoch began about one million years ago and ended about 8-10
thousand years B .C. It is believed that unusual solar activity and repeated glaciation activities occurred
during this epoch. As a result of which ice formed in the polar regions and on the mountains in the
temperate parts and the high altitudes. When the ice retreated during interglacial periods it left lakes, like
the Swiss Lakes, Lakes of Northern Italy, English and Socttish Lakes and the Great Lakes between Eastern
Canada and the United States.
During these glacial .and interglacial periods, various activities resulted in the destruction and
redistribution of many animal and plant species and adaptive modifications in those plants and animals
which survived. He rbaceous plants continued to evolve.
Many Pleistocene mammals such as saber-toothe d tigers, mammoths, elephants, wolves, elks and true
horses attained very large size, but few like saber-toothed tigers, ground sloths and mammoths could not
withstand the cold and became adapted to cold climate by developing hair or woollen fur. The fossils of
ancestral forms of man and apes have also been dug out from the Pleistocene rocks .

1023. 2. Recent Epoch
The peri od of last 10,000 years of the Earth's history is called Recent Epoch. It includes the time after last
glaciation. During this span ice receded, forests spread over in Europe and North Africa. Deserts grew up
and new vegetation of herbaceous plants became dominant. The modern species of man, Homo sapiens.
evolved and has undergone much cultural evolution during this epoch but no biological evolution is indicat
ed during this epoch.
The p alaeontologie records make it impossible to doubt that the present species arise from previously
existing ones . Desp ite certain gaps, this chronicle sequence of fossil plant and animal life exhibits an
orderly deve lopment from the simple to the comp lex forms . For many lines of evolution, especially the ver
tebrates, the successive steps are well recorded in the fossils .
KE Y TERMS
• Archaeozoic • Coenozoic
• Era • Epoch
• Mesozoic • Ostracoderm
• Pala eozoic • Precambrian
• Choanichthyes
• Geological time scale
• Proterozoic
• Placoderm
REVIEW QUESTIONS
I . Write an essay on geological division of the Earth's crust with the predominant fossil contents of each .
2. Describe the various geological divisions of Earth's crust giving examples of the chief fossils found in
each .
3. Describe in a tabular form the 'Geological Divisions' of the Earth's crust along with the ir salient
features and fauna .
4. Give an account of geological periods and indicate the groups of animals characteristic of those periods.
5. Describe the characteristic features of the Mesozoic Era.
6. Write short notes on the following:
(a) Dinosaurs (b) Origion of birds
(c) Recent Epoc h (d) Archaeozoic Era.
FURTHER READINGS
I. Benton, M.J., 2003. When Life Nearly Died: The Greatest Mass Extinction of All TIme, Thames and
Hudson, London.
2. Benton, M.J., 2004. Vertebrate Palaeon tology, 3rd ed ., Blackwell, Oxford, U.K.
3. Carroll , R.L., 1988. Vertebrate Palaeontology and Evolution , Freeman, New York.
4. Cavalier-Smith, T., 2004. Onl y six kingdoms of life. Proc. Roy. Soc. B, London. 271, 125 1-1262.
History of Life on Earth iii 479
5. Cutty Rogers, K.A., and 1.A. Wilson (eds), 2005 . Sauropods : Evo lution and Palaeobiology. University
of California Press, Berkeley, CA.
6. Erwin, D.H., 2006. Extinction : How Life on Earth Ended 250 Million Years Ago, Princeton University
Press, Princeton . Nl .
7. Hall, B.K., (ed.) 2007a. Fins into Limbs: Development, Evolution, and Transformation, The University of

1024. Chicago, Chicago .
8. Hallam, A., 2005. Catastroph es and Lesser Calamities, Oxford University Press, Oxford, England.
9. Hartwig, WC . (rd.), 2002. The Primate Fossil Record. Cambridge University Press, Cambridge,
England.
10. Hou, X-G., R. 1. Aldrodge, 1. Bergstrom, 0.1. Sivetor, and X- H. Feng, 2004. The Combrian Fossils of
Chenigjiang. China: The Flowering ofEarly Anima l Life. Blackwell Science, Oxford, U.K.
II . Hughes,c.,and R. Eastwood, 2006. Island radiation on a continental scale: Exceptional rates of plant
diversification after uplift of Andes . Proc. Nat/. Acad. Sci., U.S.A., 103, 103, 10339.
12. Kemp, T.S., 2005, The Origin and Evolution of Mammals . Oxford Univeristy Press, Oxford and New
York.
13. Kielan-laworowska, Z., R.L. Cifelli, and Z.-X. L40, 2004. Mammals from the Age of Dinosaurs: Origins,
Evolution, and Structure, Columbia University Press, New York. 14. Levinton, 1., 2001. Genetics, Palaronto
logy, and Macroevolution , 2nd ed., Cambridge University Press, Cambridge, England.
15. Raup, D.M., and Steven M. Stanley, 2007. Principl es of Palaeontology. 3rd. ed., W H. Freeman and
Company, New York.
16. Skeleton, P. (ed.), 2003. The Cretaceous World. The Open Univeristy, Milton Keynes, England.
17. Valentine, 1.W, 2004. On the Origin of Phy la, The University of Chicago Press, Chicago .
18. Weishampel, D.B., P. Dodson and H. Osm61ka (eds.), 2004. The Dinosauria, 2nd ed. University of
California Press, Berkeley, CA.
DOD

1025. 20
Fossils and Fossil Records
20.1 EARTH'S STRUCTURE
A supposed cross-section of Earth shows an innennost layer, the core, and an outer layer, the crust. In
between the two is a middle layer of mantle. The core is further differenti ated into two regions:
• inner core of solid iron, about 800 miles in radius.
• outer core of liquid iron mixed with sulfur or silicon and about 1,300 miles thick.
Mantle is the hot, partly molten porti on of Earth . It is about 1,800 miles thick and comprises
approximately four-fifth of the Earth's volume. Its density-increase s with its nearnes s to the core . Becuase
of radioactivity, pressure and localised healing and cooling, mantle undergoes repeated melting and
cryrstallizations.
The crust is the outer layer o f Earth. It forms the Earth 's surface. The thickness of the Earth 's crust is not
uniform and varies from 5-25 miles in thickness. It is composed of light and heavy materials. The forrner is
rich in silicon and alum inium, and the latter is rich in silicates, iron and magnesium.
Th e Earth's crust has been subjected to a wide range of complex internal force s within the Earth . These
forces often bring striking chang es, called revolutions in the Earth's crust , like elevating or lowerin g of
the land masses. The land masses are further subjected to ceaseless action of wind, frost and denudation
and other forces. The comb ined action of all these forces leads to the formation of new mount ains and
rocks by way of different processes.
20.2 CLASSIFICATION OF ROCKS
Th e rocks can be clas sified as follow s:
I. Sedimentary rocks
2. Igneous rocks
3. Metamorphic rocks
Fossils and Fossil Records Iil 481
"""",;';;;"';;';;;;;;.
, ,
,,, ...~" , '
Mantle
,,
, , "
,
,
'
'
, '
,
'
'
,
,
,

1026. ,

1027. '
. ';
.............,, ' ' ' '
,
,
'
'
, :.I Asthenosphere
"
"
" ,," core,,,, ,,
Inner,
,,
,
,
"
I
,
core
, I ,
-. 7-35 miles
-. 60 miles
-. .... 155 miles
+ 1,800 miles
+ 3,100 miles----~
+ 3,960 miles
1
FIG. 20.1: Section through earth to show its core, mantle, asthenosphere, lithosphere and earth crust.
Lithosphere is formed of relatively rigid plates that can slide over fluid-like, deformable mantle and
lithosphere .
20.2.1 Sedimentary Rocks
Var ious proce sses like weathering and denudations in nature carry away the surface material of land and
deposit it in the form of sedimentation in the river beds, lakes and in shallow seas. This sedimentation
material forms the cover of the sedimentary rocks.
The nature of the sediment in a sedimentary rock depends upon the nature of eroded material and the
climate of the eroded place . Wind, ice and flowing streams are the chief transporting agents .
20.2.1.1 Formation of Sedimentary Rocks
Sedimentary materials become compact rocks by the following processes:
1. Compaction: In this proces s water is squeezed out and the individual particles are pressed together by
the weight of the overlying sediments. Mud is transferred into shale by this proce ss.
2. Cementation: The mineral matter held in solution by underground water is deposited between the grains
to bind them together. The cementing material includes calcium carbonate and silica including iron oxide ,
clay and gypsum.
3. Recrystallisation: It enables small grains of minerals to grow into large and stronger ones or new
minerals to be formed in the open spaces between other grains. In due course of time, these are replaced by
more stable material s.

1028. 4. Chemical Alterations: These include replacement of inorganic iron compounds by organic matter,
destruction of organic matter by the activities of bacteria and bottom-dw elling animals.
20.2.1.2 Composition
The inorganic constitu ents of sedimentary rocks include mud and red clay along with deposits of alumin
ium, iron, mangane se and magnesium in higher concentration and calcium, potassium and sodium in lower
concentration.
The or ganic compon ents of a sedimentary rocks include various organic oozes like Globiger ina ooze,
Diatom ooze and Pteropod ooze. These oozes include small pelagic planktonic shelled anima ls. On the
death of these anima ls, their shells sink slowly, settle down on the floor of the ocean and form sediment.
Fragmental and Precipitated Rocks: Sediment ary rocks composed of particles of sediment from an earlier
source constitute fr agmental, clastic, detrital or mecha nical rocks.
Pr ecipitated rocks are formed in the following two ways:
1. By Inorganic Chemical Processes: Mineral matter dissolved in water settles down at the bottom as a
chemical or inorganic precipitate. The solution s that leached from the land are abundant in sodium
chloride, calcium sulphate, silica, carbonates of calcium and magnesium, and compounds of phospho rus,
barium, manganese and iron.
2. By the Action of Plants and Animals: Living organisms extract silica, calcium carbonate and phosphates
from fresh and sea water for making protective and hard structures such as bones, shells and teeth. But it is
difficult to determine in which manner a particular sedimentary rock is actually formed.
20.2.1.3 Classification of Sedimentary Rocks
The main sedimentary rocks are of the following types:
1. Conglomerate Rocks: Cemented gravel is termed conglomerate. In such
rocks quartz and chalcedony are especially abundant. Glacial deposits form rough,
coarse conglomerates, called tillites. When the gravel is relatively unworn rubble,
with sharp edges and pointed comers, it is called sedimentary breccia . 2. Sandstone Rocks: These may be
composed largely of gypsum , coral or quartz.
Heavy placer minerals such as magnetite , rutile and zircon are abundant in black
sands or yellow sands. Placer minerals are heavy and very resistant to weathering.
They become concentrated in running water or along beaches. The red, brown or
green colour of sandstone rocks is due to the presence of iron. In arkase variety
feldspar is prominent in addition to quartz. Graywacke sa ndstone consists largely of dark rock fragments,
usually slates or fine-grained igneous rocks.
3. Shale Rocks: These are the most frequently occurring sedimentary rocks on all the continents. These are
composed of mud (silt and clay) . In some rocks mica and quartz are also present to some extent. Sandy
shales are termed arenaceous shale, while organic matter makes black carbonaceous shale.
4. Carbonate or Limestone Rocks: These are most common of all the nonelastic sedimentary rocks.
Limestone is largerly built by the accumulaltion of shells and skeletons of organisms which take in calcium
carbonate from sea water in order to make their resistant parts. The open spaces are then filled by the
deposits of the precipitated matter from water. Calcite is the chief mineral component of limestone which
effervesces in acid. Cha lk is a soft, porous limestone.

1029. Both organic and inorganic agencies contribute calcium carbonate . The organic agencies are the shells,
carapaces and other organ ic tissues of the animals which supply CaC0
3
to the carbonate rocks. The fauna
mainly comprises the sponges, archeocyathids, hydrocorallines, bryozoans, tubicolous worms and other
oysters along with calciferous algae.
At a particular temperature, pressure, concentration of dissolved salts and pH of water, the calcium
carbonate yields limestone on precipitation.
5. Anhydrite Rocks: These rocks change into gypsum in the presence of moisture and reverse change takes
place when gypsum is heated to drive off its water content.
6. Rock-Salt Rocks: The enormous quantity of halite or rock salt dissolved in the oceans is ample to explain
the thick layers of this material that have deposited throughout geologic history.
7. Coal Rocks: These are also sedimentary rocks because these are found in layers.
8. Dolomite Rocks: They resemble limestone in many ways. These are formed when magnesium replaces
part of calcium in limestone. Dolomite is harder, heavier and less soluble in acid than calcite. It effervesces
in cold acid only when scratched or powdered.
9. Gypsum Rocks: These are found in the form of thick beds of sedimentary rocks with gypsum being the
major constituent.
20.2.2 Igneous Rocks
These rocks are formed by the solidification of cooling magma that is pushed up from the mantle, through
cracks in the Earth crust or from the lava from valcanoes.
20.2.2.1 Mineral Contents
Mineral contents in an igneous rock depend principally on the chemical composition of magma or lava.
Acidic rocks have a high content of silica. Quartz and feldspar predominate in them. These rocks are of
light colour and low specific gravity. Examples: Granite and Rhyolite.
Gra nite rocks are used in buildings and monuments. The colour of granite (white, gray, pink or red) is
largely determined by the colour of the feldspar. Presence of large amounts of certain minerals gives rise to
different varieties of granites such as biotite granite, horn blende granite or pegmatite.
Granite originates both from a molten state and by the transformation of sedimentary rocks by a process
called granitisation.
20.2.3 Metamorphic Rocks
These rocks are formed at depth , under great heat and pressure, by the alteration of either igneous or
sedimentary rocks without melting. The transformation or metamorphism is the resu lt of a changed
geologic environment characterised by the development of new textures, new mineral s or both . These are
often so unlike the former ones that it is usua lly difficult to determine the nature of the original rocks.
Most metamorphic rocks are similar in composition to the rocks from whic h they were derived.
20.2.3.1 Texture
Metamorphic rocks are foliated (leaflike), wavy or banded due to parallel arrangement of elongated
minerals. Such rocks tend to separate along the foliation roc k cleava ge.
it·i-'••1,_. Kinds of Meta morphic Rocks
-
S.No. Rock Name Mineral Composition and Structure
(A) Parallel Structure

1030. 1. Gneiss Coarse texture: Feldspar. quartz. other silicate minerals chiefly mica and amphibole.
2. Schist Coarse texture: Mica and other platy or elongated silicates with minor amounts of quartz and
feldspar.
3 . Phyllite Intermediate texture: Micaceous rock representing a transition from schist to slate .
4. Slate Very fine texture: Micaceous minerals with quartz and other impurities.
(B) Massive Structure
5. Granulite
6. Quartzite
7. Marble
8. Serpentine
9. Soapstone
10. Amphibolite
11. Hornfels
12. Ecologite
Feldspar and other s ilicates
Quartz grains and quartz cement
Calcite or dolomite
Serpentine
Talc
Hornblende granite
Clay
Pyroxene and garnet.
20.2.3.2 Factors of Metamorphism
Heat, pressure and fluid s act together to bring about changes during metamorphic process of rocks. Heat
from within the Earth, molten bodies of rock and pressure and friction speed up chemical activity. Move
ments of the crust are more effective in altering textures. Water and gas supply the mobility for the required
changes to take place and carry eleme nts from the magma to facilitate chem ical changes .
20.2.3.3 Cycling of Rocks
O ver a long period , the above three types of rocks undergo various changes and transform from one form
to other as shown in Fig. 20.2.
• Magma changes into igneous rocks by crystallisation.
• Erosion of igneous rocks by the actio n of water, wind and chemical reactions produce sediments which
get depos ited in layers and form sedimentary rocks .
• Sedimentary rocks undergo metamosphosis because of heat, pressure or chemical interactions and change
into metamorphic rocks .
I Lithification 'I
IErosion 1
Sedimentary rocks 1Crystallization

1032. 1
Melting
1
I
FIG. 20.2: Rock cycle that shows interconversion of different types of rocks .
20.3 FOSSILS (Latin; Fossilium, Something Dug Out)
Records of evolution in the rocks or in the form of fossils, i.e., "The records of the rocks", can be compared
to the records written by man, because these present a written documentary of the evolution of certain
animals, animal groups and even of plants. These geological records are written in the rocks in the
language of fossils which are the preserv ed remain s of animals, plants or their parts found in the various
strata of Earth. The word fossil literally means anything dug out of the Earth. This anything in the language
of biology means any type of remains of prehistoric organisms or almost anything that gives the evidence of
the presenc e of organisms which lived in the past.
20.3.1 Definiton
Sir Charles Lyell defines fossils as:
"An y body or traces of body, animal or vegetation buried and preserved by natural means".
It means the fossil may be an entire organism which got buried in the snow in the remote past, a mould or
cast of the entire organism or its parts or unchanged parts or their replica, foot prints or even the imprint of
a leaf on a stone.
20.3.2 Type of Fossils
1. Body Fossils: The fossils of hard parts of an organism, such as shell, tooth or bone are called body
fossils. These provide detail s of shape and function s of parts or actual organism. These occur in all shapes
and sizes and range from microscopic sea dwellers to huge terrestrial dinosaurs.
2. Subfossils: These are remains of animals and plants preserved in rocks less than 10,000 years before.
These also include remain s of bison trapped in frozen ice, in peat bogs or ancient man mummified in caves.
Subfo ssils were formed after the last ice age during Holocene Epoch.
3. Microfossils: These are the fossil remains of microscopic animals and plants which are usually less than
O.S mm or lIS0th inch in size. However, the skeletal deposits of these organism s may assume a diame ter of
10 em (4 inches) or so.
4. Macrofossils: These fossils are larger than 1 em in size. These include fossils of more advanced plants
and anima ls such as clams corals or skeleton of vertebrates.
5. Unusual Fossils: These fossils are formed by the combination of events and conditions which result in all
or most of the organ isms getting preserved in the rock. The famous deposits of Solenh ofen Limestone of
Southern Germany and Burgess Shales of Canada contain unusual fossils of:
(a) Mammoths dug from Siberian wastes and
(b) Remains of Archaeopteryx: the first bird from Solenhofen
10
Bavaria (Germany).
6. Trace Fossils: These are fossils of foot prints and trails left in mud by the organisms that lived in past.
Dinosaur's foot prints, worm trails and clam burrows are all trace fossils. It means trace fossils are formed
as a result of day to day activities of the organisms such as walking, crawling, burrowing, boring or
feeding.
7. Coprolites: These are also trace fossils. These are fossils of droppings of animal s or faecal matter. These

1033. may vary in size from tiny faecal pellets of sea snails to large coprolites of crocodiles, dinosaurs or
mammals. These are found in association with the animal fossils who made them. The study of fossil excreta
may provide valuable information pertaining to the food habits of the fossil forms .
8. Bioclast: These are fossils or fragments of fossils enclosed in sediments. This term is usually applied to
thin sections of fossils under microscope.
9. Burrows and Borings: Some animals live in the burrows, tubes and holes in the ground, wood or rocks
for shelter or in search of food. The burrows may be later filled with the sediments and preserved. They are
also regarded as fossils. Several fossil shells and woods have borings made by some other organism. Such
borings are also considered as fossils.
10. Gastroliths: These are found in abundance in the body cavities of certain reptiles. These structures are
believed to have been of some use in grinding the food contents in the stomach of some extinct reptiles.
11. Pseudofossils: Many objects of inorganic origin closely resemble the forms of organic origin, and are
found in the sedimentary rocks. These are referred to as pseudofossils.
20.3.3 Formation of Fossils
Formation of fossils includes a succession of several lucky events under specially favourable circumstances,
and a number of media help in the preservation of animals or animals parts. Unless one of such events
happens, the dead organisms are not fossilised. If an animal dies on an open hill slope or in an open plain
or in a dense forest, the flesh is at once attacked by predatory animals, scavengers, insect larvae and
bacteria, etc. All the flesh and organs quickly disintegrate. Even the bones and shells are exposed to these
destructive forces and are decayed. The wood, leaves and fruits of plants suffer a similar fate.
But, sometimes some of these terrestrial animals either die in the sand near the stream or in a stream bed
and soon become buried in the sediment and water. Their soft parts gradually degenerate and the bones are
gradually replaced by mineral matter. Even by the eruptions of volcano , the animals and plants found in its
vicinity are buried in its ashes . Petroleum springs, amber and resins from the trees, sand and ice also
preserve the organisms unchanged. The different processes of fossilisation are discussed in details as under:
20.3.3.1 Entire Organism Preserved
Under exceptionally favourable conditions, the entire anmials might get preserved in remarkably good
condition, retaining even the finer details of soft parts . These methods of preservation are:
488 !il Evolutionary Biology
1. Preservation in Ice or Permafast: In nature's cold storage warehouse, notably in the Arctic Tundra of
Siberia, the entire animals might get frozen and preserved in ice in remarkably fresh condition for
thousands of years. The bones as well as flesh remain unchanged because bacteria and other destructive
forces fail to work at such low temperature. Frozen specimens of this type are known only from Siberia, an
permafrost, where ground is permanently covered with ice and temperature is always below freezing point.
Woolly mammoths from Siberia are preserved this way, some even with their stomach content intact. Their
flesh was so well preserved that it could be fed to dogs thousands of years later. The two ice fossils obtained
were:
• In 1790 from Lena Delta
• In 190I from Siberia
2. Fossils in Petroleum Springs and Asphalts: Organisms are also fossilised
in oil saturated soils. Anima ls trying to cross petroleum, springs, pools of sticky tar or viscous asphalt get

1034. entrapped and become entombed. The remains of extinct rhinoceros, insects, birds and small mammals have
been recovered from the waxy, oil saturated soils. The best example of such preservation is at Rancho La
Brea, now situated in Los Ange les.
3. Preservation in Resins and Ambers: Usually insects get entangled in sticky secretions of the trees, like
resin secreted from the conifer trees growing along Baltic Coast over 30 million years ago from Mesozoic
and Coenozoic Era. On exposure, resin hardens and changes to amber, and with it the trapped insects and
small insect eggs and larvae are preserved for ever. Such fossils are so perfectly preserved that their
colours and even the minutest histological details are as clear as they are seen in freshly fixed specimens.
I
FIG. 20.3: A fossil fly preserved in amber from Baltic forest of Europe during Oligocene Period about 40
million years ago.
4. Preservation in Peat: The partly decomposed vegetable matter, called peat produces water-logged
conditions . It lacks oxygen and bacteria both. Animals or plants, on getting buried in this aseptic medium ,
are preserved . Peat fossils include bodies of Iron Age people in Denmark , and ' Pete March' from peat
deposit of Stafford shire.
20.3.3.2 Petrifaction
Petrifaction or Petrification is the commonest method of fossilisation. It includes burial of organisms in the
sediments that are continuously deposited on the floor of oceans or other large water bodies. Some of the
buried plants and animals turn into fossils and get embedded in the rocks, while others are destroyed. The
rocks, formed by the deposition of sediments, are known as sedi me ntary rocks. They are also called
stratified rock s because these are formed by the depos ition of sediments in layers.

1036. Living fish Sediment from river
Recent
Land raised above level
sedimentary rock
~---~
Fish skeleton partly buried in the sediments More recent sedirAnt collecting
Older sedimentary rock C
imentary rock imentary rock imentary rock imentary rock
~"
Older se
dimentary
,....""'"
Fish skeleton rock fossilised B
o
FIG. 20.4: A. Diagram to illustrate formation of fossil by the process of petrifaction B. Two stages in the
formation of fossils ; C. and D. Steps in the exposure.
W ater from rivers flows in the sea. The river wat er seeps sand and mud from the land and deposits them in
the sea, wher e these sink to the bottom. When an aquatic organism dies or a terrestrial animal accidentally
drown s or the recently dead remains of land-dwellers are washed into a lake, river or ocean by floods, may
by chance get cove red quickly by sediments. After the burial , most organisms j ust rot away but some
become deeply buried in the deposits. The soft parts of the body like mu scl e s, nerve s and internal organ s
gradually decay and are carried away by the seepage of water. The organic matter in bones, teeth or shells
of animals and leaves, trunks and roots of plants are preserved. Within the sedim ent layers, the hard parts
may retain much of their original compo sition, but their organic constitu ents disintegrate leaving the
structure porous.

1038. W ater seeps into the interior of these porous sturctures and mineral matter dissolved in water is slowly
deposited in the pores. Thus organic substance of hard parts is replac ed particle by particle with silica or
lim e
FIG. 20.5: Two fossil fishes from Green River Formation near Kemmerer during Eocene Epoch , preserved
by petrifaction.
490 ~ Evolutionary Biology
dissolved in water . The process proceeds , gradually and sometimes even the outlines of cellular structures
and almost all the histological details of various tissues are preserved . This process of fossilisation to
preserve the finer details is called histom etabasis (G. histos, tissue ; metabasis, exchange) .
As the process continues, more and more sand and mud keeps on collecting over the earlier deposits in
definite layers. With the passage of time say, after thousands or millions of years, under the influence of the
weight of sand so added over them year after year, the lower layers of sediment shrink and harden into
rocks. Such stratified sedimentary rocks abound on every continent. Whenever such rocks are explored by
man or eroded by water and wind , the fossils get exposed . Their presence indicates that the region was the
bottom of an inland sea or ancient lake.
Carbonisation
Carbonisation is the preservation of organic materials like chitin, scleroprotein , cellulose and lignin of
buried animals and plants in the petrified rocks. During carbonisation carbon residue forms a film in the
rock preserving the outline of thebody and sometimes the details of soft anatomy of the organism . Some
fossils of ichthyosaurs are carbonised.
Pytrisation
In pytrisation the soft parts of the buried plants or animals are replaced by pyrites formed under reducing
conditions within the sediments . Traces of cephalopod arms outlined by pyrites from lower Devonian
deposits from West Germany are formed by pytrisation .
The foss ils preserved by petrifaction are called petrified fossils. Both externa l and internal structural
details are preserved in these fossils.
By the process of petrifaction hard parts like bones and teeth of vertebrates, shells of molluscs, exoskeleton
of arthropods are preserved as fossils. The complete and uninjured invertebra te fossils recovered from
limestone rock deposits in Glass Mountains in Texas afford the best examples
of petrifac tion. From these preservations it
is possible to deduce the animal's posture
and style of walking. Scars on bones indi
cate the position and size of the muscles.
Fossils of such nature have been obtained
from the Pliocene beds of England.
Petrifaction of Soft Parts: Genera lly
petrifaction preserves onl y hard parts,
but at times, under spec ially favoura ble
conditions the soft tissue, like muscles or
organs, are preserved by the replacement of

1040. th eir organic material with mineral material FIG. 20.6: Glass Mountains in Texas.
molecule by mo lecule . The replacing minerals might be calcium carbonate, silica or iron pyrites . The
replacement is molecule by molecu le and so accurate that even the finer details are nicely preserved. For
examp le, a petrified muscle of a shark , about 300,000,000 (Three hundred million) years old was so well
preserved that not only every individual muscle fibre but their cross-striations could be seen in the sections
under microscope.
20.3.3.3 Compression
Th ese are mainly plant .fossils. They are formed by the compression of buried plant s, animal s or their
parts . The buried parts become flat because of compression caused by the pressure from the overlying
sediments. In case the compressed parts or structures are flat, they do not show any appreciab le change
except becoming a part of rock. Compression fossils conserve only the external morphology but do not
provide internal detai ls. The fossil plants from carbonifero us period are compressed fossils.
20.3.3.4 Moulds and Casts or Incrustation Fossils
Natural moulds and casts of terrestria l animals are formed by the hardening of material surrounding the
buried organisms. Their bodies disintegrate and are removed by seepage of the ground, leaving hollow
cavities. These form the moulds of the buried organi sms and depict the exact external features. The soft
animals like jelly fishes, wing s of insects , leaves, etc., are usually preserved by this method. Sometimes, the
holes are filled with natura l deposits of minerals which subsequently harden to form perfect and natural ca
sts of the original objec ts. If the natural casts are not formed , the mou lds discovered are filled with plaster
of Paris or some plastic compound to produce artificial casts. Fossils of Pomp eii city which was buried in
the volcanic ash from Mount Vesuvius in A.D. 79, present moulds and casts of the men and their domestic
anima ls.
Incrustation fossi ls do not contain original organic material and are without cellular detai ls or interna l
anatomy.

1042. A B c o FIG. 20.7: Different types of fossils: A. Petrified fossil formed by petrifaction. In some cases even
the finest details of soft parts are also preserved . B. Mould and cast fossil or incrustation fossil showing
only external form . C. Compression fossil in a sedimentary rock. D. Impression fossil of a leaf in a
sedimentary rock
49 2 ~ Evolutionary Biology
20.3.3.5 Impressions
Similar to casts and moulds are the impre ssions left by the vanished objects or their parts upon the
surrounding materials. These impressions are very much similar to the impressions of a ring on sealing wax
and are possibly formed when an organism or its parts come in contact with the soft clay. Impressions of
leaves of plants, feathers of extinct birds, wing membranes of flying reptiles, skin of dinosaurs , etc., have
been uncovered in the form of fossil impressions in the stones.
20.3.3.6 Tracks and Trails
While moving, the foot-prints or tracks of the organisms are left in the soft, moist mud. These get hardened
into rocks, preserving the prints.
By studying series of these foot-prints , following informations could be gathered: (a) Shape and size of foot
(b) Length of the limbs (c) Foot posture (d) Type of gait of the moving animal
(e) Body structure and body proportion
In 1948 , the tracks of an amphibian were discovered near Pittsburg, Germany from Pennsylvanian Period.
Trails are impressions left in the sedimentary rocks by the body parts. Trails of many molluscs , worms,
jellyfishes, sea urchins, etc., have been discovered in the sediments.
20.3.3.7 Mummies
In desert s, the bodies of dead animals or plants become quickly dehydrated by hot dry winds and are
preserved as mummies. After mummification the remains may get buried under the shifting sand where
mineralisation may occur. Soil blown by winds as duststorm, sometimes, becomes deposited in thick layers
over the remains of deceased animals or whole plants and are preserved as impression in the rocks. The
skin of some dinosaurs has got preserved this way.
20.3.3.8 Coal Balls
Petrified plant organs with spherial shape are called coa l ba lls because of their black colour and presence
in coal deposits. These are formed by the infiltration of calcium carbonate, magnesium carbonate, iron
sulphide, etc., in the buried plant parts. Presence of these substances prevents conversion of plant organs
into coal. Coal balls provide important informations regarding plant communities of geological past.
20.4 EXPOSING FOSSILS
Fo ssils are formed and occur in the deeper layers of Earth's crust. Their presence can be noted only if
Earth surface is dug deeply, which is not practically possible. But several natural changes occurring in the
Earth's surface unearth the fossils.

1044. EXAMPLES:
1. Due to folding and overthrusting of the Earth 's crust, mountains are thrust up and as a result older rocks
from deeper layers are exposed and if they contain fossils, these are also exposed.
2. Rivers while coming down to plains and coursing towards sea often cut deep gorges through many
sucessive strata, specia lly during flood time their activities are increased . When water recedes the eroded
Earth surface may expose rocks containing fossils.
3. As a result of tension rocks may crack and slip leaving deeper layers exposed.
4. Glaciation may remove superficial rocks.
All the above processes in addition to several other geological phenomena expose the underlying rocks
which contain fossils and make them available for study.
20.5 INTERPRETATION OF FOSSIL RECORDS
Th e Earth's crust containing fossils can be compared to the record book which contains history of
evolution of living beings written in the language of fossils. The layers of rocks can be compared to the
leaves of the book. These contain the story of life and living beings but the sequence of events is not
regularly arranged . If it were possible to find a place where deposition of sediments had been continuous
since the formation of the Earth, its present structure, the strata would form a complete geological column ,
and the included fossils would furnish a fairly good record of the forms of life that had existed during that
period . Although , some deposits are thousands of feet thick, no such complete geological column is known.
The study of fossils reveals the following informations about life in the past:
I . The fossils in different strata are different from one another, but fossils in adjacent layers are more alike
than those farther removed.
2. The more recentl y formed fossils are more alike to existing species than those in the lower strata.
3. Presence of same type of fossils indicates that these sediments were laid down at approximately the same
time under similar conditions.
On the above basis it has been possible to correlate the deposits all over the world into one chronological
sequence and a geological column or geological time scale has been constructed through these
correlations. The absolute age of Earth is estimated to be about 4.5 billion years. Though , the rocks
containing earliest known fossils are found to be 3.3 billion years old, the total span is divided into five
major divisions called eras. These are Archeozoic, Proterozoic, Palaeozoic, Mesozoic and finally Coenozoic
eras. The boundaries of successive eras are determined by the occurrence of major geologic revolution s.
Eras in tum are divided into periods and periods into epochs.
In addition to information about life in the past, fossils reveal certain other facts. The discovery of the fossil
remains of marine organisms like corals and sea urchins

1046. 494
~
Evolutionary Biology
in India an d in Himalayas indicates that at one time India and Himalayas must have been covered by
ocean and were thrust up to their present towering heights. Fossils of palms and alligators in Dakotas and
musk oxen in Arkansas are indicative of wide fluctuations in the climatic conditions of past.
20.6 LAW OF SUPERPOSITION
T he law of superposition proposes that the lower strata of a geological formation was the first to be
deposited and is oldest. The new fossils are formed in the rocks laid over them in due course of time and in
continuous succession.
20.7 WILLISTON'S RULE
20.7.1 Definition
S.M. Williston, an American palaeontologist, has formulated a principle based upon the study of fossil
records and the anatomi cal studies of living forrris. The rule states that du r ing th e evolution of lineage,
serially homologou s parts tend to reduce in num ber but get more and mo re differentiated.
EXAMPLES:
I. In primitive arthropods, the trilob ites, the appendages are present in large number and are more or less
identical arthropods on the other hand , possess in shape and structure. Modern
fewer appendages , which exhibit a great deal of speciali sation in structure and function. For example, in
crayfish the head appendages are modified for feeding , thoracic appendages for manipulation of food and
walking, and the abdominal appendages for swimming. Members of different orders and suborders of
Crustacea exhibit the same tendency.
2 . In fishes, the maximum number of bones in the skull is counted to be 150, whereas in mammals only
about 28 skill bones are present.
3. In the transition from primitive reptiles to mammals, the teeth changed from numerous, homodont type to
a few and heterodont type which are specialised for different functions.
4. In dicotyledonous Angiosperms, there is a decrease in their specialisation to be differentiated into sepals
and petals.
20.7.2 Exceptions
Of course, there are so many examples in favour of Williston 's rule, a few exceptions have been found :
I . The rule is applicable only to segmented organisms and cannot be applied to organisms lacking
segmentation.
2. The teeth in piscivorous cetaceans show an increase in number and decrease in specialisation. It is just
the opposite what the rule states.
Antenna
Chelate leg ,.....r-..+- Antennule
Chelate leg
Chelate leg
Chelate leg
Chelate leg
A Chelate leg
B

1047. FIG. 20.8: Diagram illustrating Williston's rule of reduction in the number and
complexity in the structure of serially homologous parts. A. A primitive and relatively generalised trilobite;
B. A crayfish with highly differentiated segmental appendages.
3. In snakes the number of vertebrae has increased considerably and these are not so much differentiated as
in lizards or in other groups .
4. Cacti flowers present an increase in the number of perianth parts and a decrease in their differentiation.
5. Increased specialisation of parts is in direct benefit to the organisms because it increases the competency
to perform certain tasks. For example, modified thoracic appendages and the walking legs in crustaceans
(crayfish) will be more efficient for crawling than the undifferentiated appendages of trilobite s or lobsters.
Natural selection always favours
uals and each part becomes more and more
the more competent individspecialised. This is achiev ed by the division of labour. But natural selection
does not favour decrea se in number.
Therefore, Williston 's rule has not been accepted and even its formulation is questioned.
20.8 COPE'S RULE
20.8.1 Definition
Edward D . Cope is an American palaeontologist. He made extensive study of fossil mammals. During his
course of study of fossil mammals he was struck by the number of different lineages that independently
produced giant forms in Pliocene and Pleistocene Epochs. He summarised his observations in the form of a
rule or law which is now known as Cope's Rule.
496
~
Evolutionary Biology
According to Cope's rule "Organisms have a tendency towards increase in size du ring their evolution."
EXAMPLES :
The generalisation was based on observations on fossil mammals. Evolution of horse, camel and all other
herbivorous mammals exhibits increase in size and some of them even attained gigantism.
I. Organisms other than mammals such as turtles, crocodiles, etc., exhibit the same phenomenon.
2. Dinosaurs provide the most interesting example of attaining gigantism.
20.8.2 Exceptions
Th ere are many obje ctions to Cope's rule. Cope's observations were based on mammal s but a number of
mammalian lineages do not show the tendency toward s increase , e.g.. insectivores and insectivorous bats.
It is physically impossible for mole to be very large, because that will create difficulty in burrowing.
Similarly, the flying habit imposes limitations on body size. Many mammalian lineages exhibit decrease in
size. For example, ungulates and carn ivores attained gigantism in Pliocene and Pleistoce ne but are now
showing decr ease in size.
It is now presumed that increase in size in different group s of mammals may be on account of different
reasons. In Pleistocene mammals, it could be a device to reduce body surface: body volume ratio so as to
conserve heat in the cooling environm ent. In carnivorou s animal s increase in size could facilitate the
capture of large sized herbivores on which they fed. In egg laying animal s this could be a device to
increase the fecundity (more eggs in a large body).
Walking Legs
Walking Legs

1049. 1st Pair of Walking Legs
Walking Legs
Walking Legs
Walking Legs
Walking Legs
FIG. 20.9: Differential growth rate or allometry as exhibited by the forelimbs of male beetles of the species,
Euchirus longimanus.
20.9 ALLOMETRY (DIFFERENTIAL GROWTH RATE)
Durin g development, various parts of the body of an organism exhibit differential growth rate and these
rates change from time to time. The study of differential growth rate was named heterogon y (Gr. unlike
development) by Huxley, but is well known by the term allometry (Gr. differential measurement).
Th e phenomenon of differential growth of different parts at different times is a matter of everyday
observations and is also exhibited by the fossils. Few of the examples are given below:
EXAMPLES:
l. In human foetus the head of the body, specially the decelerated rate, whi le the increased rate.
grows much more rapidly than the rest legs. After birth the head develops at a
lower portions of the bod y grow at an
2 . In a certain species of beetle, Euchirus longimanus, forelegs exhibit differential growth rate in
comparison to rest of the body. In this species sma ll males have large forelegs in comparison to males of
other species . But larger males of the species have forelegs that are much larger than those of the smaller
males, i.e., the growt h rate of forelegs is much greater than that of the body.
3. Allometry is ex hibited in the antlers of deer, Irish Elk, Cervus antiques. It exhibits positive allometry. The
species grew steadily large throughout the late Pleistocene Period and their antlers attained immense size.
It is often said that the antlers became of such an immense size that the Irish Elk became extinct, because it
could not hold up its head.
Allometry i s very useful in ontogenetic and phylogenetic studie s. General body outline can be analysed by
cartesian transformation , a special aspect of allometry. The outlin e of a primitive member of the group is
drawn upon a rectangular grid, is distorted by stretching particular parts in one direction or another. The
results stimulate the outlines of related species with different factors for all allometric growth. This provides
a support that such evolutionary changes are based upon mutations of genes influencing allometry.
20.10 DETERMINATION OF AGE OF FOSSILS OR
DATING OF FOSSILS
The study of fossils reveals the trends in evol ution and sometimes the ancestra l history of animals of past.
For this, it is essential to arrange the fossils in chrono logical succession. It needs approximately accurate
estimation of age of the fossils or the rocks in which these are contained. The dating of rocks is called
geochronology.
Palaeontologist s use following three methods for dating of fossils: I. Stratigraphy
2. Biostratigraphy
3. Radiometry
20.10.1 Stratigraphy
Stratigraphy consists of 'analysis of the succession of strata that have been laid one after the other in a

1050. more or less regular succession.'
Thi s is based on the concep t that lower the strata, the older are the rocks and the fossils contained. It
means lowest strata are the oldest and the uppermost strata is most recently formed.
20.10.1 .1 Drawbacks of Stratigraphic Methods
Stratigra phy is a crude method of determining age of fossils and rocks. It does not give the actual age but
provides only the relative age of fossils. Stratigraphy establish es a sequential relationship between different
rocks and the fossils contained therein, but not the exact time of their formation. This method can be used
only for those places where erosion had cut deep gashes and has exposed various strata and their fossils.
Other drawbacks of this method are:
I. The correct sequence of rocks might get disturbed at places due to overthrusting and other changes in the
Earth's crust. For example, in Sierra Nevada Mountains, the younger rocks have got covered with older
rocks.
2 . Floods cause erosion of rocks. The elevation of Earth crust at places forms mountain and disturbs
sequence of strata.
3. Deposition of strata is not uniform in all the places.
20.10.2 Biostratigraphy
Thi s method consists of recognising strata on the basis of fossils of flora and fauna contained. By
comparing the fossils, the strata in different parts of the Earth are identified.
20.10.3 Radiometry or Radioactive Clock Method
This met hod of dating fossils was introduced by Boltwood in 1907. It is called radiometric dating. It is
based upon the disintegrating property of radioactive isotopes of some elements, such as Uranium-238,
Uranium-235, Thorium-232, Radium-226, Potassium-40, Iodine-131 and Carbon-14.
The r adioactive isotopes are present in the igneous rocks along with the normal atoms in extremely small
quantities. They are unstable and emit electrons from their outer ring at a steady rate. Finally they change
into stable nonradioactive elements in a specific time. The rate of disintegrati on remains unaffacted by any
changes in the environment or by the presence of any other radioactive elements.
Each radioactive element has its half life which means one gram of a radioactive element changes into half
a gram (0.5g) in a specific time.
Half life is defined as ' the time taken by a specific radioactive element to decay to one half of its original
value.'
Table 20.2 provides a list of radioactive elements and their half life.
r
ii.·i:tliWl,.,J Half Life of some radio isotopes Elements
1 . Hydrogen-3
2. Carbon-14
3. Phosphorus-32
4. Calcium-45
5. Iron-59
6. Cobalt-60
7. lodine-131

1051. 8. Radium-226
Radio Disintegrate Isotopes into
3H 2H
14 C 12C
32p
P
45Ca Ca
59Fe ?
60CO ?
131
1
?
226Ra 205Pb
9. Uranium-238 238U 206Pb
10 . Potassium-40
11. Rubidium-87
12. Uranium-235
40 K 8Ar
87Rb 878 t
235U 207Pb
13. Thorium-232 232Th 208Pb
Half-life
19 days
5,730 years or 5.7 x 10
3
14.3 days
164 days
45.1 days
5.3 years
8.1 days
1,620 years
(1.6 x 10
3)
4.5 billion years
(4.5 x 109)
1.28 x 10
8
years
47,000million years
7.04 million years
(7.04 x 10
8)
13.9 billion years
1. Uranium-Lead Method: In this method radioactive uranium-238 (238U) is measured in the rocks.
Uranium breaks down into lead and helium at a very slow but constant rate . This is measured in units of
time called half-life. One-half of the total numb er of uranium atoms break down into lead in 4.5 billion
years. It means half life of 238U is 4.5 billion years. By accurately estimating the uranium-238 (238U) and
lead-206 (206Pb) in the given rock, the age of rock can be calculated as follows:
I

1052. ------ of a gram of lead (Pb)
7, 60,00,000,000
At this rete 'U' gram of uranium will yield

1054. I x U
grams of lead in I year
7, 60,00,0 00,000
In T years the uranium will yield
T x I x U grams of lead
7,60, 00, 000,0 00
Therefore, the amount of lead (Pb) produced by a given quantity of Uranium U, in T years, can be denoted
by the following equation:
T xU
Pb =
7,60,00,000,000
It means that time taken in number of years to produce this quantity of lead is:
-
Pb
x 7,60,00,000, 000 years
U
In thi s way, we can determine the age of uranium crystal and hence the age of the rock in which it is
located, provided we know the lead-uranium ratio.
As the uranium-238 (238U) atoms disintegrate, a series of new radioactive substances are formed and
ultimately end up in lead (206Pb).
By this method the oldest rocks are found to be pre-cambrian rocks which are 3,300 million years old.
"'Uranium
----' 226
Radium
----.
2
18
Astatine .
218Polonium"---' --210
Polonium--.

206Lead
FIG. 20.10: Steps in the disintegration of
238
superscript Uranium into 206Lead.
Recen tly some more geochemical methods have been developed. 87Rubidium yields Strontium;
232Thorium yields 208Lead and 235Uranium yields 207Lead. Each parent element has its own
characteristic half-life ranging from 126,000,000 years to 6,00,0 0,000,000 years.
Drawbacks of Radioactive 238U Method: In spite of perfection, the radioactive method has following
shortcomings.
• It is limited to those igneous rocks which contain the radioactive mineral.
• It is not known whether the helium or lead resulting from the disintegration of uranium is lost or not since
the time of deposition of uranium in the rock.
2 . Radioactive Carbon Method: Radioactive carbon (14C) method is quite accurate for relati vely recent
fossils. It was discovered by W. F. Libby (1946) . 14C is created from 14N by cosmic ray bombardment in
Earth's upper atmosphere. The natural rate of production of 14C is fairly constant. Its half life is 5730 =
5.7 x 103 years.
The radioactive isotope of carbon
than 0.1% in air, surface waters and
14C occurs in minute quantities, i.e., less living organisms. It is being continually produced in the
atmosphere by the action of cosmic rays on nitrogen and oxygen nuclei, and rate of 14C production is con
stant. There is an equilibrium between the production of 14C and los s of 14C due to radioactive decay. The
living organisms absorb 14C either as CO2 or as organic molecules throughout life. After death no more
carbon is taken in from the atmosphere. but 14C continues to decay. Therefore, amount of 14C gradually
decreases to one-half every 5.6 x 10
3
years until about 40,000 years it is no longer detectable. By

1055. measuring the emiss ion of 13-rays the age of fossil or of the organic matter is determined. For example, if
the amount of 14C in a fossil mammalian bone is 1I4th of the original
amount and half-life of 14C is 5.6 x 10
3
years , the estimated age of fossil bone will be 11.2 x 10
3
years. The
age of fossils as old as 10.0 x 10
4
years can be determined. 14C method has restricted application, because:
• It can be used to determine the age of actual remains of organisms. It cannot be used on fossils in which
organic matter has been replaced by minerals.
• It can only be used to determine the age of younger fossils that are not older than 40,000 years.
However, 14C method has proved useful
~-particles
FIG. 20.11: Radioactive decay of radioactive carbon (14
c)
into Nitrogen 14N.
in determining the age of young fossils from late Pleistocene rocks very accurately. 3. Potassium-Argon
Method: Recently, pota ssium- argon
method has been introduced to estimate the age of fossils .
Ordinary potassium contains 0.0 I% of radioactive isotopes,
which gradually disintegrates into calcium and argon. Its half
life is 11.6 x 10
9
years. Very recently this method was employed
to determine the age of earliest known fossils of hominoids in
East Africa.
Although, so widely used, the radioactive methods have a drawback, because
Anderson (1971) has reported that the decay rate of radioactive materials might be
affected by the adjacent atoms and other forces .
20.11 SIGNIFICANCE OF STUDY OF FOSSILS
The fossils have been described as the written documentary of the process of evolution contained in the
rocks in the language of fossils . The stydy of fossils provides following clues:
20.11.1 Fossils as Stratigraphic Indicators
The stratigrapher obta ins clues to the age of rocks from the fossils they contain. It is due to the fact that
there is a definite relation between the fossi l con tents of a rock and the position of this rock in the geologic
column. Older fossils are found in the bottom strata and the younger fossi ls are laid on the top of the old
strata. This principle of superposition enab les the stratigrapher to est imate the age of a particular rock.
Sometimes, the stratigrapher determines the sequence of the rock strata based on the law of faunal or floral
succession. According to this Principle fauna and flora are distinctive for each portion of Earth's history
and their fossils are formed one after the other in a definite order. Based on this principle, stratigrapher
determines the rock strata when it has been disturbed by deformations of the Earth 's crust. Sometimes, the
use of fossils to demonstrate the relationship of certain rock units is referred to as palaeontologie
correlations described below:
I . Guide Fossils: Fossils which characterise certain strata are called the guide fossils. These are the
remains of the rapidly evolving species that lived only for a short duration in the geologic history but had a
widespread distribution when alive.
2. Fossil Assemblages: Sometimes rocks contain more than one species of fossil assemblages. Under such
situations, the stratigrapher determines first the types of organism s involved in those assemblages which
lived during the time when the rock was deposited. Secondly, he also determines the relative abundance of

1056. each of the species .
3. Stage of Evolutionary Development: Palaeonto-logical correlations can also be made on the basis of
similarity of the evolutionary development between the groups of organisms appeared in the same strata of
rock. This type of correlation is seen in the phylogeny of horses. The record begins with Hyra cotherium, the
dawn horse, which was about the size of a small dog. It is the primitive known horse that appeared in
Eocene Era. The later horses which appeared in the successive periods of Coenozoic Era were larger in
size.
The rel ative primitive remains of Hyracotherium are used to correlate the Eocene strata, while that of the
more advanced horse, Pliohippus were used to correlate the Pliocene strata.
20.11.2 Evidence in Favour of Organic Evolution
According to the theory of 'Organic Evolution', the more advanced forms of today had evolv ed from the
simpler and more primitive ancestral forms living in the past. The fossils of the organisms in the older rocks
are found to be more simple than those contained in the new rocks or those living today. The fossils
contained in the older rocks differ considerably from the living forms and those contained in the younger
rocks appeared to be more closely related to present day forms . This succession of fossils clearly indicates
that life evolved gradually from simpler forms to more complex forms.
20.11.3 Fossils and the Prehistorical Life
The fossils provide useful information about the animals and plants living in the past. The giant dinosaurs,
ammonoites and trilob ites are known from their fossil records only. Fossils have also helped in determining
the relationship between different plant groups and the animal groups .
20.11.4 Pedigree of Certain Animal Groups
The evolu tionary history of certain animal groups is so completely depicted by the fossil records that
almost a completely unbroken sequence of different ancestral forms has been obtained. Pedigree s of horse,
elephant and camel can be quoted as examples of such fossil records .
20.11.5 Fossils and Palaeography
With the help of fossi ls it is possib le to reconstruct ancient geography of Earth. By observing the nature of
fossils in a particular area , it is possi ble to depict the habitat of that area. For examp le, presence of fossi
ls of trees or stumps indicates the terrestrial environment. Simi larly, the fossils of cora ls and echinoderms
are suggestive of marine environment.
20.11.6 Fossils as Climatic Indicators
Fossils also help in determining the climatic conditions of the geological periods . For example, fossils of
ferns and associated plants such as magno lias suggest a warmer climatic condition. The glacial or colder
conditions are indicated by the presence of such fossils as musk ox and reindeers.
20.11.7 Fossils as Indicators of Oil and Minerals
Fossils are useful in locating the important underground resources like ores, coal, oil and gas in the
sedimentary deposits. Geo logists use the fossils extensively to date the strata above and below the rocks
which contain valuable minerals. Many valuable deposits of radioactive minerals have been located in the
sedimentary rocks on account of the fossils only. Some specimens of fossil wood and bones of certain fossil

1057. reptiles and mammals have been found to contain uranium. The limestone and the sandstone beds are
formed largely due to the deposition of fossils. Trigonia sto ne of the lower Cretaceous deposits is a well
known example of such stones . This stone also contains large number of casts and moulds of marine clams
and snails .
Our knowledge on microfossils has greatly benefitted the petroleum industry. The majority of microfossils
comprise the remains of the invertebrates like foraminifers, radiolarians, sponge spicules, echinoid and
holothurian fragments, cryodonts, bryozoans, microscopic brachiopods and molluscs. But all of these,
Foraminifera is the most important group .
20.12 INCOMPLETENESS OF FOSSIL RI:CORDS
20.12.1 Difficulties in Fossilisation
No doubt that fossil records are so helpful and important in understanding the process of evolution and in
knowing the past life, 'the greatest drawback is their incompleteness. The reasons for their incompleteness
may be discussed as under:
I . Majority of animals do not get proper conditions for fossilisation. They are destroyed by predators or
scavengers, or are exposed to air for disintegration.
2. Generally, the hard parts of the body are fossilised. But majority of animals do not have any hard parts .
This means most of the protozoans (except foraminifers) , majority of coelenterates, worms and helminths
did not have a chance for fossilisation .
504 ~ Evolutionary B
iology
3. Of the several methods of fossilisation, the best chances are offered to animals living in the ocean, the
animals living in fresh water have the next best chance and the terrestrial animals have least opportunity of
changing into fossils.
4. All periods in the Earth history were not equally favourable for the formation and preservation of
sedimentary rocks. It is now established that the general level of continents has fluctuated. During glacial
ages the continental shelves were largely exposed, while during warm ages the low land areas and the
continental shelves were submerged. During submergence newly deposited stratum together with its fossils
was protected and preserved by the deposit ion of additional strata. So, the fossils of the period of
submergence were more abundant than those of the period of elevation.
20.12.2 Destruction of Fossils
Of the animals which are fossilised, only a f ew escape the irony of destruction. These are expos ed to
several natural destructive forces which can be summarised as under: I. The fossi ls present in the deeper
rocks with thousands of feet of other deposits above them are crushed and distorted. The rocks are exposed
to tremendous pressure and heat which may result in the alteration of deposits and the destruction of fossils
due to recrystallisation of minerals of the rocks and the fossils.
2. Due to the upheaval during mountain formation, changes occur in the strata and the rocks rise up in the
form of peaks thousands feet above the ground. This exposes the rocks and the fossils contained in them to
destruction by natural forces.
3. River s flowing down from the mounta ins carry rocks which are finally turned into sand. This destroys
the fossils contained in the rocks.

1058. 4. Rivers may cut deep gorges through many successive strata, thereby destroying the fossils .
5. Erosion by wind and water wear away rocks and the high mountain peaks are thus worn down to low
hills. One can never imagine how many pages of chapters from the fossil records might be spoiled this way.
6. Glaciation carries away superficial rocks leaving the deeper layers exposed for erosion.
7. Crack s in the rocks containing fossils due to tension may result in the destruction of fossils.
8. Sliding or slipping of the crust may disturb the sequence of fossils and may result in destruction of some
of them.
20.12.3 Difficulties in Exploring the Fossils
Of the fossils which escape destruction only a few are noticed by the palaeontologists because of several
practical difficulties. The se are:

1060. I. It is not practically possible to dig out the deeply situated fossils which are present in the older rocks and
are buried under the younger rocks.
Fossils and Fossil Records lil 505
2. Sedimentary rocks are chiefly formed under the sea and so also the contained fossils which remain
unnoticed and even their digging is not practically possible.
3. Major portion of the Earth is covered with sea, a part is covered with thick jungles and some portion is
ever covered with ice. All such places are not accessible for the palaeontologist, where he can go and
collect the fossils.
4. High mountain peaks are also unexplorable and even if the fossils are exposed there , they are left
unnoticed and unread .
5. Some areas are thickly inhabited, where digging is not possible. Our knowledge about the fossils has
been very superficial and incomplete and will remain so in due course. The story of evolution contained in
the sedimentary rocks in the form of fossils has several pages and chapters missing and unrecorded, while
some others are disturbed. Even though fragmented , these records present a clear testimony of the fact of
evolution and considerable details have been worked out in many lines of descent.
20.13 EVOLUTIONARY RATE THROUGH FOSSIL RECORDS
With the advent of a number of methods to estimate the absolute age of fossils it has been possible to
calculate the rate of changes of a particular taxa and whole faunas and floras. Simpson (1994, 1953) has
made outstanding contribution in the study of evolutionary rates and has made important conclusions about
the modes of evolution. Measurement of evolutionary rates is most accurate if these are expressed as rates
of changes in genomes. The average evolutionary rates are estimated by measuring genetic differences
between species that diverged during known times.
Estimation of rate of the change in some molecules like proteins and nucleic acids have helped in tracing
degree of resemblances and divergence between closely related groups e.g., degree of divergence between
apes , man and monkeys. However, in the fossil records, the estimation is restricted to morphological
changes, which may be evaluated in various ways. Table 20.3 presents some approaches to the estimation of
evolutionary rates by using fossil records.
The measurement of the rate of change may include a morphological character , complex morphological
characters or the total morphological aspect of lineage. These measurements in some known time will
provide estimated evolutionary rates during that time .
20.13.1 Evolutionary Rate of a Single Character
Ziegler (1966) studied the range of variation in the rib strength (the ratio of rib height to rib width) within a
lineage in Wales during the last ten of fossil silurian branchiopod of the genus Eocoeloa million years. The
estimation of time was inferred
from radioactive dating of rocks . The evolution of this character was noticed to be most rapid between 413
and 412 million years ago. Changes in other morphological characters were not taken into consideration.
The following observations represent a gradually changing sequence of ancestral and descendant
populations in a single lineage . These populations can be divided into four morphospecies:

1062. 50 6 [i] Evolutionary Biology
(a) E. hemisphaerica between 413-415 million years
(b) E. intermedia 412-4 13 million years
(c) E. curtisi 406-412 million years
(d) E. sulcata 405-406 million years
r_ii'i-'1.1,.' Approaches to Estimation of Evolutionary Rate by Using Fos s il Records
A. Morphological Change
1. Single Character Rate The rate of morphological change of one character which mayor may not
correspond with rates of other characters.
2. Complex Character Rate Rate of evolution of any number of characters, the rate of increased differences
corresponds (approximates) to the avera ge rate of morphological change within a lineage.
B. Taxonomic Change
1 . Time Phyletic Rate Rate of origin of morphological difference, sufficiently distinct to merit recognition of
new species within an evolving lineage.
2. Group Rate
(a) Appearance
(b) Duration frequencies (c) Turn over rate
Rate of origin of new taxa.
Distribution of average life of taxa.
An average of both appearance and disappearance rate of taxa.
20.13.2 Evolutionary Rate of Character Complexes
Westoll (1949) studied morp hological changes in 21 characters of lungfishes. The primitive condition of
each character is taken as a standard and each character is scored according to how closely it matches the
standard. The total score of all the characters for any given lungfish is a measure of the degree of change
from the standard. When total scores of a series of fossil lungfishes are plotted against time it gives the rate
of changes of the whole complexes of characters.
Fig . 20.12 illustrates the degree of departure of lungfishes from the primitive standard in a time series. For
body characters, the rate of change is low at first (the line being horizontal to the base line). It greatly
accelerates in the late Devonian, but reduc es again in Carboniferous and remains low until today.
Characters of the head changed rapidly at first nearly completing the phase of rapid evolution by the late
Devonian, after wh ich it slowed down. The difference in the rate of evolution of the head and body is
because the diet of these animals shifted first from a biting to grinding type and influenced their head
character. This later influenced the swimming habits from swift to sluggish type. The above group shows
that:
(a) The evolutionary rates vary from lineage to lineage within a given lineage and from time to time.

1064. 100
90
80
70
l!?
CD
ti 60
~
l'll
50
s:
o
CD
40
~
·E
30 >
it "0
20
o co
10
0
------
Fig. 20.12: Morphological evolution of character complexes in eighteen fossils and three living lungfishes.
The curves indicate the rate of disappearance of primitive character complexes for head, body and both
combined: A. Dipnorhynchus; B. Dipterus; C. Pentiandia; D. Scaumenacia; E. Fleurantia; F.
Phaneropleuron; G. Uronemus; H. Ctenodus; I. Sagenodus; J . Conchopoma; K. Ceratodus; L.
Epiceratodus; M. Protopterus and N. Lepidosiren.
(b) The evo lutionary rates vary among different characters or character complexes within the same lineage
at a given time.
(c) From the above facts it can be concluded that selection acts differently on different functions. This is
called mosaic evolution.
20.13.3 Evolutionary Rate of Taxonomic Group
The rate of taxono mic change can be measured as the rate of change of taxa within a clade or of the
number of taxa belonging to a clade at any time. The duration of taxa may be taken as an estimate of
evolutionary rates. The shorter the duration, the faster is the evolutionary rate. Such estimates may be made
at any taxonomic level. However, its use at species level is not possible because of the poor fossil records at
this level. This is commo nly used at the level of genera or families or other taxonomic levels.
20.13.4 Time Phyletic Rate
Species can disappear e ither by extinction or by evolving into a descendent species. The appearance of
new forms by evolution within a single lineage through specific time causes pseudodiversification and the
disappearance or extinction of some species evo lving into a descendent spec ies pseudoextinction. At fami
ly level,

1066. pseudoexti nctions account for about 5 percent of all extinctions and never more than 20 percent of
extinctions. The pseudodiversifications have the same rate. This sort
508 ~ Evolutionary Biology
of p seudodiversification or pseudo extinction rate is called time phyletic rate. For example , Fig. 20.11
shows the approximate pseudod iversification i.e., the total number of diverse group s of placental mammal
s present at a specific time. The diversity varies through time. There are eight more or less successive
genera in equid evolution from Hy racotherium to Equus during 60 million years. It gives an approx imate
average rate of about 0. 13 genera per million years.
20.13.5 Types of Evolutionary Rates Simpson has recognised three types of evo lutionary rates:
I. Horotelic Rates: Horotelic rates have the average distribution of dura tion frequen cies within group s.
See the curve showing duration frequency in Fig. 20.12. The horo telic rates may include both rapidly and
slowly evolving lineages. These represent the usual pace of evo lution within an adaptive zone includ ing
faster and slower episodes as evolutionary opportunities.
2. Bradytelic Rates: The se are exce edingly slow and are detected wh en slow ly ev o lvi ng lin ea ge s
exceed the number expected on the basis of horotelic distributions. These lineages survive long. Bradytely is
achieved by lineage s whose adap tive zones happen to endure indefini tely, and are flexibly adapt ed.
3. Tachytelic Rates: These include rapid evolutionary changes, faster than the horotelic rates. These rates
are observed in forms or group s with first invasions of
5
.
?:' 40
~
Q)
is 20
60 40 20 o Millions of years
FIG. 20.13: Rates of appearance (broken line) of families of terrestrial placental mammals and their
diversification (solid lines) during Coenozoic Era. TIme phyletic rate for mammals is about six families per
million years.
rJl K
rJl
KK
to Q)
'0
:::l Q)
~ 1 ~ 1
.0
Q; E.0 :::lE Z:::l
Time ~ Time ~
Z
A B
FIG. 20.14: A. The equilibrium model of population density; B. The equilibrium model of species density. K
represents equilibrium population size and KK- equilibrium diversity.

1068. new adaptive zones or these represent evolutionary acceleration along phyletic lines, i.e., tachytelic rates
are exhibited in megaevolution.
• Allometry
• Coprolite
• Horotelic rate
• Metamorphic rocks
• Petrifaction
• Sedimentary rocks
KEY TERMS
• Bradytelic rate
• Cope's law
• Igneous rocks
• Palaeontology
• Phyletic rate
• Subfossils
• Bioclasts
• Gastroliths
• Microfossils
• Pseudofossils
• Radioactive clock
• Tachytelic rate
REVIEW QUESTIONS I. Discuss different types of evolutionary rates.
2. What is phyletic rate of evolution? Explain the terms pseudo-diversification and pseudo-extinction.
3. Differentiate between the terms horotelic , bradytelic and tachytelic rate of evolution.
4. Explain, how fossil records can be used for estimating evolutionary rates.
5. Write short notes on the following :
(a) Evolutionary rate of a single character
(b) Evolutionary rate of character complex
(c) Time phyletic rates
(d) Types of evolutionary rates.
6. What are rocks? Give an account of different types of rocks.
7. What are fossils? How are these formed in nature? Name any two fossils and mention in each case the
rock and the class to which these belong.
8. Write an essay on the significance of fossils.
9. Write an essay on fossils.
10. What is fossil? Describe the processes of fossilisation. Enumerate the methods for determination of the
age of fossil.
II. How are the fossils formed? With the help of fossils you have studied, discuss their importance in
understanding organic evolution .
12. Define fossils. How are they formed? Comment on their significance . 13. Describe the various types of
fossils and discuss their significance In biological studies .
14. Write short notes :
I. Petrifaction 2. Histometabasis
3. Pseudofossils 4. Cope's rule
5. Williston's rule 6. Allometry.

1070. DOD

1071. 21
Origin and Evolution of Horse
The modern horses belong to the order Peris sodactyla, suborder Hippomorpha, family Equidae and the
genus Equus. The specific name of our domestic horse is Equus cabalus. The family Equideae is about 60
million years old.
21.1 PLACE AND TIME OF ORIGIN
--
The histo ry of evolution of horse dates back to the beginning of Eocene Period of Coenozoic Era and
covers a period of abou t sixty million years . The primary centre of their evolution was Great Plains region
in North America, from where some of the specie s have spread out to Europe and Asia from time to time.
But the complete sequence of evolutionary events as found in North America is not found in Europe and
Asia. The first known fossil of horse from Europe was Hyracotherium and the contemporary fossil of horse
from America was Eohippus. By the close of Pleistocene Period horses in North America became extinct
due to reasons unknown, but those who migrated to Old World survived and flourished.
21.2 EVOLUTIONARY TRENDS
The fir st known ancestors of horse were fox-like animals living on moist ground and browsing soft leafy
vegetation. With the change in climatic and physical conditions of the surroundings and the nature of food,
they were exposed to various challenges and passed through different evolutionary phases evolving new
features for better adaptability to the changing conditions. The history of evolution of horse presents
definite changes in certain organs in a particular direction. These directional changes are known as
evolutionary trends. The following evolutionary trends are exhibited in the phylogeny of horse :
21.2 .1 Changes in Limbs
1. Overall increase in size
2. Lengthening of the limbs
Origin and Evolution of Horse ~ 511
3. Gradual enlargement and elongation of third digit
4. Perfection of hoof on third digit
5. Reduction in the number of digits from 4 or 5 digits to one in each foot
6. Development of spring mechani sm
7. Change of foot posture from plantigrade to unguligrade
21.2.2 Changes in the Skull and Teeth
8. Elongation of the preorbit al region of the skulls.
9. Loss of canines.
10 . Development of high crown s on the molars and premolars. Their conversion into continuously growing
grinders which are more suitable for grazing. Thus the brachydont dentition was replaced by hypsodont
dentition.
Interosseus medius muscle
and hoof.
and hoof.

1072. Interosseus tendon
and hoof.
v IV III
Fetlock joint
Sesamoid bone
Sesamo id
ligament
Tendon of flexor profundus
muscle of digit { Hoof
FIG. 21.1: A. Forelimbs of Hyracotherium and B. Equus showing difference in the number of digits , size of
ligament, spring mechanism and hoof.

1074. Maxilla
(upper jaw)
and hoof. and hoof. and hoof. and hoof.
and hoof.
Crown
and hoof.
Lower Molars
of Lower Tooth Canines
FIG. 21.2: Teeth of modern horse. Note the high crowns on molars and premolars.
II . Premolars becoming molariform (molar-like in appearance).
12. Appearance and increase of diastema, a space between incisors and grinding teeth
21.2.3 Changes in Brain
13. Increase in the size and complexity of cerebral hemispheres and cerebellum 14. Olfactory lobes hidden
under cerebrum.
21.2.4 Change in Neck
15. Elongation of neck to enable horse for grazing on ground grass .
21.3 CHARACTERISTICS OF MODERN HORSE
I . Neck and head long and slender to minimise the resistance during running
2. Skull large and brain box spacious
3. Incisors long and crowned, adapted for chopping and first premolar reduced
4. Canine absent; diastema long
5. Premolars molar-like with deep crowns and adapted for grinding
6. Feet long and one-toed, i.e., unguligrade
7. Brain well developed with a higher sense of intelligence
8. Radius and tibia are long while the humerus and femur are short
9. Ulna is fused with radius and fibula is a splint bone
10. Size of modem horse is 6' and 4" at shoulder
II. The back is arched
21.4 PHYLOGENY
21.4.1 Horses in Eocene
1. Hyracotherium and Eohippus: Evidences indicate that modem horses have descended from order
Condylarthra, an order of five-toed hoofed mammals or ungulates, which is now represented only by extinct
forms. The first known horselike animal which forms the starting point in the Equine evolution is
Hyracotherium from London Clay and was the old world horse. Its contemporary new world horse was
Eohippus. Its fossils were obtained from Big Hom Basin in North America. It is commonly known as the
"dawn horse". Cooper (1932) has suggested that Hyracotherium and Eohippus were very similar to each
other and must be called Hyracotherium.
The dawn horse appeared in the beginn ing of Eocene Period and its remain s are abundant in the deposits
in Wyoming . Although horse-like, it was very much different from our modem horse. It was a small ,
browsing animal of the size of fox-terrier.
Origin and Evolution of Horse [i] 513
Its skull and neck were short . The back was arched and flexible. The ulna and fibula were stout and well-
built and were free from radiu s and tibi a respecti vel y. Th e hindlimbs we re moderately longer. The limbs
were digitigrad e. The forelimbs possessed four digit s (2, 3, 4 and 5th) and hindlimbs possessed three
digits. The first and fifth digits in the hindlimbs were represented by splints. The third toe in each limb

1075. indicated the incipient sign of predominance. All the toes touched the ground and were supported by a pad.
Tiny hoofs were present on each digit in incipient stage . The characteristic springing mechanism was
absent.
Th e preorbital portion of skull was not elongated so that the orbits were placed midwa y measuring from
front to rear. Dentiti on was brachydont. The total numb er of teeth was 44. The cheek teeth (premolars and
molars) were moderately specialised for grinding, i.e., these were with low crowns and pronged roots and
their surfaces were covered by round ed tubercle s or cusps of enamel. The cerebral hemispheres were small
and smooth, and they neither covered the olfactory
II IV
Hoof
IIIIII
FIG . 21.3 : Metacarpal and metatarsal bones of (A) Forelimb and (B) Hind- limb respectively in
Eohippus.
bulb s nor the midbrain.

1077. Olfactory bulb
Hyracotherium
Olfac tory bulbs hidden
Cerebrum (Smooth surface)
Convoluted
surfaceof -N~~;::;;j9t~~-1J
cerebrum
Cerebellum
Mesohippus
Equus FIG. 21.4: Diagram showing the relative increase in the size and complexity of brain in horse .
H yracotherium was a forest-dweller and used to brow se on soft vegetation. It was better adapted to walk
on the soft floor of the forest and could escape its enemies by hiding.
Sev eral lines of descent differing in few detail s rediated from Hyracoth erium and flourished throughout
Eocene Period but became extinct in early Oligocene in Europe and North America. Some other forms
which replaced Hy racotherium are Orohippus and Epihippus.
2 . Orohippus (Mountain Horse): Its fossils were recovered from Bridger Beds, New Mexico in the middle of
Eocene Period . This horse was a little higher than Eohippus. Its forelimb s retained four toes and the
hindlimbs had three toes but tiny vestiges of the two toes in the hindlimbs were lost. The middle digit in both
the limbs was prominant. The third and fourth premolars exhibited a trend towards molarisat ion, but
Orohippus was still a browser.
3. Epihippus: It was slightly larger than Orohippus. The forelimbs were with four and hindlimbs with three
digits . The vestiges were totally absent. Last two premolars were molar-like in appearance. It was still a
browser residing in jungles. These forms became extinct by the end of Eocene and were replaced by
Mesohippus.
21.4.2 Horses in Oligocene Period
Although there were several lines of descent of horses in Oligocene Period , but the one leading to equine
evolution is represented by Mesohippus and Miohippus in Oligo cene.
1. Mesohippus (Intermediate Horse): Meso hipp us appeared in Oligoc ene Period and was of the size of
sheep. It measured from 18-24 inches in height. The neck was short and less flexible than the earlier forms.
The trunk was long and slender
Calcaneum and back was more arched . The legs were
lon g and slender and the number of toes in both fore limbs and hindlimbs was reduced to three. But fifth
digit in forelimbs was represented by a splin t. All the three digits touched the ground but the middl e one
was larger to bear most of the body weight. Both fore as well as hind limbs exhibited lengthening tendency
due to the elongation of metacarpals and metatarsals. The ulna and fibula, although distinct, were slender.
The ligament s were slightly better develop ed as compared to those in Hyracotherium. The feet had pad
under the toes showing beginning of hoof.
The preorbital portion of the skull exhi- bited the tendency of elongation and as a
Astragalus Carpals
Tarsals

1078. Metatarsals
Metacarpals
Hoof
FIG. 21.5: Metacarpal and metatarsal bones of forelimb and hindlimb respectively in Mesohippus.
Restoration Fore-foot Dentition Skull & bra in
CD c
CD o
.9 en "iii
a::

1079. ~~
o J
Large
J <.:)
cerebral
~
hemispheres
Equus One-toed Hypsodont
'
CD c
CD o .Q
a:: one-toed Hypsodont
-..
Merychippus Three-toed Brachydont
CD
cs
o
.~
o M
esohippus Three-toed Brachydont
@
~
Diastema
CD c
2l
o w
~
~ Skull B
rain
Hyrac otheri um Four-toed Brachydont
FIG . 21.6 : Diagrammatic representation of the ancestry of horse.
r esult a space appeared between the incisors and cheek teeth. This space is calIed diastema. The dentition
was stilI brachydont, i.e., molar s were stilI low crown ed but the last two premolars were molariform. The
structure of teeth and feet was suggestive of their forest dwelIing habit.
Th e brain of Meso hipp us exhibited marked features of advance over the brain of Hyra cotherium. The
oppo ssum-like configuration of brain was lost. The cerebral hemispheres were enlarged and convoluted
and brain had the appe arance of horse brain.
2. Miohippus: In the late Oligocene, Mesohippus was replaced by another slightly more advanced horse-
like form, named Miohippus. It was much like Mesohippus

1081. 516 iii Evolutionary Biology
in appearance but somewhat large in size. Although, both forelimbs as well as hindlimbs were three-toed
but the toes were broad and spreading. This indicates that Miohippus was also a forest dweller. The cannon
bone (the greatly elongated and fused metacarpal or metatarsal bones in the limbs of hoofed mammals) in
both the limbs was in contact with the outer ankle bone . The teeth were still low crowned and it was still a
browser.
21.4.3 Horses in Miocene
Miohippu s appeared to be the ancestor for several equine forms which radiated from it in the Miocene, and
early Pliocene Period. The closest descendents were Anchitherium, Hypohippus and Megahippus. They
were three-toed browsers living in forest and represented an enlarged version of Miohippus. They appeared
in North America but soon migrated to Old World (Europe and Asia). They thrived in Miocene and Pliocene
Periods and became extinct without any descendent. These three forms represented a gradual increase in
body size and modifications of cheek teeth.
The three-toed horses which were on the direct line of descent of Equus are represented by Parahippus and
Merychippus in Miocene Period.
1. Parahippus: It was on the direct line of equine evolution, who descended from Miohippus in early
Miocene Period . Its legs were still three-toed but exhibited marked elongation. The third digit in fore and
hind limbs became more predominant than in its ance stors . Although, all the three digit s touched the
ground, only the median one was effective in locomotion. The side toes were slender.
The snout was considerably elongated so that the orbits were shifted behind and lied close to the occiput.
Premolars were molar-like and formed the dental battery. The premolars and molars bore low crowns but
the molars of upper jaw changed into grinders by the deposition of cement between the cusps. The dentition
in Parahippus was of hypsodont type.
The fossils of various species of Parahippus presented the complete spectrum of various transitional stages
from three-toed browsers (Miohippusj to "three-toed grazers" iMerychippusv which represents the next
stage in Equine evolution.
2. Merychippus (Ruminant Horse): Merychippus lived in the middle and upper Miocene Periods and
became extinct in Pliocene Period . It represented the first three-toed grazer feeding on grass and,
therefore, marks the transition from primitive browsing horse to the modem grazing horse . It was adapted
to live on western grassy

1083. plains which appeared due to the replacement of the forest as a result of continental elevation and
comparatively drier atmosphere.
Although, Merychippus still had three toes , the side toes (2nd and 4th) were reduced and no longer touched
the ground. The central toe ended in a large convex hoof, like that of Equus. The ligaments of the muscles
were highly developed. The limb s were with well developed cannon bone and the muscles formed an
efficient spring mechanism which of course restricted the free movement but added to the speed. The foot
pad was absent. It means the feet were adapted to walk on comparatively hard ground,
Th e preorbital region was more elongated and diastema was we ll developed. The teeth were high crowned
(hypsodont) and fully cemented grinders, which were more suitable to grind the harsh siliceous vegetation
of plains. But the milk teeth were low crowned and uncemented. The cerebral hemispheres were large and
convoluted with fundamental pattern of fissures , resemb ling that of modem horse.
Me rychippus marked the completion of transition from browsing to grazing . The next form which evolved
from Me rychippus in late Miocene and became contemporary to it was Protohippus . It was mor e or less
like Me rychippus but its cheek teeth were high crowned and cemented and adapted for grazing .
21.4.4 Horses in Pliocene Period The climatic conditions in Pliocene Period became
Astragalus Carpals
Tarsals Splint
Metatarsals
Metacarpals
Hoof
III III
FIG. 21.7: Metacarpale metatarsal bones of forelimb and hindlimb respectiv ely in meryehippus.
m ore drastic and land bridge s were formed resulting in the change and migration of the fauna. The
grazers became more numerous and widespread and several lines of descent evolved from Merychipp us .
The common grazers of Pliocene Period were Hipparion , Neo hipparion and Nannipus. All the three were
three-toed grazers having high crowned cheek teeth and were of the size of pony. All of them became extinct
in late Pliocene or early Pleistocene Period. The direct descendent of Mery chippus which gave rise to
Equus in Pleistocene Period , was Pliohippus, commonly known as Pliocene horse.
1. Hipparion: The fossil remains of Hipparion are found in Pliocene Period from China to Western Europe
including India, Central Asia, Greece, Spain , etc., and also in Afric a up to the Cape. They actually evolved
in North America but migrated to Old World. They were the last three-toed ancestors of horse and were of
the size of pony about 40 inche s in height. The teeth were straight but with cusps adapted for grinding. In
early Pleistocene their offshoot gave rise to Stylohipparion , but the line became extinct in upper
Pleistocene.
2 . Pliohippus (Pliocene Horse): It was the first one-toed horse whose fossil remain s are found from upper
Miocene and Pliocene Period. It is on the evolutionary line leading to Equus . Pliohippus was of the size of
modem pony about 40 inches in height. The most striking change was the reduction of the side toes, 2nd and
4th. Some species had tiny side toes, while in others these were represented by splint bone s as in Equus.
The crowns of the upper molar s were similar to those of modem horse, but the pattern of ridges was not so
elaborate. Pliohippus exhibited trend for the increase in size, increase in preorbital length of skull and
increase in the size and complexity of molar teeth . Pliohippus was succeeded by the members of modem
horse (genus Equu s).

1085. Period South America North America OldWor1d Recent Pleistoce ne
Pliocene
Miocene
Oligocene
Eocene (Extinct\
(Extinct)L ~ ~__--"'EqZs
Styloh~arionEquus" 1'--.

"T' ~
(Extinct) Pliohippus
Hippadion ......

1086. \ /
. ~ ";/

1087. Hlppanon V.--+ Hypohippus
MeryChiPPUS~
Megahippus ·
(Extinct)
A~~--1 V
Ancnithetium

1088. ~~~o,\Jj
Miohippust( Mesohippus
• "
EPihiPPUS~
Oroh

1089. ...
~
ippus'".,.---.J Hyracolherium (Eohippus)
Fig. 21.8: Phylogeny of horse.
3 . Plesippus: Fossils of P/esippu s have been recovered from Texas . They are more or less like modem
horse and of the size of Arab ian horse. Their teeth had long and less curved crowns.
21.4.5 Horses in Pleistocene Equus (Modern Horse)
Equu s includes the modem horse which appeared towards the end of Pliocene Period. The transition from
Pliohippus to Equus involves further increase in size and some changes in the anatomical details. The first
representative of Equus appeared in late Pliocene Period and achie ved world-wide distribution during
Pleistocene Period. It is about 60 inches in height and has lost the first and fifth digits entirely and the
second and fourth digits are represented by splints so that the

1091. e ntire weight of the body is balanced by the third digit alone. It bears well-developed hoof. The crowns of
the molars are much elongated and possess complicated enamel ridges, adapted to feed on dry and harsh
grass. The brain is enlarged and
Tarsals
..~n--" Carpals
1---4- - Splint Bone
IV Metatarsals ______ - Metacarpals
IV
II
III
III
FIG. 21.9: Skeleton of forelimb and hindlimb of Pliohippus.
th e cerebral hemispheres have groo ved surface. Thus, the evolution of horse has resulted in the
development of an intelligent, long-legged, swift-running animal , which is suited to live and feed in open
grass lands.
Th ough North America was the stage of equine evolution, horses became extinct on this continent on the
close of Pleistocene Period due to certain unknown reasons. The speci es of horse found there are
secondarily introduced by man.
At pr e sent , true wild type of horses are present only in Asia (The Asiatic wild ass-Equus hemionus and Pr
zewalsky's horse-E. przewalskii and, in Africa, the African wild ess-E. asinus). The varieties found in North
Ameri ca are not the wild horses but the descendants of dome sticated horses brought over from Europe.
- Astragalus
Nav icular
Mesocuneiform + Entocuneiform Tarsals
Carpals Entocuneiform
Metacarpals
Phalanges
FIG. 21.10: Skeleton of forelimb and hindlimb of Equus .
21.5 SIDE LINES
In outlin e, the phylogeny of horse just presented indicates a single line of succession from earliest known
equine (Hyraco therium ) to present day horse (Equus). Long before the fossil record s were compl eted,
equine evolution was considered to represent changes in a regular progression or along a definite line. This
type of evolution is known as orthogenetic evolution or orthogen esis (Gr. ortho , straight and genesis,
production). But this is an over simplification of the concept of how evolution of horse occurred.
Actually , at each level from Hyracoth erium to Equus, adaptive radiation had occurred and numerous
horse like forms evo lved, thrived and became extinct except for Equus. Miohippus, for examp le, was
ancestral not only to Merychippus, but also to Anchitherium and Hypohippus (The three-toed browsing,
forest horse and to three other lineages. Similarly, Merychippus was ance stor to Hipparion , Protohippus
and Pliohippus. Of them only Pliohippus gave rise to Equu s and others became extinct. Even from
Pliohippus, a number of forms, like Equu s, Hippidion and Onohippidium diverged and underwent indep
end ent adaptive radiation. Thi s sugges ts that the process of evolution is chiefly based on adaptive modific
ation s. Durin g evolution changes appear in various direction s but the evolutionary shift continues in one
direction as long as it introdu ces an impro ved adaptation. This is called directional evo lution.

1093. KEY TERMS
• Brachydont • Diastema
• Orthogenesis • Plantigrade
• Unguligrade
• Hypsodont
• Spring mechanism
REVIEW QUESTIONS
I . Trace the palaeontological history of modem horse.
2. Write an account of the fossil history of horse.
3. Give an account of ancestry of horse.
4. Describe the evolution of horse.
5. What are the evolutionary trends exhibited in the phylogeny of horse?
6. Write the characteristics of modem horse.
7. Show the phylogeny of horse by a chart.
8. Summarise the evolutionary trends exhibited in the evolution of horse.
9. (a) Comment on the factors responsib le for evolution of horse. (b) Describe fossils of horses during
Pliocene Period.
10. What is directional evolution?
II . Write short notes on the following :
(a) Equus
(c) Eohippus
(e) Mesohippus
(g) Parahippu s
(b) Orthogenesis
(d) Orohippu s
(f) Miohippus
(h) Merychippu s.

1095. DOD

1096. 22
Origin and Evolution of Man
22.1 INTRODUCTION
Human species is the outcome of the same natural evolutionary process that has shaped all other animals
or plants. Humans are the most recent arrival on the Earth. They represent the latest adaptive advancement
in the animal kingdom, and occupy a unique position in the living world. The course of human phylogeny
can be followed only from the fossil records. Though the fossil records are fragmentary, the
palaeoanthropologists have been able to piece them together and draw an almost complete phylogeny of
primates and of modem man , Homo sapiens sapiens.
The early stages of human evolution have been studied by comparative anatomy of fossils and also by the
comparative biochemistry of present day humans, apes and other primates. Information on the later stages
in human evolution are based on artifacts that include stone tools, pottery, fire-hearths and the fossils of
other animals along with human fossils .
22.2 SCIENTISTS ASSOCIATED WITH HUMAN EVOLUTION
• Isaac de Ia Payrere, a Frenchman, made a study of large collection of chipped stones, gathered in French
countryside in 17th century. His book was publically burnt in 1655.
• Johann Friedrich Eper in 1771 found human bones associated with the remains of extinct cave bears at a
site in Germany.
• John Frere in 1790 found unfamiliar stone tools, hand axes and extinct animals from the same beds at
Hoxne in England.
• P. C. Schmerling in 1830 found many stone artifacts along with bones of rhinoceroses and mammoths and
two human skulls from Belgian caves. These finds went unnoticed.

1098. • Jacques Boucher de Perthes, a French custom s official, interested in archaeology, found flint objects and
tools like axes in the gravel banks near Abbeville in Northern France.
• Scandinavian archaeologists, led by Christian Jurgensen Thomsen (1788-1865) and Jens Jacob Asmussen
Worsaae (1821- 85) proposed that excavations in Denmark and throughout Europe could be organised
chronologically into three successive period s: Stone age, Bronze age and Iron age.
• English palaeontologist, William Buckland (1784-1856) in 1823, discovered a human skeleton in a cave.
• J . McEnergy, a catholic priest, in 1829 found flint tools and bone s of ancient animals below a cave floor
on the South Coast of England.
The di scoveries of all the above persons went unnoticed.
• Lubbock suggested that the stone age period needs to be divided into two distinct period s: the
Palaeolithic (when man used crudely chipped flint implement s) and the Neolithic (when man developed
finely crafted tools).
• Charles Lyell (1830-1833) studied tool-rich layers in Sommee Valley and suggested them to be at least
1,00,000 years old.He wrote the book 'Geological Evidences for the Antiquity of Man'.
• Thomas Henry Huxley in 1863 pub lished a book 'Man's Place in Nature' and established that
evolutionary development of men and apes has taken place simultaneously.
• Charles Darwin in 1871 wrote 'The Descent of Man' and explained the origin of man from apes .
• Eugene Dubois, a Dutch doctor, in 1880s obtained fossils of skull cap , lower jaw fragment and thigh
bones of Java man in the East Indies .
• Otto Schoetesack in 1907 published the account of a fossil human jaw obtained from a quarry. It was
found to belon g to Homo heidelbergensis.
• Several fossils of Neanderthal skeleton were discovered from some French village.
• Charles Dawson, a lawyer and amateur palaeontologist, excavated in 1908 and 1912 a cran ium similar
to modem man and a part of ape-like lower jaw from a gravel pit at Piltdo wn in Sussex, England. It was
named Eoanthropus dawsoni or Piltdown man. In 1953, it was shown to be a hoax.
• Raymond A. Dart in 1920s obtai ned a fossil skull of a child Taung from South Africa. It was fossil of
Australopithecus africanus and declared it to be human ancestor.
• Davidson Black in 1927 discovered skull jaws, few limb bones and some artifacts of Peking man from a
Great Cave near Beijing in China. It was named Sinanthropus p ekin ensis.
• Gustav Heinrich Ralph von Koenigswald, a German palaeontologist, in 1930s discovered a number of
fossils of Pithacanthropus.

1100. Origin and Evolution of Man ~ 523
• Robert Broom and his student John T. Robinson collected a number of fossils of australopithecines from
caves in South Africa during 1930s and I940s. They argued that australopithecines were the direct
ancestors of modem humans.
• Franz Weidenreich in 1930s studied fossils of Peking man (Homo pekinensis) in China.
• George E. Lewis in 1934 discovered fragments of upper jaw of Ramapithecus from Sivalik Hills in India.
• L. S. B. Leakey and Mary Leakey in 1930s described fossils from excavations at Olduvai Gorge near Lake
Victoria, Kenya, East Africa. These fossils were identified as Zinjanthropus boisei and Homo habilis.
• White et al. and Wood in 1994 discovered fossils of Ardipithecus ramidus and Ardipithecus kadabba from
East Africa.
• Senut et al. in 200 I obtained fossils of another ancestor common to man and chimpanzee from western
Kenya . It was named Orriorin tugenensis.
• Brunet et al. and Wood in 2002 discovered fossils of very first common ancestor of man and chimpanzee.
This is named Sahelanthropus tchadensis. The fossils were obtained from Chad in Central Africa.
22.2 TIME OF ORIGIN OF PRIMATES AND MAN
According to palaentological records, evolution of primates started about 75 Mya (million years ago)
during Late Cretaceous from some tiny arboreal, insectivorous, shrew-like mammal. Genetic studies show
that primates diverged about 85 Mya. They lived in luxurious forests during Eocene Period of Coenozoic
Era. About 55 Mya, primate lineage split into Prosomian and Anthropoid stocks.
In Late Oligocene, about 30-25 Mya, when evergreen forests were replaced by dry Savannah grasslands,
ancestors of modem apes and human separated from anthropoid apes. Some of these tree dwellers came
down to the ground and-adopted bipedal habit. The common ancestor of man and apes appeared about 20
Mya in Africa. Around 14 or 15 Mya Orangutans diverged from Hominidae family. The earliest Hominin
(Hominids) were Sahelanthropus or Orriorin. Ardipithecus and Australopithecus appeared about 7 mi11ion
years ago. According to primate molecular clock, some time between 6 and 7 Mya two offshoots diverged
from a common primate ancestor. They developed into Chimpanzee line and human line.
Humanisation (i.e. appearance of genus Homo) started about 4 to 5 Mya. The earliest fossils of genus
Homo were Homo habilis. They lived about 2.3 Mya in Tanzania and coexisted with australopithecines.
Genus Homo erectus appeared around 1.5 to 1.0 Mya. Three Hominin species coexisted approximately
600,000 years ago, namely Homo heidelbergensis, Homo neanderthalensis and Homo sapiens.
The oldest fossils of Homo sapiens were obtained from Ethiopia. They were 160,000 to 195,000 years old .
Cromagnon man belonging to Homo sapiens replaced heidelberg and neanderthals about 35,000 or 40,000
years ago.

1102. 524 IiJ Evolutionary Biology
22.4 PLACE OF ORIGIN OF MAN
T he fossi ls of prehum an and ancestral human forms have been obtained from widely diverse regions of
Africa, Asia and Europe which indicate that man's centre of origin was probably in Afric a and Asia. More
precisely man has originated in Africa and migrated to central Asia, becuase:
• The oldest known hominin fossils are obtained from Africa , Asia, China, Java and India (SivaIik Hills).
• The number of domesticated animals and plants is maximum in Asia.
• A number of migration s of animals have occurred in the past from Africa to Asia, Europe and America.

1104. ~;-cffI!e:=....-,"'"""-;$"'7' Earliest modern human (Homo sapiens)
• Australopithecus fossil discoveries o Paranthropus fossil discoveries
• Australopithecus and ardipithecus fossil discoveries
Ind ian Ocean
FIG. 22.1: Homi nin fossil sites in Africa.
Origin and Evolution of Man Ii] 525
• Asian culture appears to be the oldest.
• Climatic condi tions in Africa and Asia and nearby places were most conducive for human evolution .
• Rich fossil beds are discovered from Rift Valley in East Africa, where Hominid fossils have been found.
These areas are: OIduvai Gorge, Lake Victoria and Lake Natron in Tanzania and Lake Turkana in Kenya .
22.5 PRIMATE HERITAGE
Modem man , Homo sapiens sapiens. belongs to class Mammalia, order Primates and suborder
Anthropoidea. Primates is a large and varied group . It includes tree shrews, lemurs, lorises, tarsiers,
monkeys, apes and man. All of them are basically adapted for arboreal life and have certai n features in
commo n such as binocular vision, manual dexterity, sensitivity, brain size and upright posture, etc.
22.6 SPECIAL FEATURES OF PRIMATES
T he special features of primates are:
1. Binocular and Stereoscopic Vision: In primates, eyes are located on the front
of the head, both looking forward so that these can focus on the same objec t from
slightly different angles.
The evolution of binoc ular visio n occurred in early development of arboreal
primates in Palaeocene Period. This permitted stereoscopic vision, accurate depth
perception and judgeme nt of distance while j umpi ng from branch to branch. This
also enable d them to attack accura tely and capture highly mobile prey. 2. Visual Acuity: The retina in the
eyes of primates, in addition to rods, has cones
for colour vision and greater visua l acuity. The cones are specia lly concen trated at
IM:I."!I Characteristics of Primates
1 . Opposable thumb with grip for power and precision
2. Hands can rotate at 180·
3. Eyes forwardly directed, lying together on the face with parallel optical areas.
4. Increased number of rods and cones with their own nerve cells
5. Increased sensory and motor areas, deeply fissured large brain
6. Cerebrum expanded; foramen magnum ventral
7. Reduced snout, flattened face
8. Longer gestation period and increased parental care
9. Living in groups, corporate activities
10. Replacement of claws by nails at the tips of digits

1106. ---+ Grasping limbs
---+ Rotating forelimbs
---+ Stereoscopic vision
---+ Visual acuity
---+ More intelligence
---+ Skull enlarged
---+ Reduced olfaction
---+ Few offspring
---+ Social dependency
---+ Manual dexterity
526 !il Evolutionary Biology
a particular point, the central area called fovea. It is the point of clearest vision. The light rays are focused
on this point to form the clearest image.
3 . Manual Dexterity and Sensitivity: The evolution of binocular vision led to manual dexterity. The primate
s could use their hands for various activites other than just for walking.
4. Grasping Limbs: In primate s, the thumb or big finger is opposable to rest the fingers, turning the
forelimb s into wonderful grasping organs. These can have a grip of tree branch es while climbing and can
pick up objects and tools.
5 . Rotating Forelimbs: In primates, hands can rotate at 180
0
because radius and ulna bones of arm can
twist about each other on the wrist. Additional flexibility comes from the ability of upper arm to rotate in
the shoulder socket (glenoid cavity) enabling them to swing on tree branche s.
Primate evolution was linked to grasping hands and binocular vision which led to large brain and bipedal
locomation.
6. Replacement of Claws by Nails: In more advanced primates the tips of digits have flat nails instead of
claws. This exposes bare surface at the end of digits rich in nerve endings. This provides manual dexterity.
7 . Large Brain: The visual centre s in the brain are expanded in accordance with the visual advancement.
The sensory and motor areas in the cerebrum are extensively developed and the surface of cerebrum is
deeply fissured . This helps in accurate control of sensitive hand 's manipulation and faculty of thinking,
reasoning and analysis.
8. Enlarged Skull: To accommodate expanded cerebrum, the cranial cavity is enlarged . The cranial part of
skull is enlarged and the facial part is shortened. The foramen Magnum is shifted ventrally.
9. Reduced Olfactory Sense: Sense of smell is poorly developed and accordingly the olfactory lobes of the
brain are reduced in primate brain.
10. Fewer Offspring: Primates give birth to fewer young ones and their young ones are dependent on
parents for long, having longer gestation period. Long period of adult and child interaction has helped in
cultural evolution .
11. Social Dependency: Living in groups has led to the evolution of socia l interacti on and corporate
activities .
22.7 EVOLUTION AND ADAPTIVE RADIATION IN PRIMATES

1107. Primate s are presumed to have descended from some small arboreal insectivorous mamma ls in late
Cretacesous Period about 75 million years ago (Mya) . They were small shrew-like mammals with small
face and long tail. They lived in luxurious forests. Early in their evolut ionary history, about 55 Mya, the
primate lineage split into two main branches leading to the two present day suborders, Prosom ii and
Anthropoide a.

1109. Lemurs J\Prosimians
Lorises Insectivorous
Arboreal
Anthro
poids
98 66 55 36 24 5
'-------y-----''-----v--'~'-------y-----'~
Late Paleocene Eocene Oligocene Miocene Cretaceous Millions of years ago (mya)
FIG. 22.2: Phylogenetic tree of primates: Primates evolved from ancestral arboreal insectivorous
mammalian stock some 75 Mya. Anthropoids diverged from primates about
55 Mya. New and old world monkeys evolved as separate lineages about 20 Mya. Lineages leading to
African apes and humans branched off about 7 Mya.
Prosomians (before monkeys) continued to follow arboreal and nocturnal habit. They evolved into lemurs
and loris.
• Anthropoids (human-like) evolved into following four groups in Africa and Asia: 1. Tarsiers: They
originated about 55 Mya.
2. New World Monkeys: Th ey are found in Central and South America . They include marmo sets and
spider monkeys. They are with a long and prehen sile tail.
3. Old Wor ld Monkeys: They lived in Africa and Tropical Asia. Example are baboons and monkeys. They
are arboreal but without prehen sile tail.
4. Hominoids: Apes (Gorilla, chimpanzee, orangutan) and man. Man resembles old world monkeys and
apes and differs from new world monke ys in following features:
• Similar to old world monkeys, man has narrow nose with nostrils close together.
• It has two premolar teeth instead of three on each side in both jaws.
• It has a bony canal connecting external ear with middle ear.
• Man and apes are without a tail. In some old world monkeys the tail is reduced. New world monkeys have
long and prehensile tail.
Between 20-30 million years ago ancesto rs of apes and man were spread out in tropical forests of Africa ,
Asia and Europe. Around 18 Mya, a group of primates from Africa diverged and became ancesto r to
hominids (human and apes). They are now called Dryopithecines . The diversification occurred due to
splitting up of woodlands into isolated grasslands. These further evolved into African apes and humans.
22.8 COMPELLING CAUSES OF EVOLUTION OF MAN
Accordin g to the distinguished anthropo logist, Sherwood Washburn, divergence of human and apes from
their primate ancesto rs must have occurred after the development of brachiation because they still retained
certa in brachiating characters such as broad trunk, flexible arms and strong collar bone. What led to their
return to ground must be the climatic change s and increased competition among tree dweller due to
expandin g populations and shrink ing forests.

1110. 1. Shrinkage of Forests due to Arid Climate: 18 million years ago, the Pliocene Period was characterised
by continental elevation and consequently increased aridity of climate. With this increased aridity, heat and
tempering of tropical conditions, the forests dwindled and shrinked away towards the equator. These were
replaced by widely spread Savanna grassl ands. Dart presum ed that the gradual shrinking of forest and
reduction in the number of trees comp elled tree-dwelling forms to get down to the ground.
2. Expanding Ape Population: During Miocene , populations of monkeys and apes increased manifo ld in
African forests. Due to popu lation pressure, apes like dryopithe cines from the forest fringes migrated to
Savanna grasslands.
Onc e on the ground, they rapidly acquired such adaptations as were necessary to ensure survival in the
new habitat. The adaptations included prolonged childhood; retarded maturity of the skull and hence
greater increase of the brain ; and finally the adaptation to live in open count ry. These changes framed out
individuals which are called humans.
22 .9 IMPACT OF THE DESCENT FROM TREES ON
PRIMATE ORGANISATION
The de scent from trees introduced following changes in the organisation during human evolution:
• Assumption of erect progre ssion.
• Liberation of hands from their ancient locomotory function to become organs of mind, i.e., to be used for
holding , grasping and manipulating.
• Increased intelligentia.
• Shortening of snout to provide unimpeded view by frontal eyes.
• Hunting for food (dwindling of forests minim ised the availa bility of food and necessitated the search for
food sources).
• Need of clothing particularly during winter.
• Freedom from climatic restrictions, assuming omnivorous diet, clothing and use of fire led to their
dispersal to new habitats and origin of new races.
• The development of communal life.
22.10 EVOLUTIONARY TRENDS DURING HUMAN EVOLUTION
From extensive comparisons of DNA sequences, it is now well established that humans are most closely
related to chimpanzees and that our next nearest living relatives are gorillas. But humans are the only great
ape that is fully bipedal. They have evolved simultaneously from some common ancestral stock for quite
some time. After this human ancestors assumed bipedal gait and upright posture, while apes still remained
knuckle walkers. During their evolution, humans acquired following special features:
22.10.1 Bipedal Locomotion
Humans walk exclusively on hindlegs. The forelimbs or hands are freed from locomotion and are used for
various other functions, such as grasping, manipulation of tools and collecting and carrying food while
walking and carrying offspring.
Whether bipedal gait freed forelimbs completely to be used as hands or use of hands for other purposes
compelled man's ancestors to assume bipedal stance need to be answered. Day (1986) suggested that
bipedalism was influenced and enhanced by need for improvement in food acquisition, predator avoidance,
warning display, reproductive success and energy saving.

1111. 1. Improved Food Acquisition: Early hominins lived in dry grasslands of Savanna that provided seasonal
food supplies. This compelled our ancestors for an omnivorous diet, food searching for more time and over
long distances. Bipedal locomotion enhanced long-distance food search and enabled them to carry the
gathered food for their family and children and for storage . Bipedalism also enabled primitive man to feed
on carcases from migratory herds of ungulates.
2. Improved Predator Avoidance: Bipedalism added to the height and improved hominin's ability to see long
distances over tall grasses and to wade in deeper water to seek protection from predators .
3. Improved Reproductive Success: Lovejoy (1981) proposed that bipedalism offered three important
advantages in terms of reproductive success .
• Bipedalism provided a stable home base and a closer mother-infant relationship.
This improved infant survival.
• It reduced chances of infant injuries because mothers became less mobile.
• It reduced intrauterine life of foetus and also the spacing between the births.
Because of this, parents were able to care the offspring more. 4. Economical Energy Expenditure: Bipedal
locomotion is much more economical in energy expenditure than quadrupedal locomotion.

1113. FIG. 22.3: Difference in the shape and position of skull, vertebral column and pelvic girdle in A. man and
B. Chimpanzee.
Apes are still tree dweller. Their arms are longer than legs and they still walk on all the four arms.
22.10.2 Upright Posture
The bipedal gait resulted in upright posture necessitating major changes III the musculo-skeletal system.
These changes are :
• For balancing the body, the hindlimbs or legs became longer.
• The abdominal region became short and thorax became broad and flat.
• Because of shortening of abdomen, the lumbar vertebrae are 4-5 in man but in apes they are 6-7 in
number. In man sacral vertebrae are fused.
• A lumbar curve has developed in man which is absent in apes.
• The iliac bones of the pelvic girdle in man are broad and expanded to form basin-like structure which
accommodates internal organs of the abdominal cavity.
• Tail is absent.
• The sku ll is balanced on the verteb al column straight instead of being placed at right angles to the
vertebral column as in apes .
• The occipital condyle and foramen Magnum in man are directed downward, whereas in apes these are
directed backward.
22.10.3 Forelimbs
The incr eased brain power must have been meaningless without the ability to use it. In this respect
evolution of hands is another major event in the evolution of man. The forelimbs evolved into efficient
grasping and manipulating devices for making tools, hurling weapons and carrying objects. These provided
the very means by which man is able to put his ideas into new creations.
FIG. 22.4: Diagram showing difference in the pelvic girdle of A. Ape, B. Australopithecus, and C. Man.
A B
c o
FIG. 22.5 : Use of forelimbs by ancient humans for grasping and manipulating tools and hurling weapons .
A. Borer used for making hole B. Hand axe used for skinning and cutting carcases, C. Knife-like blade
hafted on a spear .for throwing on the wild animals and D. Dentiiculate tool for making other tools.
A Primitive Hand -Axe was a pick-lik e tool, rather crude ly wor ked on both surfaces and with a dairly
sharp point. It may have been used to dig edible roots and tubers from the ground.
22.10.4 Opposable Thumb
As the hands evolve d, instruments forkilling prey such as spea rs, bows and arrows , and simple too ls for
cutt ing up carcases were developed. This resulted in the lengthening of the thumb so that it became opposa
ble to fingers.
Evolution of opposable thumb enabled the ancestors to grip, hold and manipulate objects with precision .
This led to the evolution of hunting way of life.
22.10.5 Brain and Cranial Cavity
M an's most characteristic feature in which it differs from all its primate relatives is his extraordinary large
and complex brain and high intelligence . This has enabled him to think rationally, foresee the outcome of
his actions and solve problems . Evolution of social life, group living, communication power, cooperative

1114. hunting and food gathering led to such an enlargement of cerebral hemispheres.
To ac commodate a large brain, the cranial cavity is more volurnnous averaging from
1350-2000 c.c. or even more. In apes (the closest relatives of man) cranial cavity is
450-600 c.c.
The large size of frontal lobes has resulted in the development of high forehead which is slanting in apes.
Modem humans are part of an evolutionary group, called the hominins, that includes all the upright-
walking or "bipedal" primate species. Cranial capacity is a measure of the volume of brain tissue that can
fit inside the cranial cavity. Directional selection for cranial capacity is seen in the ever-increasing cranial
cavity in this group of hominins, all of whom are now extinct, except for ourselves. Going from left to right,
all the names of the species shown are Australopithecus afarensis (the "Lucy" species), Australopithecus
african us, Homo habilis, Homo ergaster; Homo erectus and Homo sapiens (ourselves).
Study of chemistry of human brain provides insight for the rapid increase of brain in humans and not in
apes.
Cranial capacity ( the volume of the cranial cavity)
has increased in hominins over time
~ H. sapiens'--<V"~
H. ergasler
1400
()
iil
1200 ~
A afarensis 1000 ~

1116. '"
.
BOO -< n
c
600 0" o·
s
:::l 400 3'
CD (jj
200
~
3 2 Present Earliest fossil record (millions of years ago)
FIG. 22.6: Directional selection: Evolving toward increased brain size: A. stands for Australopithecus and
H. for Homo
Origin and Evolution of Man [j] 533
Brain is a fat -rich organ with about 60 percent lipids. These are formed of omega-3 and omega-6
polyunsaturated fatty acids. Animal brains, livers, fish and shellfish (molluscs) are very good source of
these fatty acids. Plant food does not contain them and human body cannot synthesise them fast enough
from fatty acids obtained from plant.
It is presumed that the dramatic increase in the brain during human evolution occurred in those ancestors
which lived near lakes in Africa, because they could receive fish and molluscan food, Chimpanzees
remained in the forest, ate fruits and nuts and could not obtain omega fatty acids. Their food did not
support development of large brain. Moreover, early humans were hunters. They obtained omega fatty acids
from animal food that supported brain growth.
22.10.6 Face
• The ape's face is protruded out into a sort of muzzel on account of broad and long teeth row with
protruding canines of the lower jaw. This is known as prognathous type. In Homo sapiens, face is
orthognathous.
• In apes a simian shelf is present but it is absent in man (simian shelf is a sort of ledge of bones extending
backward from the symphysis of lower jaw).
• A chin is present in man, formed by the forward extension of lower margin of the lower jaw, but it is
absent in prognathou s face.
A
B

1118. FIG . 22.7: Skull restoration and brain of A. Homo erectus and B. Homo sapien (man) showing difference
in the facial region , teeth , skull, the position of occipital condyles and size of cranial cavity.
534 ~ Evolutionary Biology
• In apes heavy ridges of bones project over the eyes (the eyebrow ridges or supraorbital ridges). These
ridges are not present in man.
22.10.7 Teeth
The shape, size and arrangement of teeth is different in man and apes.
• In man dental arch (arrangement of teeth) is rounded parabola, but in apes these are arranged in a
straight-sided U.
• The canines are large and projecting in apes forming tusks but not in man.
• The incisors are large in apes but small in man.
• In apes a simian gap (diastema) is present in the upper jaw between incisors and canine. It accommodates
elongat ed canines of the lower jaw.
22.10.8 Improved Food Acquisition and Omnivorous Diet
Early ho minins lived in patchy environment of mixed woodland and dry grass lands that provided seasona l
food supply. Though evolved basica lly from herbivore ancestors, their new environment dictated
omnivorous diet, food search over long distances and storage of food. This was possible because of their
bipedality.
22.10.9 Intelligence
Because of highly developed cerebral cortex and cerebral hemispheres, the number of neurons and cross
connections between them have increased a lot and human intelligentia reached highest level. This
bestowed in humans the ability of thinking, planning, reasoning or logic and expression of emotions.
22.10.10 Binocular Vision
Humans have developed large forwardly directed eyes with great power of accommodation. They possess
binocu lar and stereoscopic vision enab ling them in-depth
Incisors

1120. FIG. 22.8: Diagram showing difference in the dental arch of .A. ape and B. man.
perception and distance estimation. Retina of eye contains photosensitive rods and colour sensitive cones.
22.10.11 Olfactory Sense and Hearing
Olfactory lobes are less developed. Acoustic efficiency is also less developed.
22.10.12 Breeding Capacity
In human beings birth rate is reduced and polyembryony is lost. Breeding cycle evolved in women by the
development of menstruation, whereas males developed continuous fertility.
22.10.13 Loss of Body Hair
Because of protected living and hunting habits, body hair gradually reduced and are present in vestigial
form. Presumably, hunting in a hot climate placed stress on the body's cooling system. Loss of body hair
facilitated heat loss.
22.10.14 Social and Cultural Organisation
In the emergence of man , the development of speech has been extremely significant. It has great
significance in the expression of feelings and ideas. Now it is a means of communication of complex
information between individuals.
All the differences between modem apes and man indicate that both have evolved from some common
ancestor simultaneously but on two different paths. Apes took to brachiation and man's ancestors came
down to ground habit.
22.11 EVIDENCES FROM MOLECULAR BIOLOGY IN SUPPORT OF HOMINID EVOLUTION
FROM APES
Advancement in the field of molecular biology in the past two decade has solved the degree of closeness
between different primates and has helped in tracing their relationship. Comparison of amino acid
sequences in homologous proteins (such as haemoglobin, cytochrome c, serum albumen) and cross
reactivity or precipitation test between such proteins of man and apes has clearly indicated close
relationship between man and great apes, and amongst the apes. Chimpanzee is closest to man. The
relationship between the two is described in terms of immunological distance (1.0.) between their
homologous proteins and the time of their divergence. Some of the similarities are as follows:
22.11 .1 Structure of Haemoglobin
Comparison in the actual sequences of amino acids in the polypeptide chains of haemoglobin of man,
chimpanzee and monkey shows that there is absolutely no

1122. difference in the sequencing of 141 amino acids in each a-chain and 146 amino acids in each l3-chain of
haemoglobin of man and chimpanzee. There are only 2
differenc es out of 287 amino acid s between haemoglobin of man and chimpanzee and betw een that of man
and monkey.
22.11.2 Molecular Structure of Other Proteins
M olecul ar structure of cytochrome c, insulin and serum albumin in man and apes exh ibits very little
differences. Human and chimpanzee' s polypeptides exhibit on an average 99 percent similarity. Th is makes
chimpanzee closest living relative of man .
22.11.3 Chromosomes
Somatic cells of man have 46 (23 pairs) chromosomes. In apes their number is 48. Man has evolved from an
ancestor having 48 chromosomes by the centromeric fusion of two chromosomes.
Chromosomes of man and apes have been studied with special staining techniques and it has been esta
blished that:
Chromosome-3
• Chromosomes of man and
ape s have similar banding
Chromosome-6
pattern.
• Some chromosomes of man
a nd apes have identical bands.
• Fig. 22 .9 shows third and
sixth chromosom es. Simil ar
ity in their bands indicates
that both apes and man have
evolved from some common
ancestor.
King and Wilson ( 1975) have
emph asised that in spite of large
anatomical differences , humans and
ap es exh ibit remarkable genetic
A B A B
similarity. It means that the two A B
group s presen t parallel evolution FIG. 22.9: Chromosome 3rd and 6th of man and divergin g from the
same common chimpanzee. A . Chromosome of man, ancestor. According to them , the
B. Chromosome of
chimpanzee.
two kind s of regulatory mutations have occurred during their evo lution :
I. Point mutation in the promotor or operator gene which influences production of protein but not its amino
acid sequence.
2. Chromosomal inversions , translocat ions, add ition and deletion s and also fission and fusion.
At least ten large invers ions and transl ocations and one centromeric fusion have occurred since two
lineages diverged. Chromosomal evolution has little effect on structural proteins but has dramatic effect on
development rate and consequently on body design.

1124. Most probably the timing of gene expressions associated with the development differs greatly in man and
apes. This has produced differences in the development of brain and vertebral column. The science of
establishing relation with development and evolution is called evo-devo.
22.11.4 Similarities in DNA
In 1971 , Bill Hoyer, David, Kohne and others compared the genome sequence of chimpanzee and man by
DNA-DNA hybridisation method. This indicates 96 percent similarity in the nucleotide sequence between
the DNA molecules of the two species. There exists a difference of 4 percent between the DNA of
chimpanzee and man.
22.11.5 Genomic changes in Human Evolution
Comparative genomic data of humans and c himpanzees sugges ts that most mutations in DNA associated
with unique human traits are produced by changes in the genes that regulate developmental and struct ural
protei ns. These mutat ions are associated with large brain size, cranio-sacral modification, S-shape
vertebral column with lumbar and sacral depression, reduced hair covering on the body, comp lex
behaviour and cultural features.
Differences of man from chimpanzee are attributed to gradual changes in the genome or DNA. Scientists
from United States, Belgium and France have identified 49 human accelerate d reg ions (HARs) where
marked or significant changes in the nucleotide arrangements have occurred during the last 30 million
years. The most active HAR identified as HAR-I is found to have 18 out of 118 nucleotides changed since
the evolutionary separation of humans from chimpanzees about 6 million years ago. Most probably this
region of genome is associated with the critical steps in the development of brain. However, it appears that
evolution of just one region in the genome could not make all the differences between human and chimp's
brains . It is much likely that a series of many small changes got added to produce the results. These
changes in brain development are produced during the key formative period of human brain from 7 to 19
weeks of gestation.
According to recent discoveries, human and chimpanzee's genomes are 99 percent identical and most
differences between them are due to gene regulation rather than difference in gene composition.
By comparing an array of complementary DNA (cDNA) from liver of human s, Chimpanzees, orangutans
and rhesus monkey, scientists have identified two types of genes:
• Stabilised Ge nes: These genes have remained unchanged over two million years , since the four primate
groups diverged. These genes have undergone sta bilising selection.
• Evolving Ge nes: The expression of these genes differed significantly in humans either due to the presence
of addition transcription factors, or due to silencing of certain genes by mutations. These genes exhibit
directional selection.

1126. Human Chimpanzee
Two nonsy nonymous mutations in human lineage led to unusually high rate of evolution of this protein
(FOXP2)
FIG . 22.10: Divergence of man and chimpanzee due to substitution: Synonymous (white box) and
nonsynonymous (yellow box) changes in gene FOXP2 are important in the evolution of speech and
language.
H umans and chimpanzee differ in the expression of 19 regulatory genes. These gen es tum other genes on
and off leading to so man y differences. It means divergence of chimpanzee and humans is caused ma inly
by transcriptional regulations which produce differential expression of certain genes. For example:
E volution of language and speech in man occurred due to a rare substitution mutation in FOXP2 gene
(fork head box 2 gene). This gene encodes a transcription factor. The substitution mutation occurred during
human evolution less than 200 kya .
~~.1~
COMMON ANCESTORS OF APES AND MAN IN OLIGOCENE, MIOCENE
~2.12.1
Propliopithecus
The fossil of first known ape was obtained from Fayiim deposits of Egypt. It was described under the name
Propliopithecus. It lived about 30-35 million years ago in Oligocene Period. It is represented by the fossil
jaws and teeth. No doubt more or less ape -like, Propliopithecus were short statured with monkey-like teeth.
Their den tal formula was 2, I, 2, 3. Their incisor teeth were vertical rather than directed forward and
molars had 5 cusp s each . Swinnerton presumed that the apes have directly evolved from tarsiers bypassing
the monkeys.
There are two different concepts of ma n's Oligocene ancestry:
• that Propliopithecus directly gave rise to Ramapithecus in Miocene Period.
• that Propliopithecus gave rise to Dryopithecus in Miocene, which in tum
evolved into ape s and man .

1128. 22.12.~
Aegyptopithecu5
Its fossil s were found by E. L. Simon and Richard in 1980 from Cairo. These were similar to
Propliopithecus.
22.12.3 Limnopithecus and Pliopithecus
Their foss ils were obtained from Fayiim deposits of Egypt and some other places. The se forms existe d in
Miocene and Pliocene periods. These are considered to be the ancestors of gibbo ns and orangutans.
22.12.4 Proconsul
It s fossils were discovered by Leakey (1930) from East Africa near Victoria Lake in Kenya from Miocene
deposits. Its molars had five cusps each. The face was prognathous. It walked on its four legs.
22.12.5 Dryopithecus
I n early Miocene Period about 25 milli on years ago, there existed a group of apes , collectively know n as
Dryopithecines. Dryopithecus africanus. forme rly known as Procons ul. exhibited close similarity to
chimpanzee and is considered to be a common ances tor of man and apes or a direct forerunner of man .
Although ape-like, it had arms and legs of the same length and its legs and heels indicate that it must have
assumed a semi -erect posture (knuckle walker). It has large canines and incisors, feeding on fruits and
leaves.
Dryop ithecines from India and Europe in Miocene Period appear to be the ancestors of modern apes.
Several members of Dryopithecine have been discovered from Sivaliks. Recently an almost complete lower
jaw of a Dryopithecus has been obtained from Haritalyanga in Bilaspur district of Himachal Pradesh and
is being studied by 'Chopra- Simon' team at Punjab University. Sivapithecus is another fossil ape from
Sivalik Hills.
22.12.6 Oreopithecus
Or eopith ecus exhi bited still more resemblance with man. It has broad , basin like pelvic girdle, man-like
teeth and jaws and a short man-like face. It might have walked erect. It lived in late Mioce ne and early
Pliocene period s. Although so man-like, it is regarded to represent an aberrant branch from the common
ancestors of man and ape and its simi larities to man have developed on account of parallel evolution .
22.12.7 Ramapithecus
The anthro poid fossi ls obtained from Mioce ne depo sits from Africa and Asia in- cluding India were
named after Hindu Gods, Rama and Siva as Ramapithecus and Sivapithecus. They were discovered by
Edward Lewis in early 1930s from Sivalik Hills in Northern India. The fossils were represented by a fragile
upper jaw.
Characteristics
1. Ramapithecus had reduced canines, relatively sma ll incisors and low crowned molars capped with thick
layer of enamel.

1130. 2. Thic kly enameled molars suggest that Ramapith ecus ate seeds, nuts and fruits with hard covering.
3 . Their teeth were adapted for grinding and not for tearing and crushing, i.e., their molars had flat
grinding surface similar to that of modern man.
4 . Thei r dental arch was V-shaped and not parabolic as in modern man.
5. The recent discov ery of fossilised limb bones from Pakistan suggests that Ramapithecus was quadrupeda
l and arboreal.
6. It liveded15 million years ago in Miocene.
Evolutionary Status: Earlier Ramapith ecus was considered to be the earliest man-like primate and oldest
ancestor of man.
FIG. 22.11: Palate and teeth of Ramapithecus. Note arched palate, small size of incisors and canines and
flat crowned and evenly
proportioned premolars and molars.
• But according to Elwyn Simons of Yale University, Ramapithecus marks the point where homin id lineage
separate d from pongid assemblage.
• According to Adrienae Zihlman of University of California, Ramapithecus was one of the several types of
apes that walked on all four legs but ventured into open grass fields, about more than 14 Mya.
• According to anthropologist David Pilbeam Ramapithecus and Sivapith ecus were ancestors to
Orangutan. They were apes on the direct line of ape and human evolution.
22.13COMMON ANCESTORS OF APES AND MAN IN PLIOCENE PERIOD
Fossil Humans and Origin of Homonini
According to molecular dating , the diverge nce of human and chimpanzee lineages occurred in the
Pliocene Period between 6.3-7.0 Mya. Recently obtained fossils of three African species, Sahelanthropus
tchadensis, Orrorin tugensis and Ardipithecus kadabba are considered to be earliest hominins because they
show a mixture of homonin features including bipedal locomotion.
22.13.1 Sahelanthropus tchadensis
The fos sils of Sahelanthropus tchadensis were disco vered in Chad in Central Africa by Brunet et al. (2002)
and Wood (2002). These were informally referred as Tournai and date back to late Miocene 6.5- 7.4 Mya.
The fossils included a complete cranium

1132. and a few fragmentary lower jaws and teeth. Similar to apes, these fossils had short cranial cavity and
small brain of about 350 c.c. Like hominids, they possessed brow ridges, small canines and a flat face. They
were almost certainly bipedal. This species is cosidered to be most primitive hominin, very close to the
divergence of human and chimpanzee lineages.
22.13.2 Orrorin tugensis
The fossil s of Orrorin tugensis were di scovered from Western Kenya by
FIG. 22 .12: Computer generated model of Sahe/anthropus tchadens is or Toumai the most ancient fossil of
a hominin that lived
around 7 mya in Central Africa.
Senut et al. (200 I). These fossils date back to about 6 Mya. The fossils included fragmentary arm and thigh
bones and fragments of lower jaw and teeth . They were
habit. They shared feature s with man as adapted to both bipedality and arboreal well as chimpanzee.
22.13.3 Ardipithecus kadabba
E arliest fossils of Ardipithecus kadabba and Wood (1994 ). These date back to 4.4-5.2 Mya i.e., late
Pliocene Period . The fossils were fragmentary and were named Australopith ecus ramidus. In 2005. These
were assigned to genera Ardipithecus ramidus. It was about 120 cm (3¢II ") tall and weighed about 50 kg.
The brain and skull were of the size of chimpanzee. These had australopithecine features and could live on
ground as well as on trees . They had reduced canine s and a big toe for locomotion in trees.
were disco vered by White et al. (1994)
22.14 EVOLUTION OF MAN IN PLEISTOCENE PERIOD
The Plei stocene history of man is primarily a story of progressive enlargement of brain , increase in
inteligence, and the perfection of bipedal gait. Until recentl y, the evolutionary sequence of man in
Pleistocene was thought to include many species and genera, because each new fossil was generally given a
new name. In the last fifteen years, the Pleistocene fossils of human s have been reconsidered in the light of
modern biologi cal principle and are regrouped into three. These are represent ed by Australopithecus in
early Pleistocene, Homo erectus in middle Pleistocene and Homo sapiens in late Pleistocene.
22.14.1 Australopithecus (The Southern Ape)
Australopit hecines were fully bipedal hom inids with relatively small brain. They lived from 4 to 1.5
million years ago. They were similar to primiti ve ape in certain features and to man in many others . Their
first fossil was an unusual small skull

1134. H. sapiens
Present

ecen
H.neanderthalensis
H. heide/bergensis
H. fIoresiensis
H. mauritanicus H. erectus
2
K. rudolfensis
-
Kenyan thropus pfatyops
Austra/opithecus anamensis
AT ramkius
5
ononn
6 tugenensis
Ardipithecus kadabba
Ar. Ardipithecus
Au . Austra/opithacus P. Paranthropus H. Homo
Sahelanthropus tchadensis
7 FIG. 22.13: A possible family tree of family Homoni ni including Homo sapiens (after Ian Tattersall , the
expert in human evolution).
cap of a child from a P leistocene limestone quarry near village of Taung in South Africa. It was discovered
by an Australian anatomist, Raymond A. Dart, in 1924, and was named Australopithecus meanin g '
Southern ape' (L. auster, or austal = southern + pith ekos = ape) . Later, in
skull s of Australop ithecus from caves
1930 Robert Broom found complete adult
in Sterkfontein and Swartkrans in South Africa. Several new fossil forms from Sout h Africa and Java were
described under different name s but now they all have been placed in genu s Australopith ecus and species
anam ensis. The second most ancient australopithecine was called Australopithecus afarensis. Its fossils
were found in Ethiopia in 1974. They were about 3.9 Mya old.
Characteristics of Australopithecines
Australopithecines show following characteristic features:
I . Australopithecines were short-statured measu ring 4-5 ft.
2. They walked nearly or comp letely straight on two hind legs.
3. They lived in open and had given up arborea l life.
4. Their vertebral column had a distinct lumbar curve .
5. The thighs and hips were well adapte d for erect standing and walking. The pelvics were broad and
flattened.
6. The ankle bones were designed to bear body weight during bipedal locomotion, but were intermediate
between those of human and ape.
7. The big toes in the feet were still adapted for grasping.
8. Their arms were longer than legs and adapted for brachiation.
9. The brain was small but more or less man like in shape .
10. The size of cranial cavity ranged from 450-600 C.c.
II. Dental arch was smoothly rounded parabola.
12. Incisors and canines were sma ll and man-like, but larger than those of modem man.

1135. 13. A simian gap between canines and premo lars as found in apes was absent. 14. Face was prognathous,
jaws protruding and chin absent.
IS. Eyebrow ridges projec ted over eyes .
16. They were hunters and canni bals.
Australopithecines represented man with ape brain and are commonly described as ape man. They lived in
small groups of just a few dozen individuals. Types: Two distinct species of Australopithecines evo lved from
A. afarensis lineage. These are:
1. Gracile Form or
Australopithecus africanus:
/
They were lightly built slender
forms; about 1.2 metres ta ll
and weighed around 25-30
kg. Their jaws and teeth were
similar to ours but molars were
larger and stronger.
These gracile forms were
named A ustra lopithecus
FIG. 22.14: Restoration of head and skull of
africanus. They supplemented Australopithecus.their
diet with animal food (i.e.,
became omnivorous) and consequently their molars became sma ller than those of robust form.
2. Robust Form or Australopithecus robustus: They had heavier body structure . They were about 1.5
metres tall and weighed almost twice the gracile form. They had massive cheek teeth (molars) indicating a
pure vege tarian nature. They were initially
named Paranthropus P robustus and P boisei but are now called Australopithecus robustus and A. boisei.
They perished without evolving further.
It is presumed that these two species of Australop ithecus coexisted at the same time and in the same region.
The vegetarian A. robustus perished without leaving any descendent s, while the omn ivorous A. africanus
evolve d into hominins. Presumably, the change in diet directed the evolution towards 'tool-using bipedal
human'.
S ome other Fossils from Africa and Java
Some other fossils obtained from East Africa and Java were earlier placed in separate genera but are now
called Australopithecus. These are: 1. Paranthropus (Beside-Human): The fossils of three species of this
genera
were identified . These were named Paranthropus becuase all the three species are monophyletic and form
an independent side branch during human evolution. They became extinct without evolving further.
These forms were much stockier than gracile australopithecines, and heavier on an average about 20
pounds . Their skull was much broader and more robust with massive cheek teeth and large cheek-bones .
These forms had tremendous biting power and their food included large seeds and coarse plant material.
2. Zinjanthropus: Its fossils were found from Tanzania (East Africa) by Leakey in 1959. They had extremely
massive jaws and enormous teeth and were named 'Nutcracker man'. They lived about 1.8 million years
ago. Many scientists now regard them as East African Australopithecus robustus . Others believe that it
should be assigned to a new species , Australopithecus boisei.
3. Meganthropus: Its fossils were found in Java.
22.14.2 Early Homo

1136. Th e genus Homo diverged from Austra lopithecines (either A. afarensis or A. africanus) about 2.5 Mya in
Africa . The earliest African Homo fossils are assigned to two species H. rudolfensis or H. habilis and H.
ergasters.

1138. ABC FIG. 22.15: Three Homanin s A. Paran thropus, more ape-like but bipedal. B. Homo ergaster, the
early human, more man-like. C. Homo neanderthalensis, recent human fossil that lived in Europe in
proximity with Homo sapiens for thousands of years.
22.14.2.1 Homo habilis (Handy Man or First Tool-Maker Hominid)
Homo habilis lived in early Pleistocene about 2.5 million years ago. Its fossils were found from Olduvai
Gorge by Mary Leakey and Louis Leakey in 196()-{j I and then by Richard Leakey (1972) , the son of Mary
and Louis Leakey along the eastern shore of Lake Turkana in North Kenya.
Characteristics of Homohabilis
They showed following characteristics
I. The body and brain ofHomo habilis were larger than those ofAustralopithecus. 2. He walked fully erect,
but its arms were long and legs short like ape ancestors . 3. His hands were similar to ours and had manua
l dexterity.
4. He used his hands in making stone too ls, which were used in digging underground food and killing sma
ll animals.
5. His tools were crude choppers, made by removing flakes along one side of a pebble . They had an irregu
lar cutting edge .
6. The size of its cranial cavity reached about 700-800 c.c.
7. It was omnivorous and had begun hunting .
8. Homo habilis lived in small bands or groups with stab le camp sites. It was leading settled life to some
extent.
9. It also showed sexual division of labour and also developed communication with visual signals and
simple audible sounds.
22.14.2.2 Homo ergaster
Fossils of Homo ergaster appeared about 2 Mya. The forms were more like those of modern humans. This
limb proportions in these lineage gave rise to two
branches: (i) Homo erectus and (ii)
Homo heidelbergensis.
22.14.2.3 Homo erectus
(The Forerunner of
Modern Humans)
Homo erectus was the first hominid
species to leave Africa and spread out
in Asia and Europe . Its fossils were
obtained from diverse sites ranging
from Olduvai Gorge in Africa to
Java, Algeria, China, Hungary and
Germany. These fossi ls date back
from 8,00,000 to 3,00,00 years. Its
first fossil was discovered in 1891
by Eugene Dubois from Java. It
was named Pithecanthropus erectus

1140. FIG . 22.16 : Hominid tools: Hand axes, Flint implements, Needle-like implements of bone and antler
meaning 'ape-man that walks erect'. Simi lar fossils were found in a cave, Choukoutien, near Peking in
China and were named Sinanthropus pekinensis. On account of their similarities with Java ape-man they
were renamed Pithecanthropus pekinensis. But Mayr (1950) replaced these names by Homo erectus so that
Java ape man is known as Homo erectus erectus and Peking man as Homo erectus pekinensis. They are
described as ape-like man that walked straight.
Characters of Homo erectus
I . Homo erectus had ske leton much like ours but more primitive skull with a cranial cavity of about 1000
C.c.
2. They were hunters and food gatherers.
3. They used stone tools for hunting and butchering deers, antelopes and even large ferocious anima ls such
as bears, wild oxen and elephants.
4. Their tools included hand axes, chopping tools, flakes, points, cleavers and even scrapers. They had
started using bone and wooden tools as well.
5. Signs of organised or cooperative hunting have been found in Europe.
6. They had also learned the use of fire for cooking.
7. Their face was still unlike that of modem humans with large brow ridges over the eyes, slightly
protruding face and no chin .
Homo erectus has left clear records of a developing culture and associated with the oldest or Abbevillian cu
lture. They were nomads who roamed wide ly in small groups of extended families. The people of those
times had begun to clothe themselves with anima l skins and some of them moved into caves for protection
from cold. They also used some kind of rud imentary language.
Features Responsible for Evolution of Homo erectus
1. Because of omnivorous diet and eat ing cooked meat, the molars of Homo erectus were comparatively
reduced.
2. Cultural adaptations to temperature and cold climate and use of tools for hunting large migratory
animals provided stimulus for rapid development of modern human features like larger brain and
accelerated growth of human culture.
3. Disappearance of estrus cycle in the course of hominid evolution acted as a factor in binding of male to
the family and stabilisation of family.
(Homo erectu s erectus = Pithecanthropus erectus): Its fossils1. Ja va Man
(some teeth, sku ll cap and femur bone) were found in 1891 by Dubois on the bank of Solo River in eastern
Java. Its fossils were
some 5,00,000 years to 1.5 million years ago.
found in the Pleistocene deposits It had a cranial cavity abou t 940
c .c. intermediate between that of Australopithecus (600-700 c.c.) and modem man and modem man 1600
c.c.). It was more than five feet tall and weig hed 70 kg. Its forehead was low and slanting, the face was
prognathous, and the jaws were massive with huge teeth . The chin was absent but bony eyebrow ridges
present over the eyes were heavy. Their molars were sma ller but frontal teeth (incisors and canines) were
large and more promi nent indicating omnivoro us diet.
Origin and Evolution of Man !j] 547

1141. 2. Peking Man (Homo erectus pekinensis = Pithecanthropus pekinensis or Sinanthropuspekinensisn Its
fossils were discovered from caves near Peking in 1920 by Davidson Black. The fossils included numerous
skulls , jaws and postcranial bony fragments from the limestone caves near Choukoutien. They lived most
probabl y about 1,50,000 to 50,000 years ago. Peking man was very similar to Java man with heavy bony
eyebrow ridges , low slanting forehead and chinless face. But its cranial cavity was much larger than Java
man ranging from 850-1200 c.c. and averaging 1075 c.c.
In addition to them , Gigantopithecus and Meganthrop us are described to be related to Java and
Peking ap e man.
FIG. 22.17: Restoration of head of Java man.
22.14.3 Recent Homo: Homo sapiens
Th e recent species of Hom o dates from 1.2 million years ago to present. They evolved from Homo erectus.
The several transitional forms of the lineage from Homo erectus to Homo sap iens are represented by fossils
from Africa and Europe (Germany, France). These are described under different names as Homo
neanderthelensis, Homo heidelb ergensis, swanscombe man, etc. But, since they appear to be very similar,
now they are all grouped under Homo sap iens.
Transitional Forms
FIG. 22.18: Restoration of head of Peking man.
Some transitional forms connecting Homo erectus with Homo sap iens have been uncovered from Europe .
1. Steinheim Skull: It is a fairly complete skull found from Steinhein in German y. Its cranial capacity is
estimated to be about 1000 C.c. It had puffy eyebrow ridges and a low forehead. Its face was protruded but
was relatively straight and tucked in under the brain case. The back of the skull was rounded and the
molars were modem but the incisors were rather large.
2 . Swanscombe Skull: This is known from three bones which form roof and back of the brain case. The
bones are unusually thick. The cranial cavity is estimated to be about 1320 c.c. In other features it
resembled Homo sapiens.
The abo ve two skulls were obtained from the second interg lacial period from late middle or early upper
Pleistocene.
3. Fontechvade Skulls: These skulls were discovered from Southern France from the third interglacial
period. The skull bones are unusually thick, but the skull lacked heavy eyebrow ridges and cranial capacity
even more than 1400 C.c.
4. Ehringsdone Skull: It was found from Germany and had cranial capacity 1450 cc. The skull had a fairly
high forehead but with heavy eyebrow ridges. Thus, it resemble d Neanderthal man in eyebrow ridges and
H. sapiens in forehead .
22.14.4 Homo sapiens During Glacial Period
These fo ssils indicate that during the second and third interglacial periods there lived an assemblage of
people that might have been the ancestors of both Neanderthal and Homo sapiens. Now all of them together
with others have been included in Homo sapiens . Therefore, it will be wise to call these forms as early
Homo sapiens . Following distinct types of Homo appeared in the course of evolution of present species of
humans and became extinct.
1. Heidelberg Man: It is known only from a massive lower jaw, which was found from Heidelberg,

1142. Germany. The jaw is large and heavy and lacks a chin. Teeth are like those of modern man. Heidelberg man
is regarded as an ancestor to Neanderthal man and is believed to be contemporary of Homo erectus.
2. Neander thal Man: Neanderthal man existed in the late Pleistocene period and its fossils were found in
the Neanderthal Valley in Germany. Previously it was named Homo neanderthalensis but according to
modem concept these are known as H. sapiens neanderthalensis. They arose some 1,30,000 years ago and
flourished in Europe, Western Asia and North Africa, until 25,000 years ago. They lived in proximi ty to
modem humans for thousands of years.
• They were quite similar to modem human, but were heavily built with outwardly curved thigh bones.
• The skull bones were thick, forehead was low and slanting and their eyebrow ridges were heavy.
• The lower jaw was deep with no chin . The cranial capacity was about 1450 c.c. roughly equal to or
slightly larger than that of modem man.
Cultu rally, they were more evolved. They showed ritualistic burial ceremonies of deads . They used more
symmetrical and sharp tools. Stone tools were more fine, made from flakes.
Inven tion of long wooden spear
with a sharp stone tip was an
advance ment in hunting. They killed
the large animals either by group
attack or by driving them into pit
traps. Neanderthals possessed knives
to butcher the carcasses and used fire
for cook ing and for warmth. They
used animal hides for crude clothing .

1144. They buried their dead . They had FIG. 22.19: Restoration of skull andthe concept of life and death and head of Neanderthal
man.
followed rituals. Neanderthals made
no progress either in agriculture or in domestication of animals.
Origin and Evolution of Man Ii] 549
It is be lieved that the second branch from Homo ergaster evo lved into Homo heidelbergensis. From Africa
they migrated to Europe and gave rise to Neanderthal man-Homo neanderthalensis. In Africa, a branch
from H. heidelbergensis evolved into Homo sapiens.
Reasons for Extinction of Neanderthal Man
Several theories have been put forward to explain Neanderthal's disappearance:
• Some scientists suggest that Neanderthals were wiped out by the invading
modern Homo sapiens (the prehistoric genocide).
• Other scientists believed that Neanderthals intermarried with the invading
Homo sapiens. This is the reason modern human beings have certain genes
of Neanderthal origin.
• Some scientist have suggested that Neanderthals could not compete with
the more technologically advanced new-comers and simply died out due to
displacement.
3. Cro-Magnon Man: During last 30,000 years or even more, Homo sapiens close to modem man lived in
Europe and also in other parts of the world. The forms living in Europe were described as Cro-M agnon Ma
n. These succeeded Neanderthals and became extinct about 20,000 years ago. They were much more
advanced than the Neanderthals and belonged to Homo sapiens.
Cro-Magnons were about 180 em in height with a large skull , broad face, rounded forehead, narrow nose
and a prominent chin. They lacked eyebrow ridges. The cranial cavity was about 1600 c.c. They were swift-
footed, cave dwelling forms and expert hunters. They were conversant with art and could sketch pictures of
their contemporary animals . They made tools from finely chipped stones. The tools consisted of spear-
heads and arrows. They made ornaments from ivory and decorated their body. They knew the use of hide of
animals . They did not know agriculture and domestication but exhibited some cultural advance and had
some religious burial ceremonies.
Cro-Magnons expressed themse lves through painting and sculpture. At least 12,000 years ago, they had
learnt to make paints out of clays, animals fats and metal oxides.
It is now believed that Neanderthals did not become extinct but merged with Cro-Magnon on account of
interbreeding and hybridisation. CroMagnons actually represent forms that migrated from some other part.
Further evolution of man after Cro-Magnon involved the evolution of culture rather than that of anatomy.
Techniques of manufacturing stone tools improved with time . The entire period through which man has
improved the techniques of constructing instrume nts starting from stone
FIG. 22.20: Rock painting of a reindeer hunt from a cave in Eastern Spain.
bits till now has been divided into Palaeolithic, Mesolithic and Neolithic Ages.
FIG. 22.21: Restoration of skull and head of ero-Magnon man.
Cro-Magnons represented only one of the many populations develop ed the Upper Palaeolithic cultures.
Similar cultures of humans that

1145. were thriving in Asia and Africa. All these people still lived by hunting and gathering. But many of them
had started settling down in permanent communities where food was abundant.

1147. Comparison of cerebral cortex in the brain of primate and man
Somat ic sensory and motor
Somat ic sensory and
motor
Prefrontal cortex
Prefrontal Effects on midbra in limbic system in primates and man
Prefrontal cortex
FIG. 22.22: Diagram to show enlargement of cerebrum in human brain during human evolution from
primates and the impact of prefrontal cortex on midbrain 's limbic system , which governs many emotional
responses .
4 . Homo sapiens sapiens: Evolution of modern man from Cro-Magnon is mainly a cultural and technical
evo lution . It has witnessed:
1. Advances in technologies.
2. Domestication of cattle, dogs and sheep, etc .
3. Agriculture practice
4. Establi shment of towns.
5. Extraction of metals from ores and their use in arrows, potteries and even ornaments and jewellery.
About 3000 BC, the first alloy bronze was also prepared.
22.15 CULTURAL EVOLUTION OF HUMANS
The recent events in hum an evo lution include dominance of cultura l evo lutio n, social life, transmission
of learned behaviour, tool mak ing and techn ological revo lution. These include:
1. Cultural Revolution: This ena bled human beings to live in groups and leading a protected life. It enabled
a large number of ind ividuals to survive.
2. Tool Making Revolution: The use of tools by humans resulted in improved hunting and food gathering
techniques. This resulted in the fir st population explosion, which occurred about 20,000 years ago.
3. Agricultural Revolution: The beginning of settled life, use of agricultural practices, improved farming
and domestication of animals was another step of advancement. This resulted in second population
explosion about 6,000 years ago .
4. Scientific Industrial Revolution: This is characterised by the improved techniques of food production,
industrialisation and medicine. This has made human life more comfortable and has saved him from
sufferings. This resulted in third population explosion about 300 years ago.
22.16 IMPACT OF EVOLUTION ON HUMAN BRAIN
Increase in the size and com plexity of br ain an d development of certain menta l capac ities during human
evo lution over last four million years was due to the impact of:
• Increased num ber of neurones for increased capac ity of processing informations.
• Deve lopment of intricate neuronal circuits to facilitate refinement of neural function.
E volution and advancement of these human abilities led to increased neura l circuits in the prefrontal
cerebral cortex bett er speech and better motor control. These centres are located in the left cerebral hemi
sphere. Wynn has suggested that evolution of a more human-like tool behaviour, long term memory and
problem solving ability evolved long before the appearance of language.
22.17 HUMAN RACES The modem man appeared 11-10 thousand years ago in Asia near Caspian Sea.
From here it migrated in three direction s and formed three distinct races:

1148. 1. White or Caucasoid Race: The white race spread to West along shores of Mediterranean Sea in Europe,
South-West Asia and North Africa . Caucasoids have pale skin, narrow and often long nose and straigh t,
wavy or curly hair.
2 . Black or Negroid Race: This race evolved in Africa and Malaysia. Negroids have dark brown to black
skin, frizzly or kinkly hair, broad flat nose and usually thick spread nostrils.
Geological Division Lower
Pleistocene Archaeological
Division
Lower
Palaeolithic
(Old stone age)
Duration Events
2 million years
Middle
Pleistocene
Upper
Pleistocene Middle
Palaeolithic
Th ousands of years ago 800
600
400
200
80
Homo erectus appeared.
Man l earnt use of fire.
Hunting of elephants.
Man begins to make artificial shelters from branches.
Neanderthal man emerged in Europe; ritual burials.
Holocene (Present epoch)
Upper
Palaeolithic 60 40 30
Mesolithic
( Mi d d le stone age)
Neolithic
(New stone age)
Copper age Bronze age
20
10,000 Years

1149. B.C.
9 ,000
8,000
-!.
6,000
4,000
3,000
Iron age 1,400

1151. Cro-Magnon man appeared in Europe ; large scale dispersion of human popula- tion; man reaching
Australia .
Fine tools from stone flakes.
Bow and arrow invented; dog and sheep domesticated; settled life began.
Agric ulture cultivation of wheat , barley, etc.; town city established; loom invented and cattle
domestication began.
Use o f copper began.
Bronze was used to make tools; silk moth domesticated ; potter wheel invented. Iron used in Middle East;
language and scripts evolved.
3. Mongoloid Race: This race spread in China and North-East Siberia. Mongoloids have a flattened face,
straight but coarse hair, nose flat at the root and with moderately spread nostrils .
22.18ARCHAEOLOGICAL DIVISIONS OF PLEISTOCENE AND HOLOCENE PERIODS
Genus Homo is presumed to have evolved in Lower Pleistocene period about 13 million years ago and was
represented by Homo erectus. The period from this time till date is divided into three stone ages. These are:
22.18.1 Palaeolithic
It is the old stone age. During this priod man was a nomad migrating in groups from place to place in
search of food and using stone tools for killing wild animals for food and protection. It extended from
13,00,000-20,000 years. Homo erectus inhabited during this period in Africa, Europe, and nearby places. It
had learnt the use and control of fire and primitive stone tools and had begn to make artificial shelters from
branches . Neanderthal man appeared in the middle Palaeolithic and Cro-Magnon man in upper
Palaeolithic in Europe.
22.18.2 Mesolithic
It is middle stone age which extended from 10,000 years to 9,000 years . CroMagnon man lived during this
period and had developed bow and arrow in Europe and domesticated dog in America and sheep in Middle
East.
... Migration of early
Modem Humans
_ Landmasses in
18,000 BC

1153. FIG . 22.23: The three major migrations the early human population undertook from Africa to 1. Europe, 2.
Northeast Asia and America, and
3. Southeast Asia and Pacific islands.
22.18.3 Neolithic or New Stone Age
It extended from 8,000 years ago and has show n maximum evo lution of human race. Neolithic culture
started with domestication of cattle, strategic farming and use of metals. It is divided into copper age,
bronze age and iron age. Christian Era falls in iron age in which present human race is living.
22.19 MONOPHYLETIC OR POLYPHYLETIC ORIGIN OF MAN There are two main views about the
origin of modern man (Homo sapiens sapiens from Homo erectus]. These are:
22.19.1 Single Origin Hypothesis or Noah's Ark Model or Replacement Hypothesis or Monophyletic Origin
Thi s hypothesis was proposed by William Howells. According to this hypoth esis, modern human (Homo
sapiens sap iens ) has evolved in a single locality (Africa) about two million years ago. From here
ancestors migrated to all other contin ents of Old World and replaced all other extant populations of Homo
sap iens. This hypothesis is called 'Noah's Ark model' or 'Out of Afr ica model' . Accord ing to this model,
replacement of older population s of archai c man (Homo sapiens) includ ing Neanderthals occurred with
the mixing of gene pool s. Hence , it is also
Europe Africa Asia
Europe Africa AsiaPresent

1155. Homo sapiens
1
0.5 milli on
years ago
Homo erectus
1
1 million
years ago
.. Oul of Africa " or 'Noah' s Ark'model " Candelabr a" mod el Sin Ie mono h letic oli in of Homo sa iens in
Africa, Multiple origins of Homo sapiens from and replacement of Homo erectus in Europe. Africa and Asia
Homo erectus populations in Europe, Africa, and Asia A B
FIG . 22.24: Two models for the origin of Homo sapiens . A. Monophyletic origin: Homo sapiens originated
in one locality in Africa, migrated to other continents and replaced Homo erectus Europe , Asia and Ame
rica. B. Multiple origin : Homo sapiens originated
in different localities in Africa , Asia and Europe independently but simultaneously.
called 'Replacement Hy pothesis'. Since, there was just little mixing of gene pools of resident and colonising
populations, the subspecies remained more or less genetically distinct from archaic populations.
Ev ide nces in Suppor t of Single Origin: There are several points to support this hypothesis:
1. The study of mitochondrial DNA (mtDNA) has shown that mtDNA sequences in all non-African sources
are variants of African sequence. This similarity indicates African origin of all present human races.
Moreover, there are no non-African mtDNA types.
2. Variability in mtDNA sequence among African populations suggest that these are the oldest mtDNA
populations among modern humans. The non-African populations are their deri vatives.
3. Age of most common mtDNA ancestor is older than the age at which population bearing ancestral
mtDNA diverged.
4. Nei and Roy Cho udhury calculated genetic distances between 26 human populations for 29 differe nt
nuclear genes and supported sing le origin hypot hesis in Africa, followed by population spread leading to
geographic divergence.
5. Mountain and coworkers and Batzer et al. also supported African ancestry based on polymorphism in
human populations.
It is now inferred that modern humans originated in Africa; and a population that left Africa branched off
into three monophyletic groups due to three major waves of migration from East Africa, namely:
• To North Africa, Europe, and Central Asia
• To Northeast Asia and America.
• To Southeast Asia and South Pacific.
22.19.2 Candelabra Model Multiple Origin or Polyphyletic Origin
This hypothe sis was presented by Lewin (1987) and Thomson (1992) . According to this model , human
evolution has occurred almost simultaneously in different unconnected localities in the Old World. Most
probably, Homo erectus had spread throughout the Old World. Several populations in different regions of
the Old World evolved independently to become Homo sapiens which further evolved into modern humans
(Homo sapiens sapiens) show ing para llel evolution. This is called multiple hypothesis of huma n origi n. It

1156. is also called ' Ca nde labr a model' (i. e., branched candle stick model or lamp with several lights model).
Ev ide nce in Support of Multiregional Origin: This hypothesis is supported by the continuity of anatomical
features in Chinese and Australian humanoid fossils . Fossil characters such as eyebrow ridges and cranial
size seem to have progressed from Homo erectus to Homo sapiens in a fairly continuous sequence, pointing
to their independent parallel origin.
Obj ections to Multiregional Origin: assumption that popula tions that evolved This hypothesis is discarded
on the in parallel over lon g periods cannot
accumulate simi lar mutations and similar amount of variability.

1158. 556
~
Evolutionary Biology 22.19.3 Multiregional Origin of Humans
Based on genetic data, Templeton (1997) proposed multiregional origin of modem man. He called this a
'trellis-mode l, i.e., the human ancestors were geographically mobile. Hence, there was interbreeding and
gene exchange between ancestral human populations. There was in and out movement from Africa . Still
genetic differences between African and non-African populat ion increased because gene exchange depends
on proximity.
Some anthropologists (like Jorde et al., 1998) believe in African origin of Homo sapi ens with some mixing
of ancestral populations.
22.19.4 Conclusion
According to anthropologist, Wo lpoff (1989), modem human populations have evolved through
interpopulational gene flow which has enabled the evolving groups to reach the same Homo sapiens
sapiens level. Thus he supports multiregional evolution of man. However, molecular evidence seems to
support single origin or replacement hypothesi s. Futher additions to molecular evidence may specifically
prove single origin hypothesis.
22.20 PUNCTUATED EQUILIBRIUM IN HUMAN EVOLUTION
Accordin g to the theory of punctuated equilibria, evolutionary changes in hominids occurred in rapid
bursts. These punctuated episodes were separated by long periods of stasis in hominid lineages during
which little or no morphological change took place. This is described as rectangular pattern instead of tree-
like pattern as found in the evolution of different group s.
Although the primate fossil record is fra gmentary, in combination with available immunological evidence,
it provides sufficient evidence that the hominoid and cercopithecoid (old world monkey) lineages got
separated about 30 million years ago. Simon (1976) estimated that this split occurred around 32 to 49
million years ago. Also split between African apes and humans must have occurred more than 9 million
years ago. According to Gillespie , the divergence between man and chimpanzee occurred between 5 to 10
million years ago.
Future of Man
Cultural and industria l evolution today has reached that stage that man can change his environmental
conditions. This has reduced the impact of biological evolution on human races. Anthropologist Shapiro,
has imagined that the man in future will be tall, slim and without body hair. Their skull will be dome-shaped
and brain will be large. Their life span will increase and the 5th digit in the feet will be lost. This future
race of man is named Homo futuris.

1160. • Anthropoids
• Australopithecine
• Chimpanzee
• Dryopithecus
• Gorilla
• Heidelber man
• Lumbar curve
• Orrorin tugensis
• Primates
• Visual acuity
KEY TERMS
• Apes
• Bipedalism
• Cooperative hunting
• Eyebrow ridges
• Hominins
• Human accelerated
• Noah's Ark Model
• Prognathous
• Punctuated equilibrium
• Ardipithecus kadabba
• Brachiatoon
• Cro-magnon man
• Gibbons
• Hominoids
• Homo candelabra model
• Neanderthal man
• Proconsul
• Human accelerated regions (HARs)
REVIEW QUESTIONS I. What evolutionary trends are seen during human evolution ?
2. Summarise major causes that favoured bipeda lism.
3. (a) What advantages did bipedalism offered to early hominin s?
(b) What were the anatomical changes introduc ed and perfected in human organisation for bipedalism?
4. (a) What do you mean by out of Africa concept about human evolution? (b) Who proposed this concept?
(c) With reasons justify whether this concept is right or wrong .
5. What reason can you suggest for the apparent absence of speciat ion in the recent evolutionary history of
mankind? Do you think that evolution in the future will be promoted if attempts are made to divide mankind
artificially into reproductively isolated populations, which might evolve into separate species?
6. Discuss the origin and evolution of Homo sapiens.
7. Discuss palaeontological in support of man.
8. Give an account of the fossil history of man.
9. Trace the trend in the human evolution with particular reference to recent fossil records.
10. (a) Why human ancestors shifted to omnivorous diet?
(b) What was the impact of omnivorou s diet on human evolution?
II . Describe evidences from molecular biology to show man and chimpanzee are closest relative .

1161. 12. Describe significance of Ramapithecus and Sivalika Hills with reference to human evolution .
13. Write short notes on the following:
(a) Place of origin of man
(b) Charac teristics of man

1163. 5 58 ~ Evolut ionary Biology
(c) Evolutionary trends in ape-like form to human form
(d) Cultural evolution of man.
1 4. Which came first in human evolution , large brain or upright posture? Support your answer.
IS. Give various theories to explain the disappearance of Neanderthal man.
FURTHER READINGS
I . Ann Gibbon s, 2009-10 , ' Ancient Skeleton May Rewrite Earliest Chapters of Human Evolution', Science
Magazine, Archived.
2. Betzer, M.A., M. Stonekin g, M. Alegria. Hartman , H. Bazan, et. aI., 1994. African Origin of Human
Specific Polymorph ic Alu Insertion s, Proc. Natl. Sci., USA, 91, 12288-12292.
3. Carlson, B., 2005. Human Embryology and Developmental Biology, 3rd ed. Mosby, Philadelphia, PA.
4. Fleagle, lG., 1988. Primate Adaptation and Evolution, Academic Press, San Diego, CA.
5. Hall, K. Brian, B.H. Grimsson, 2008. Strickbergers Evolution , Jones and Bartlett Publ., Sudburg,
Massachusetts.
6. Hammer, M.F., T Karafet, A. Rasanayagam, E.T Wood, et aI., 1998. Out of Africa and Back Again:
Nested Cladistic Analysis of Human Ychromo some Variation, Mol. BioI. Evol., 15, 427-441.
7. Harvati, K., and T Harrison (eds.), 2006. Neanderthals Revisited: New Approaches and Perspectives,
Springer, Netherla nds.
8. Henke, w. , and 1. Tattersall (eds.), 2007. Handbook of Pala eoanth ropology, Vol. I: Principles, Methods
and Approaches, Vol 2: Primate Evolution and Human Origins, Vol 3: Phylogeny of Hominins. Springer-
Verlag, New York.
9. Jones, S., R. Martin, and Pilbeam (eds.), 1992. The Cambridge Encyclopedia of Human Evolution,
Cambrid ge University Press, Cambridge, U.K. 10. Kingdon, J., 2004. Lowly Origin : Where, When and
Why Our Ancestors First Stood Up, Princeton University Press, Princeton, Nl.
11. Moore, K.L., and TVN. Persaud (eds.), 2003. The Developing Human, 7th ed., w.B. Saunders, New York.
12. Russell, M., 2003. Piltdown Man: The Secret Life of Charles Dawson and The Worlds Greatest
Archaeological Hoax, Tempus Publishing, Gloucestershire, U.K. 13. Samiento , Esteban E., 2010. Comment
on the Paleobiology and Classification of Ardipithecus ramidus, Science, 328.
14. Shipman, P., 2001. The Man Who Found the Missing Link: The Extraordinary Life of Sugene Dubois,
Simon and Schuster, New York.
IS. Wood Bernard, Harrison, Terry, 20 11. 'The Evolutionary Contest of the First Hominins', Nature, 470
(7334) pp. 347-352.
16. Wood, B., 2005. Human Evolution: A Very Short Introduction, Oxford University Press, Oxford, UK.

1165. D OD
Glossary
A
A crocentric: Chromosome whose centromere lies near one end (between metacentric and telocentric
locations).
Ada pta tion: Any trait that replaces other variants by means of selection for greater fitness (reproductive
success).
Adaptive landscap e: A model devised by Sewall Wright that describes a topography in which high fitnesses
correspond to peaks and low fitnesses to valleys; each position being occupied by a population bearing a
unique genotype.
Ada ptive peak: The allele frequency, or the set of allele frequencies that provide a population the maximum
fitness in its local environment.
Ada ptive ra diation: The diversification of a single species or group of related species into new ecological
or geographical zones to produce a large variety of species and groups.
Adaptive valley: The set of allele frequencies at which the mean fitnessof a population is minimum.
Ada ptive value: The relative reproductive success (relative fitness) of an allele or genotype as compared to
other alleles or genotypes .
Ada ptive zone: Similar ecological niches occupied by a group of related species.
Allometry: Differential growth rates of different body parts; during development one feature may change at
a rate different from that of another feature, resulting in a change of shape.
Allopatric speciation: Speciation of geographica lly separated populations of a species due to absence of
gene flow between them. Allopatric: Species or populations which occur in different geographical regions.
Allopolyploid : An organism or species that has more than two sets (2n) of chromosomes (i.e., 3n, 4n, etc.)
derived from different ancestral groups.
Allozyme: The particular form (amino acid sequence) of an enzyme produced by a particular allele at a
gene locus when there are different possible forms of the enzyme (different possible amino acid sequences),
each produced by a different allele.
Alternate splicing: Splicing of different sets of exons producing different nature transcripts of the mRNA
from the same gene. Different mature transcripts encode different proteins.
Altruism: Behaviour that benefits the reproductive success of other individuals because of an actual or
potential sacrifice of reproductive success by the altruist.
A nagenesis: The evolution of new species within a single lineage in the absence of branching (as opposed
to cladogenesis). See also Phyletic evolution .
Analogous structure: Structures that has similar function and superficially similar appearance due to
similar environmental pressures but has different ancestry and different basic structural plan.
Analogy: The possession of a similar character by two or more quite different species or groups without
common genetic ancestry.
Aneuploidy : The gain or loss of chromosomes from one set leading to a number that is not an exact
multiple of the basic haploid chromosome set (n) (e.g., n + I, 2n + I, 2n I, 2n 2, 2n + 3, etc.).
Aposematic: Conspicuous warning colouration in potential prey species that advertises their toxicity or
distastefulness to predators.
Archaebacter ia: One of the early cell forms that unlike eubacteria, do not incorporate muramic acid into
their cell walls and possess other distinguishing characteristics.
Archetype: The concept of an ideal primitive plan on which organisms, such as vertebrates , are presumably
based. Called by Richard Owen the "primal pattern" and "divine idea."

1166. Artificial selection: Selection process in which humans are the selective agents.
Autopolyploid: A species or organism that has more than two sets of chromosomes (polyploid) derived from
one or more duplications in a single ancestral source.
B
Ba ckgro und extinction: Slow rate of extinction of taxa at almost constant rate (opposite of sudden mass
extinction).
Balanced genetic load: The decrease in overall fitnessofa population caused by defective genotype (e.g.,
homozygotes for deleterious recessives) whose alleles persist in the population because they confer
selective advantages in heterozygous condition (e.g., heterozygote advantages).
Balanced polymorphism: The persistence of two or more different genetic forms or alleles in a population,
favoured by natural selection. (e.g., heterozygote advantage).
Bat esian mimicry: The similarity in appearance of a harmless species (the mimic) to a species that is
harmful or distasteful to predators (the model).
Big ban g theory: The concept that the Universe was born in a gigantic explosion about 10 to 20 billion
years ago.
Biogenetic law (Haeckel): The concept that stages in the development of an individual (ontogeny)
recapitulate the evolutionary history (phylogeny) of the species.
Biological species concept: The view that the primary criterion for separating one species from another is
their reproductive isolation.
Bottleneck effect: A form of genetic drift that occurs when a population is reduced in size (population
crash) and later expands in numbers (population flush). The enlarged population that results may have
gene frequencies that are distinctly different from the parental population neck (See also Founder effect).
Br achiation: Locomotion through trees by hanging from branches and swinging alternate arms from
branch to branch.
Bradytelic: A relatively slow evolutionary rate.
Burrowing animal: In aquatic forms, a bottom dweller that moves through soft benthic sediments.
C
Cambrian period: The interval between about 545 and 505 million years, marked by abundance of fossil
organisms with hard skeletons.
CamouOage: Colouration or shape of an organism that renders it inconspicuous in its environment or helps
in blending with the surroundings.
Canalisation: Evolution of internal factors that operate during development.
Catastrophism: Hypothesis that fossilised organisms and changes in geological strata were produced by
periodic, violent, and widespread catastrophic events, caused by capricious supernatural forces.
Chromosome aberration: A change in the gene sequence of a chromosome caused by deletion, duplication,
inversion, or translocation.
Clade: A cluster of taxa derived from a single common ancestor.
Cladistics: A mode of classification based principally on grouping taxa by their similar ("derived")
characters that differ from the ancestral condition.
Cladogenesis: "Branching" evolution involving the splitting and divergence of a lineage into two or more
lineages.
Cladogram: A tree diagram representing phylogenetic relationships among taxa.
Cline: A gradient of phenotypic or genotypic change in a population or species correlated with the direction
or orientation of some environmental feature, such as a river, mountain range, north-south transect,
altitude, and so forth.
Clone: A group of organisms all derived by asexual reproduction from a single ancestral individual.

1167. Coacervate: An aggregation of colloidal particles in liquid phase that persists for a period of time as
suspended membranous droplets.
Coadaptation: The action of selection in producing adaptive combinations of alleles at two or more
different gene loci.
Coenozoic era: The period from 65 million years ago to the present, marked by the radiation of mammals.
This is the third and most recent era of the Phanerozoic eon and is divided into two major periods, the
tertiary and quarternary.
Coevolution: Evolution of adaptaions in two species due to their interactions with one another or their
interdependence.
Competitive exclusion: Extinction of a population due to competition with another species.
Concerted evolution: The process by which a series of nucleotide sequences or different members of a gene
family remain similar or identical through time.
Condensation (by dehydration): The formation of a covalent bond between two molecules by removal of
H
20
.
Condylarths: The earliest herbivorous placental mammals and the ancestors of all later herbivores that
became extinct during the Miocene Period of the Coenozoic Era but whose first occurrences are in the late
Cretaceous Period of the Mesozoic.
Conspecific: Belonging to the same species.
Continental drift: The movement over time of large landmasses on the Earth's surface relative to each other.
Continuous variation: Character variations, such as height in humans, whose distribution follows a series
of small nondiscrete quantitative steps from one extreme to the other.
Convergence (also called Homoplasy): The evolution of similar characters in genetically unrelated species,
mostly because they have been subjected to similar environmental selection pressures (See also Analogy).
Creationism: Hypothesis that all species (living or fossil) on Earth were created in their present form by
some supernatural being and that they have remained unchanged; their transformation into new species
cannot occur.
C rea tionism: The belief that each different kind of organism was individually created by one or more
supernatural beings whose activities are not controlled by known physical, chemical or biological laws.
Cr eodonts: An extinct order of early Coenozoic placental mammals that were the dominant carnivores
until replaced by the modem order Carnivora during the Oligocene Period.
Cryptic: A feature that is normally not visible.
Cu sps (teeth): Elevations on the crow ns of premolars and molars. The number, shapes and positions of
cusps are inherited characters that can provide useful phylogenetic information.
Cytochromes: Proteins containing iron-porphyrin (heme) complexes that function as hydrogen or electron
carriers in respiration and photosynthesis.
D
Darwinism: Th e concept proposed by Charles Darwin that biological evolution has led to the many
different highly adapted species through natural selection acting on hereditary variation in populations.
Deleterious allele: An aberration in which a section of DNA or chromosome is lost.
Deletion mutation : A mutation produced due to loss of one or more pairs of nucleotides from the
polynucleotid e chains of a DNA molecule (gene).
Deme: A local population of a species of a local interbreeding group.
Density dependent: The dependence of population growth and size on factors directly related to the
numbers of individuals in a particular locality.

1168. Density independent: The dependence of population growth on factors such as (clima tic changes, meteorite
impacts, etc.) unrelated to the numbers of individuals in a particular locality.
Directional selection: Selection or favour of one extreme phenotype over others and causes the phenotype a
characte r to shift towards one of its phenotypic extremes.
Discontinuous variations: Character variations that are sufficiently different from each other that they fall
into nonoverlapping classes.
Disruptive selection: Selection that tends to favour the survival of organisms in a population that have
phenotype extremes for a particular character and eliminates individuals with intermediate values (cent
rifugal selection).
Divergent evolution: Evolutionary changes which two lineages become more and more different with the
passage of time.
Duplication: Instances in which a particular section of DNA or visible chromosome segment occurs more
than once.
E
Endosymbiont hypothe sis: Hypothesis that favours the origm of certa in cell organelles of eukaryotic cells
(like mitochondria and chloroplasts) due to mutual or symbiotic associa tion between early preeukaryotic
cell and bacteria.
En vironmental variance: Phenotyp ic variations among individuals caused by variation in the environment.
Eon : A major division of the geological time scale, often divided into two eons beginning from the origin of
the earth 4.5 billion years ago; the Precambrian or Cryptozoic (rarity of life forms) and the Phanerozoic
(abundance of life forms).
Epigamic selection: Selection for mating success based on appearance or bahaviour dur ing courtship.
Epigenesis: The concep t that tissues and organs are formed by interact ion between
c ells and substances that appear during developm ent, rather than being initially present in the zygote
(preformed).
Epoch: One of the categories of geological time sca le, a subdivision of a geological period.
Equilibrium (genetic): The persistence of the same allelic frequencies over a series of generations.
Equilibria may be stable or unstable. In a stable equ ilibrium (e.g., when the heterozygote is superior in
fitness to the homozygote), the population returns to a particul ar equilibrium value when the allelic
frequencies have been disturbed . In an unstable equilibrium (e.g., when the heterozygote is inferior in
fitness to the homozygotes), such disturban ces are not followed by a return to equilibrium frequencies .
Era: A divisio n of geological time that stands between the eon and the period; the Phanerozoic eon is
divided into Paleozoic, Mesozoic, and Coe nozoic Eras and each era is divided into two or more periods.
Euploidy: Variations that involve changes in the number of entire chromosome sets (n, 2n, 3n, 4n, 5n, etc.).
Evolution: Genetic changes in populations of organisms through time that lead to differences among them.
Evolutionary (molecular) clock: The concept that the rate at which mutational changes accumulate is
constant over time. To which genes or genomes this clock may apply, and whether it is really constant, are
disputed.
Exon shuffling: Formation of new genes by new combinations of exons from two or more preexisti ng genes
via recombination of intervening introns or by retrotransposition of exons.
Extinction: The disa ppearance of a species or higher taxon.
F
Family: A taxonomic category that stands between order and genus; an order may comprise a number of
families, each of which contains a number of genera.
Fauna: All animals of a particular region or time period.

1169. Fecundity: Quantity of eggs produced by a female of a population.
Fitness: Relative reproductive success as measured by selection in a particular environment: the ability of
an organism (genotype) to transmit its genes to the next reproductively fertile generation relative to this
ability in othe r genotypes in the same environment. Since there are causes other than selection that
influence genotype frequencies (e.g., mutation, random genet ic drift, migration), fitness is not the only
cause for (short term) surviva l.
Fixation: Achieveme nt of a frequency of 100 per cent (i.e., monomorphism) by an allele or genotype that
begins in a populati on at a lesser frequency (i.e., polymorphism).
Fixity of species: A concept held by Linnaeus and others that members of a species could only produce
progeny like themselves, and, therefore, each species was fixed in its particular form(s) at the time of its
creation. Fossils: The geological remains, impressions or traces of organisms that existed in the past.
Founder effect: The effect caused by a sampling acc ident in which only a few "founders" derived from a
large population begin a new colony. Since these founders carry only a sma ll fraction of the parental
population's gene tic variability, radically different gene frequencies can become established in the new
colony.
Frameshift mutation: A mutationa l change in the gene caused by the insertio n or deletion of one or more
nucleotide pairs; that changes the reading frame of all the codo ns following the mutation site.
Frameshift mutation: Shifting of the triplet codon reading frame during protein synthesis due to insertion or
deletion of one or two nucleotides in the coding sequence.
Fr eq uency-dependent selection: Instances where the effect of selection on a phenotype or genotype
depends on its frequency (e.g., a genotype that is rare may have a higher adaptive value than when it is
common).
Fundamentalism (religious) : The belief that creation stories and the many events and rules given in
religious documents are to be taken literally.
G
Gametic inco mpatibility: The inability of sperm from one species to fertilise eggs of another species.
Gene family: Two or more gene loci in an organism whose similarities in nucleotide sequences indicate
they have been derived by duplication from a common ancestral gene (e.g., the bglobin gene family, which
includes b, g, d, and e genes).
Gene family: Two or more genes with similar nucleotide sequence aligned close together on the same
chromosome.
Gene flow: The migration of genes into a population from other populations by interbreeding.
Gene frequency: The proportion of a particular allele among all alleles at a gene locus (Also called allele
or allelic fre quency).
Gene locus: The chromosomal position (nucleotide sequence) occupied by a particular gene.
Gene pool: The total of all alleles of all the genes in a population .
Gene pool : Total number of genes of sexual population.
Genealogy: A record of familial ties and ancestral connections between members of a group.
Genetic death: The inability of a genotype to reproduce itself because of selection.
Genetic dista nce: A measure of the divergence between populations based on their differences in
frequencies of given alleles.
Genetic drift: A change in the allele frequency of a small population purely by chance.
Genetic drift: Random changes in the allele frequencies or genotype frequencies within the gene pool of a
population.
Genetic equilibrium: A situation in which the allele frequencies remain constant from generation to
generation.
Genetic load : The loss in average fitness of individuals in a population because the population carries

1170. deleterious alleles or genotypes.
Genetic poly morphism: The presence of two or more alleles at a gene locus over a successio n of
generations (called balanced polym orphism when the persistence of the different alleles can not be
accounted for by mutation alone).
Genome: The complete genetic constitution of a cell or an individual.
Genotype: The genetic constitution of cells or individuals, often referring to alleles of one or more specified
genes.
Gen us (plu ra l, genera): A taxonom ic category that stands between family and species: a family may
comprise a number of genera, each of which contains a number of species that are presumably related to
each other by descent from a common ancestor. In taxonomic binomial nomenclature, the genus is used as
the first of two words in naming a species; for example, Homo (genus), sapiens (species).
Geographic isolation: The separation between populations caused by geographic distance or geograp hic
barriers.
Geogra phic speciation: See Allopatric speciation.
Geo logical time sca le: The correlation between rocks (or the fossils contained in them) and time periods of
the past.
Germinal mutation: A change in the gene or chromosome of a cell from which gamete s are formed; these
mutations are heritable (Germ-line mutation).
Germplasm: Cells or tissues in a multicellular organism that are exclusively devoted to transmitting
hereditary information to offspring, either asexually or by means of gametes (sex cells). These cells are in
contrast to the somatic cells that produce the non-germ-Iine body tissues.
Glucose-6-phosphate dehydrogenase (G6PD): An enzyme used in huamn beings during metabolism of
glucose. G6PD deficiency causes a form of anaemia in which RBCs are prone to burst.
Gondwana: The supercontinent in the Southern Hemisphere formed from the break up of the larger
Pangaea landmass about 180 million years ago. Gondwana was composed of what is now South America ,
Africa, Antarctica, Australia, and India.
Grade: A level of phenotype organisation or adaptation reached by one or more species. Distantly related
or unrelated species that reach the same grade are considered to have undergone parallel or convergent
evolution.
Gradualism: Darwin's view that evolution proceeds by small succes sive changes through intermediate
forms rather by large leaps.
Group selection: Selection acting on the attributes of a group of related individuals in competition with
other groups rather than only on the attributes of an individual in competition with other individuals. For
example, altruism may not be beneficial to the individual altruist but can be quite beneficial to a group
containing altruists. The fitness of an individual in such a group is thus, at least partially, associated with
the properties of the group.
H
Haeckel's gastrula hypothesis: The concept that metazoans developed from swimm ing hollowballed
colonies of flagellated protozoans.
Half-life (radioactivity): The time required for the decay of one-half the original amount of a radioactive
isotope.
Haplo-diploidy: A reproductive system found in some animals, such as bees and wasps, in which males
develop from unfertilised eggs and are therefore haploid, while females develop from fertilised eggs and are
therefore diploid.
Haploid: Cells or organisms that have only one set (n) of chromosomes , meaning the presence of only a
single allele for each gene.
Haplotype: A sequence of nucleotides, restriction sites, or marker genes inherited as a linked unit from one

1171. parent. Since more than a single genetic locus may be involved, a haplotype may be composed of a string of
alleles.
Hardy-Weinberg principle: The conservation of gene (allelic) and genotype frequencies in large populations
under conditions of random mating and in the absence of evolutionary forces, such as selection, migration,
and genetic drift, which act to change gene frequencies.
Heterochrony: A term Haeckel originally proposed, to describe changes in timing of an organ's
development during evolution.
Heterodont: An organism with structural and functional differences among its teeth.
Heterokaryotype: A genome heterozygous for a chromosomal rearrangement.

1173. Heterosis (hybrid vigo ur): The increase in vigour and performance that can result when two different, often
inbred strains are crossed. Since each inbred parental strain may be homozygous for different deleterious
recessive alleles (e.g., alai x ap2)' the cause for heterosis has been ascribed by some authors to the
superiority of heterozygotes (e.g., a
1a 2) .
Heterozygote advantage (superiority): The superior fitness of some heterozygotes (e.g., AIA2) relative to
homozygotes (e.g., AlAI, or A2A2).
H eterozygote: A genotype or individual that possesses different alleles at a particular gene locus on
homologous chromosomes (e.g., Aa in a diploid).
Hierarchy: A term used in some evolutionary and developmental studies to designate increasing levels of
complexity or organisation.
Hitchhiking genetic: Adaptive fixation of an advantageous mutant and the associated fixation of linked,
neutral variants.
Hitc hh iking: Change in an allele frequency due to linkage with a selected allele at another locus.
Hominid: A member of the family Hominidae, which includes humans, whose earliest fossils can now be
dated to about 4 million years ago (genus Australopithecusi. Only a single hominid species (Homo sapiens)
exists.
Hominin: Modem humans and our ancestors.
Hominoids: A group (superfamily Hominoidea ) that includes hominids (Hominidae), gibbons
(Hylobatidae), and apes (Pongidae).
Homology: The similarity of biological features in different species of groups because of their descent from
a common ancestor (i.e., homology = degree of ancestral similarity).
Homoplasy: Parallel evolution of two or more different species or groups having similar or identical
characters to carryout similar function.
Horizontal transmission: Transmission of genes between organism other than by transmission from parents
to their offsp rings (which is vertical transmission); also called lateral gene transfer. Hybrid breakdown,
inviability, sterility: Hybrids that suffer from loss of fitness and reproductive failure.
Hybrid s (hybridisation): Offspring of a cros s between genetically different pa rents or

1174. groups. ..
Ign eous rock : A rock such as basalt (fine-grained) and granite (coarse-grained), formed by the cooling of
molten material from the Earth's interior.
Inbreedin g coefficient (F) : The probability that the two alleles of a gene in a diploid organism are
identical because they originated from a single allele in a common ancestor.
Inbreeding depression: Decrease in the average value of a character, or in growth, vigour, fertility, and
survival, as a result of inbreeding.
Inbreeding: Mating betw een ge netically rela ted indiv idua ls, often resulting in incr eased homozygosity
in their offspring.
Inclusive fitness: The fitness of an allele of genotype measured not only by its effect on an individual but
also by its effect on related individuals that also possess it (kin selection).
Industrial melanism: The effect of soot and pollution in industrial areas in increasing the frequency of
darkly pigmented (melanic) forms because of selection by predators against non-pigmented or lightly
pigmented forms.
Inheritance of acquired characters: The concept used by Lamarck to explain evolutionary adaptations that
phenotypic characters acquired by interaction with the environment during the lifetime of an individual are
transmitted to its offspring.
Interdemic selection: Group selection of populations of a species.
Introgressive hybridisation: The incorporation of genes from one species into the gene pool of another
because some fertile hybrids are produced from crosses between the two species.
Inversion loop: Loop-like structure formed by the synapsis of two homologous chromosomes one of which
has an inversion.
Inversion: An aberration in which a section of DNA or chromosome has been inverted 180 degrees, so that
the sequence of nucleotid es or genes within the inversion is now reversed with respect to its original order
in the DNA or chromosome.
Isochromosome: A chromoso me with two identical arms containing homologous genes.
Iso lating mec ha nisms : Biological mechani sms that act as barriers to gene exchange between populati
ons.
Isoto pe : One of several forms of element, with a distinctive mass based on the number of neutron s in the
atomic nucleus (The number of proton s and electrons is the same in different isotopes of an element).
Radioactive isotopes decay at a rate that is constant for each isotope and release ionising radiation as they
decay.
K
Karyotype: The characteristic chromosome complement of a cell, individual, or species. Kin selection :
Selection effects (e.g., altruism) that influence the survival and reproducti ve success of genetically related
individuals (kin). This contra sts with selection confined solely to an individual and its own offspring (See
also Inclusive fitness).
Kingdo m: The highest inclusive category of taxonomic classification, each kingdom including phyla or
subkingdoms. The most common presently used classification system proposes five kingdoms: Monera
(prokaryotes), Protista, Fungi, Anim alia, and Plantae.
Knocko ut mu tation: A mutation that compl etely disrupts the function ing of the gene, also called null
mutation or loss of functio n mutatio n or amorphic mutatio n.

1175. Kn uckle-walki ng: Quadruped al gait of chimpanzees and gorillas, perform ed by curling the fingers
towards the palm of the hand and using the backs (dorsal surfaces) of the kunckles to support the weight of
the front part of the body.
K-selection: Selection based on a population be capacity ; selection is therefore theoretically

1176. rapid numerical increase. ..
La dder of nature: A concept based on Aristotle's view (the Scale of Nature) that nature can be represented
as a succession of stages or ranks that leads from inanimate matter through plants, lower animals, higher
animals, and finally to the level of humans.
La marckian inh eri tan ce: The characters acquired or lost during the life experience of an organism, as
well as characters that organisms attempt to acquire in order to meet environmental needs, can be
transmitted to offspring.
La urasia: The supercontinent in the Northern Hemis phere (comprising what is now North America,
Greenland, Europe, and parts of Asia) formed from the break up of Pangaea about 180 million years ago.
Letha l mutation: A mutation that results in the death of affected organism.
Lineage: A series of ancestral and descendant populations through time.
Living fossil : An existing species whose similarity to ancient ancestral species indicates that very
few morphological changes have occurred over a long period of geological time.
Loca l pop ulation : A group of organisms of the same species occupying the same area where most of the
individuals find their mates, also called demes or Me nde lia n populati on .
Loc us (plura l loci): The site (nucleotide sequence) on a chromo some occupied by a specific gene.
Luca: Last universal common ancestor of all living things on this Earth.
maintained at or near the limit of its carrying
for improved competi tive abi lity rather than for
M
Macroevo lution: Evolution of taxa higher than the species level (e.g., genera, families, orders, classes),
commonly entailing major morphological changes.
Macromutat ion: The concept that there are single mutations whose effects are large enough to produce an
instantane ous new species or perhaps even to produce organisms that signify the beginni ng of a higher
taxonomic category.
Marsu pials: Mammals of the infraclass Metatheria possessing a reproducti ve process in which tiny young
are born, and then nursed in a female pouch (marsupium ).
Mass ext inction: Large scale extinction at a fast pace over a relatively short geological time scale.
Mesozoic era: The middle era of the Phanerozoic eon, covering the approximately 180-million years
interval between the Paleozoic (ending about 245 million years ago) and Coenozoic.
Metacentric: A chromosome whose centromere is at or near centre.
Metamorphic rock : Rock that has been subjected to high but nonmelting temperatures and pressures,
causing chemical and physical changes.
Metapopulati on: Local population s that have gene flow and exhibit patterns of extinction and recolon
isation.
Microevolution: Evolutionary changes of the kinds usually responsible for causing differences between
populations of a species (e.g., gene frequency changes and chromo somal variations). Many evolutionists
suggest that accumulations of such changes over time are sufficient to explain the origin of most or all taxa.
Microsa tellites : Tandem repeats of short di, tri-, and tetranucleotide sequences such as
cytosineadeninecytosine-adenine-cytosine-adenine, and so on. Such loci are abundant (humans are
estimated to possess at least 35,000 loci for repeats of the C-A sequence) and mutate (change in sequence

1177. number ) at a relatively high rate.
Microspheres : Microscopi c membran e-bound spheres formed when proteinoid s are boiled in water and
allowed to cool. Some cell-like propertie s, such as osmosi s, growth in size, and selective absorption of
chem icals have been ascribed to them.
Mimicry: Resemblance of individuals in one species (mimic) to individuals in another (models).
Missense muta tion: A change is coding sequence of DNA that results in an amino acid replacement in the
polypeptide chain.
Monomorphic: A population or species that shows no genetic or phenotypic variation for a particular gene
or character.
Monophyletic: Derivation of a taxonomic group from a single ancestral lineage.
Monotremes: Egg-laying mammals, presently restricted to Asutralsia; the platypus (Ornithorhyncus i and
Echidna spiny anteater (Tachyglossus, Zaglossusy.
Mulleria n mimic ry : Sharing of a common warning coloration or pattern among a number of species that
are all dangerous or toxic to predators.
Multigene fa mily: See Ge ne famil y.
Mutagen: An agent that introduce s mutation or increases mutation rate.
Mutation r at e: The probability of appearance of a new mutation in a particular gene per gamete or per
generation.
Mutation: A change in the nucleot ide sequen ce of genetic material whether by substitution, duplication,
insertion, deletion or inversion.
Mutational load : That portion of the genetic load caused by production of deleterious genes through
recurrent mutation.
Mutua lism: A relationsh ip between different species in which the participants benefit.

1179. N
N atura l selection: Differential reproduction or survival ofreplicating organisms caused by agencies that
are not directed by humans (See Artificial Selectio n). Since such differential selective effects are widely
prevalent , and often act on hereditary (genetic) variations, natural selection is a common major cause for
a change in the gene frequencies of a populat ion that lead to a new distincti ve genetic constitution
(evolution) (See a/so Fit ness, Selection).
Neo -Darwinism: The theory (also called the Modem Synthesis) that regards evolution as a change in the
frequencie s of genes introduced by mutation , with natural selection considered as the most important,
although not the only cause for such changes.
Ne utral mu tation : A mutation that does not affect the fitness of an organism in a particular environment.
Neutra l theory of molecular evolution: The concept that most mutations that contribute to genetic
variability (genetic polymorphism on the molecular level) consists of alleles that are neutral in respect to
the fitness of the organism and that their frequencie s can be explained in terms of mutation rate and
random genetic drift.
Nondisj unction: Failure of homologous chromosomes to separate (disjoin) and move to opposite poles of
the spindle during anaphase of meiosis; results in loss or gain of a chromosom e in the daughter cells.
Nonse nse mu tation : A mutation that produces a codon that term inates the translation of a polypeptide
prematurely. Such codons were previously called "nonsense" codons but are now generally called stop
codons or chain termination codons.
Null mu tati on: A mutation that renders the gene completel y nonfunctional by introducing a stop codon
and causes prematur e termination of polypeptide chain; also called knockout mutation, amorphic mutation
, loss of function mutation or nonsense mutation.
Numerical (pheneti c) ta xonomy: A statistical method for classifying organisms by comparing them on the
basis of measurable phenotypi c characters and giving each character equal weight. The degree of overall
similarity between individuals or groups is then calculated, and a decision is made as to their classification.
O
Ort hogenes is: The concept that evolution of a group ofrelated species proceeds in a particular direction
(e.g., an increase in size) because of unknown internal or vitalistic causes rather than because of
nonmystical factors such as selection.
O rthologous genes : Gene loci in different species that are sufficiently similar in their nucleotide sequences
(or amino acid sequences of their protein products) to suggest they originated from a common ancestral
gene.
P
Paedomorphosis: The incorporation of adult sexual features into immature developmental stages.
P alaeontology: The study of extinct fossil organisms.
Palaeozoic: The first era of the Phanerozoic eon, extending from 545 to about 250 million years ago.
Paleom agn etism : The magnetic fields of ferrous (ironcontaining) material s in ancient rocks.
Am ong other application s, paleomagneti sm provides information on the position of landmasses and
continents relative to the earth 's magnetic poles at the time that the rocks were formed and thus can be
used to describe the historical movement of continents relative to each other (continental drift).

1181. 570-
~
Evolutionary Biology
Pangaea: A very large supercontinent formed about 250 million years ago comprising most or all of the
present continental landmasses (See Gon dwana, La urasia).
Pangenesis: The concept of heredity, held by Darwin and others, that small, particulate "gemmules," or
"pangenes," are produced by each of the various tissues of an organism and sent to the gonads where they
are incorporated into gametes. The increase or decrease of specific gemmules during the use or disuse of
organs was proposed in order to explain the Lamarckian concept of inheritance of acquired character.
Panspermia: The concept that life was introduced on earth from elsewhere in the universe.
Paracentric inversion: Inversion in which centromere of the chromo some lies outside the inverted segment.
Parallel evolution: The evolution of similar characters in related lineages whose common ancestor was
phenotypically different (See also Convergence).
Pa ralogous genes: Two or more different gene loci in the same organism that are sufficiently similar in
their nucleotide sequences (or in the amino acid sequences of their protein products) to indicate that they
originated from one or more duplications of a common ancestral gene (See also Ge ne family).
Parapatri c: Geographically adjacent species or populations whose distribution s do not overlap but are in
contact at one or more of their mutual boundaries.
Par aph yletic: A taxonomic grouping which includes some of the descendants of a single common ancestor,
but not all.
Parsimony met hod: Choice of a phylogenetic tree that minimises the number of evolutionary changes
necessary to explain species divergence.
Perice ntr ic inversion: An inversion in which the segment that undergoes inversion has a centromere.
Period (geological): A major subdivision of an era of geological time distingui shed by a particular system
of rocks and associated fossils. Thus, the Cretaceous Period is named after the abundanc e of chalk (Latin,
creta), and the Carboniferou s Period for the abundance of coal (Latin, carbo) in their component rock
systems.
Peripa tr ic: Populations that bud off from the geographic periphery of a parental population .
Pha ner ozoic eon : A major division of the geological time scale marked by the relatively abundant
appearance of fossilised skeletons of multicellular organisms, dating from about 590 million years ago to
the present.
Ph enetic: Referring to phenotypic characters that can be described or measured. Also a system of
classification that groups taxa by their degree of similarity for measured or numerically evaluated
characters.
Ph yletic evolution: Evolutionary changes within a single nonbranching lineage. Although new species are
produced by this lineage over time (chron ospecies) there is no increase in the number of species existing at
anyone time (See also Anage nesis).
Phyloge netic evolut ion: Evolutionary changes that produce two or more lineages that diverge from a
single ancestral lineage (Also called "branching evolution ," see Cladogenesis).
Phylogeny: The evolutionary history of a species or group of species in terms of their derivations and
relationships.
A phyl ogenetic tree is a schema tic diagram that rep rese nts th e evolution .
Plate tecton ics: The concept that the earth's crust is divided into a number of fairly rigid plates, whose
movement (tectonics) relative to each other are responsible for continental drift and many crustal features
(See also Tectonic plates ).
Pleiotropy: Instances when a single gene produces phenotypic effects on more than one character.
Plesiomorphy: Instances when a species character is similar to that character in an ancestral species.

1183. Pol ygen e: A gene that interacts with other polygenes to produce a quantitative phenotypic effect on a
character.
Polym er: A molecule composed of many repeating subunits (monome rs) linked together by covalent bonds.
Polymorphism: The presence of two or more genetic or phenotypic variants in a population. Usually refers
to genetic variations where the frequency of the rarest type is not maintained by mutation alone. (See also
Balanced polymorphism).
Polyph yletic: The presumed derivation of a single taxonomic group from two or more different ancestral
lineages through converge nt or parallel evolution.
Polyploidy: Variations in which the number of chromoso me sets (n) is greater than the diploid number (2n)
(e.g., triploidy (3n), tetraploidy (4n), etc.
Population bottleneck: A form of genetic drift in which allele frequencies change in subsequent generations
because of extremely small size.
Population genetics: Study of frequency, distribution, and inheritance of alleles in a population.
Population: A geographically localised group of individuals of a species that are found in the same place
and time and interbreed and share a common gene pool.
Preadaptation: A character that was adaptive under a prior set of conditions and later provides the initial
stage for evolution of a new adaptation under a different set of conditions.
Precambrian eon: A major division of the geologica l time scale that includes all eras from the origin of the
earth about 4.5 billion years ago to the beginning of the Phanerozoic eon, about 590 million years ago. The
Precambrian (also known as the Cryptozoic) is marked biologically by the appearance of prokaryotes about
3.5 billion years ago and small, nonskeletonised multicellular organisms in the Ediacarian Period about 50
or 60 million years before the Phanerozoic.
Predation: The killing and consumption of one living organism (prey) by another (predator).
Preformationism: The concept that an organism is preformed at conce ption in the form of a miniature adult
and development consists of enlargement of the already preformed structures.
Pre mating isolating mechanisms: Any structure, physiological function, or behaviour that prevents
interbreeding among individuals of two different populations.
Progenote: A hypothetical ancestral cellular form that gave rise to prokaryotes (archaebacteria,
eubacteria) and eukaryotes.
Prokaryotes: Organisms such as bacteria and cyanobacteria that lack histone-bound DNA, endoplasmic
reticulum, a membrane-enclosed nucleus, and other cellular organelles found in eukaryotes.
Proteinoids: Synthetic polymers produced by heating a mixture of amino acids. Some show protein like
properties in respect to enzyme activity, colour test reactions, hormonal activity, and so on.
Punctuated equilibrium: The view that evolution of a lineage follows a pattern of long intervals in which
there is relatively little change (stasis or equilibrium), punctuated by short bursts of speciation and
macroevolutionary events during which new taxa arise.
Q
Quantum evolution: A rapid increase in the rate of evolution over a relatively short period of time.
R
Race: A population or group of populations in a species that share a geographically and/or ecologically
identifiable origin and have unique gene freqencies and phenotypic characters that distinguish them from
other races.

1185. Radi oacti ve dating: The dating of rocks by measuring the proportions of a radioactive element in an
igneous intrusion and the isotopes produced by its radioactive decay.
Radioactivity: Emission of radiation by certain elements as their atomic nuclei undergo changes.
Random genetic drift: The random change in frequency of alleles in a population. These can be caused by
sampling errors that are of greater magnitude in small populations or by bottle-necks (founder effects)
when population size is suddenly reduced to a few individuals.
Random mating: Mating within a population regardless of the phenotype or genotype of the sexual partner
(panmixis).
Recessive lethal: An allele whose presence in homozygous condition causes lethality.
Red Queen hypothesis: The view that adaptive evolution in one species of a community causes a
deterioration of the environment of other species. As a consequence, each species must evolve as fast as it
can in order to survive.
Reproductive isolation: The absence of gene exchange between populations (See Isolating mechanisms).
Reproductive success: The proportion of reproductively fertile offspring produced by a genotype relative to
other genotypes (See a/so Fitness).
Robertsonian translocation: Translocation (a chromosomal aberration) in which the long arms of two
acrocentric chromosomes join to a common centromere.
Roen tgen: A unit of ionising radiation; defined as the amount of radiation resulting in 2.083 x 109 ion
pairs per cm3 of dry air at O°C and I atm pressure.
R-selection: Selection in populations subjec t to rapidly changing environm ents with highly fluctuating
food resources.
S
Saltation: Th e concept that new species or higher taxa originate abruptly because of macromutations, or
because of sudden unknown causes.
Sampling error (gene frequencies): Variability in gene frequencies caused by the fact that not all samples
taken from a population have exactly the same gene frequency as the population itself.
Sedimentary rock: Rock formed by the hardening of accumulated particles (sediments) that had been
transported by wind and water.
Segregational genetic load : See Balanced genetic load .
Selection coefficient (s): A relative measure of the effect of selection, usually in terms of the loss of fitness
endured by a genotype, given that the genotype with greatest fitness has a value of I .
Selection: All the forces that cause differential survival and differential reproduction among genetic
variants. When the selective agencies are primarily those of human choice, the process is called artificial
selection, when the selective agencies are not those of human choice, it is called natural selection.
Serial homology: Similarities between parts of the same organism, such as the vertebrae of a vertebrate or
the different kinds of haemoglobin molecules produced by a mammal. The genetic basis for such homology
can often be ascribed to gene duplication s that have diverged over time but still produce somewhat similar
effect. (See Gene famil y, Paralogous genes ).
Sexual selection: Selection that acts directly on mating success through direct competition between
members of one sex for mates (intrasexual selection).
Sibling species: Species so similar to each other morphologically that they are difficult to distinguish but
are reproductively isolated (Sometimes called "cryptic species") .

1187. S peciation: The splitting of one species into two or more new species (See Cladogenesis, Phylogenetic
evolution) or the transformation of one species into a new species over time (See Anage nesis, Phyletic
evolution).
S pecies: A basic taxonomic category or an interbreeding or potentially interbreeding group of populations
reproductively isolated from other such groups (the biological species concept) and a lineage evolving
separately from others with its own unitary evolutionary role and tendencies (Simpson's evolutionary
species concept).
S pontaneous gener ation: An early concept that complex organisms can appear spontaneously from inert
materials without biological parentage.
Sta bilising selection: Selection that favours the survival of organisms in a population that are at an
intermediate phenotypic value for a particu lar character and eliminates the extreme phenotypes.
Stasis: A period of equilibrium during which change appears to be absent; e.g., in the concept of
punctuated equilibrium (See also Stabilising selection).
Subspecies: A taxonomic subdivision of a species often distinguished by special phenotypic characters and
by its origin or localisation in a given geographical region whose members can still interbreed successfully
with the remainde r of the species.
Sympatric speciation: A speciation that occurs in populations that are not separated physically or
geographically. It occurs suddenly due to chromosomal aberration or changes in the number of
chromosomes.
Sympatric: Species or populations whose geographical distributions overlap (in the same place).
Synonymous mutation: A change in the gene 's nucleotide sequence that does not alter the amino acid
sequence in the mutated protein (codes for the same amino acid which was present in the normal protein) .
T
T achytelic: A relative ly rapid evolutionary rate.
Targeted mutations: Site specific mutations targeting specific gene/genes.
Tautomer : A nucleic acid base in which a hydrogen atom has moved from one position to another
leading to a change in its pairing relationships .
Ta utomeric shift: A reversible change in the location of a proton that alters a molecule from one form to
another.
Tectonic plat es: The fairly rigid plates composing the earth's crust whose boundaries are marked by
earthquake belts and volcanic chains. In oceanic regions, accrelations to these plates occur at midoceanic
ridges (sea floor spreading), and they are subdueted under other plates at the deep oceanic trenches.
Continental masses ride on some of these plates, accounting for continental drift and such processes as the
mountain building that occurs when these plates collide.
Teleology: The concept that natural processes such as development or evolution are guided by their final
stage (telos) or for some particular purpose .
Telocentric: Chromosomes whose centromeres are located at one end.
Tet raso my: A state when an organism or species has four copies of one or more chromosomes .
T hera psids: An order of synapsid mammal-like reptiles, which were dominant reptilian stocks during the
Permian and Triassic Periods.
Tra nsition: Base pair substitution of a purine (adenine or guanine) with another purine, or of a pyrimidine
(thymine and cytosine) with another pyrimidine .
Tra nslocation: A chromosomal aberration in which a sequence of nucleotides is moved to a different
position in the genome.

1189. Transversion : Base pair substitution of a purine with a pyrimidine, or vice-versa. Typology: The study of
organic diversity based on the principle that all members of a taxonomic group conform to a basic plan and
variation among them is of little or no significance.
T
Ultraviol et radiation: Elec tromag netic radia tion at wavelengths between about 4 and 400 nanometres,
shorter than visible light but longer than X rays. It is absorbed by purine and pyrimid ine ring structures
and is therefore quite damaging to nucleic acid (genetic material).
Unequal cro ssing over: The result of improper pairing between chromatids, causing their crossove r
products to differ from each other in the amounts of genetic material.
Unifor mitar ia nism: A concept, popularised by Lyell in geology, that none of the forces active in past
Earth history were different from those active today.
Use and disuse: A concept used by Lamarck to explain evolution as resulting from the transmissio n of
characters that became enhanced or diminished because of their use or disuse respective ly, during the life
experience of individuals (See also Lamarckian inheritance).
T
Vari ation : A term commonly used to indicate differences in the qualitative or quantitative values of a
character among individual members of a population, whether molecules, cells or organisms .
Vestigial organs: Organs or structures that appear to be small and functionless but can be shown to be
homologous with ancestral organs and structures that were larger and functional.

1191. Index

1193. :
a-la ctalbumin , 341
A.C. Allison, 244
A.I. Oparin , 417, 421
Abbe Lemaitre, 417
Abiogenesis, 421, 443, 444
Acquisition of new characters, 71 Active trends, 385
Adam, 413
Adam Sedgwick, 86, 454
Adaptability, 5, 510
Adaptation , 260, 263, 273
Adaptive convergence, 18, 373, 408 Adaptive divergence, 362, 369, 373 Adaptive heights, 236, 238
Adaptive radiation, 18, 27, 54 Adaptive trait, 285, 388
Adaptive valleys, 236
Additional theories, 86
Advantageous mutations, 146, 147 Aegyptopith ecus, 538
Agar, 109, 147
Aggressive colouration, 392
Agricultural revolution, 551
Alfred L. Wegener, 49
Allele frequency, 220, 22 1, 222 Allele substitution, 209, 211
ndex
Allometry (Differential growth rate) , 497 Alloparapatric speciation, 263, 265, 272 Alloploid s, 180, 183
Allopolyploids, 110, III , 178
Allopolyploidy, 185, 186, 264
Allotriploid hybrid, 181 .
Allotypes, 323
Alluring colouration , 393
Amber, 38, 39, 487
Amphiploid s, 181, 183
Anaerobic chemoautotrophs, 433
Anagenesis, 359, 360, 372
Analogy, 18, 24, 25
Anaximander, 6, 415
Anderson, 121
Andreas Wagner, 44
Aneuploidy, 176, 177, 188
Anhydrite rocks, 483
Antihum an serum, 61
Ape man, 476, 543, 546
Aquatic adaptations , 388
Ardipithecus kadabba , 557, 523, 540 Arenaceous shale, 483
Aristotle, 7, 16, 415
Arrhenius, 414
Artificial selection, 10, 79, 84
Asphalt, 38, 39, 488

1194. At avism of teratology, 32 Auditory stimuli, 202
Auerbach, 131
August Weismann, 10
Autogenesis, 415
Autopolyploids, 110, 178 Autopolyploidy, 179, 264, 273 Ayala, 241
:
B ackground radiations, 130
Balanced polymorphism, 242, 243 Balancing selection, 321, 322, 323 Balanoglossus, 57
Baldwin effect, 330
Barnacle goose, 4 15
Base analogs, 135
Bateson, 92, 125
Batzer, 555
Behavioural adaptations, 388, 390, 405 Behavioural isolation, 197, 202
Beneficial mutations, 234, 300
Bernhard Rensch, 379
Big-bang, 445
Biochemical disorders, 128
Biochemical mutations, 128
Biochemical theory, 417, 444
Bioclast, 487
Biogenetic, 33, 37
Biogenetic law, 33, 37
Biogeny, 42 1
Biogeographical division of the earth, 51 Biogeography, 51
Biological effect of radiations, 130 Biological evolution, 333, 374, 431 Biopoiesis,413
Biostratigraphy, 497, 498
Biotic adaptations, 404
Biston betularia, 89
Black or Negroid race, 552
Blakeslee, 95, 125, 172
Boag, 108
Body fossils, 486
Boltwood, 41, 498
Bonner, 74
Bonnet, 8
Borings, 487
Bottleneck effect, 235
Bottleneck period, 235, 292 Bottleneck phenomenon , 29 1 Brachydont , 366, 5 11
Bradyletic evolution, 359 Britten, 148
Brower, 402
Brown, 198, 396
Buffon, 8
Burrows, 72, 73, 487
:
C. Auerback , 75

1195. Calcite, 483
Campbell, 35 1
Canalisation, 233
Carbonaceous shale, 483
Carbonaria, 90
Carson's theory, 279
Casper Friedrich Wolf, 78
Cataclysmic, 275
Catastrophic mode of speciation, 278 Caucasoid race, 552
Causes of mutations, 129
Causes of somatogenic variation, 108 Centripetal selection, 3 16, 330
Cesnola, 89
Chalk, 452, 473, 483
Chance similarity, 402
Change in chromosome number, 210, 232,
264 , 275
Charles Lyell, 8, 486, 522
Chemical evolution, 3, 429, 444
Chemical mutagens, 129, 150, 375 Chemical stimuli, 202
C hemogeny, 421, 429, 443
Chemoheterotrophs, 432
Chemosy nthesisers, 433
Chemosy nthetic origin of life, 12, 75, 97,
208
Chromosomal aberrations, 97, 125, 263 Chromosomal mutations, I I, 65, 75, 109 Chromosomes, 259
Clade or lineage, 360, 385
Clades, 359, 387
Cladogenesis, 278
Classical theory of gradualism, 482 Clastic, 262, 378, 407
Clines, 427, 43 1
Coacervates, 492
Coal balls, 483
Coal rocks, 85, 94, 372
Coevo lution, 421, 443
Cognogeny, 75, 96, I II, 187
Colchicine, 404
Comme nsal adaptations, 404
Concea lment mimicry, 395
Conditio nal mutations , 128
Congenital, 108
Cong lomerate rocks, 482
Conspecific organisms, 2 17
Conspecific predominance, 204
Constitutio nal, 108
Continuous variation , 92, 106, 107, 262 Convergent, 5, 27, 54, 348
Convergent evolution , 5, 27, 54, 37 1 Conve rgent molecular evolution, 347-348 Cope's rule, 495--496
Coprolites, 40, 487

1196. Copy error mutations , 134
Cosmic evolution, 4, 417
Cosmic matter, 417
Cosmic rays, 128, 130, 420, 500
Cosmozoa, 4 14
Coyne, 211
Crick, 21, 202, 414
Cro-Magnon man, 549, 553
Cryptic behaviour, 396
Cultural revolution, 55 1
Cursorial adaptations, 388 Cuvier, 8-9, 38
Cyclic selection, 3 13, 320-321
•
D arwinism, 6, 70, 86, 9 1
Darwin's finches, 108, 30I, 364
Datura, 95, 125, 188
Da~
60, 72, 348-349
Deaminatio n, 140-1 42, 390
Degressive species, 93
Deleterious mutations, 146, 234
Deletion, 96, 97, 11 2, 151
Deletion mutatio ns, 132, 143
Demaillet, 8
Demes, 235, 260
Descent with modification, 285-287, 294,
295
Desert adaptat ions, 4, 15, 76, 299 Determinate variation, 389
Determiners, II, 66
Detrimental mutatio ns, 127
Detrital, 482
Developmental homology, 19- 20
Diatom ooze, 482
Dicke, 417
Differentia l reproduction, 99, 236, 299 Dimers, 131
Directed panspermia, 4 14--4 15
Directional selection, 249, 302, 314, 532 Discontinuous variation , 92, 107
Disruptive colouration, 391
Divergent evolution , 5, 55, 357, 369 Dobzhan sky, 12,91 , 148, 195
Dollo's law, 32
Dolomite rocks, 483
Dominant mutations, 196, 127, 328 Driesch, 75
Drosophila, 12, 75, 95, 13 1
Dryopithe cines, 528, 539 Dryopithecus, 538-539 Duplicatio n, 97, 112 Durietz, 195
Explosive, 275, 366
External teleology, 7
Extrinsic isolating mechanisms, 195 Eyre-Walker, 145

1197. : :
E . Freese, 142
E. L. Simon, 538
E.B. Bobcock, 91
E.B. Ford, 89, 242, 302
E.G. Peckhammian , 399
Early homo, 544, 548
Ecological adaptations, 389
Ecological isolation, 199, 200, 273 Ecological races (Ecotypes), 261 Ecological variation, 105
Edward D. Cope, 495
Eldredge , 277, 381, 387
Electric discharge , 420
Emboitem ent theory, 8
Encaseme nt theory, 8
Empedocl es, 6, 415
Environment, 5, 95, 105
Environm ental isolation , 199
Eobionts , 431
Eohippus , 5 10, 512
Epicurus , 7
Erasmus Darwin , 8
Ernst Haeckel, II , 37, 89
Ernst Mayr, 12, 262
Ethological isolation, 208
Etienna Geoffroy, 73
Eugene Dubois, 545
Eve, 3-11
Evidences from connecting links, 18, 55 Evidences from genetics , 18
Evidences in favour of Lamarckism, 71 Evolution in test tube, 351
Evolution ary clock, 349-350
Exon dupl ication , 341-343
Exon shuffling, 336, 341-343
F2 hybrids , 206
Faecal pellets , 487
Family atavism, 31-32
Father of taxonomy, 8
First atmosphere, 419
Fisher, 226, 241, 270
Fitch, 350
Forbidden base pairs, 136
Fossorial adaptations, 338
Ford, 242, 303, 310
Founder effect, 235, 269-270 Founder flush speciation theory, 278 Founder members, 235, 269, 291
Fragmental, 482
Frameshift mutations, 116
Francis Bacon, 7
Francis Crick, 414
Francisco Redi, 415--416
Frank 8., 248

1198. Free gene flow, 96, 218, 266
:
G-protein coupled receptors, 343 G.G. Simpson, 406
Galaxies, 417
Gametic frequency, 221
Gametic incompatibility, 204 Gametic isolation, 196, 204, 208 Gametic mortality, 204, 205 Gametic
mutations, 127
Gametic pool, 219, 309
Gametic selection, 308-309 Gametic variation, 106
Gamow,417
Gas troliths, 487
Gemmule s, 88-89
Gene duplication, 336, 338, 347
Gene flow, 96, 194, 2 18
Gene mutation , 12, 96, 109, 129
Gene pool, 91, 210, 217
Gene rearrangement, 208
Genetic bottleneck, 292
Genetic code, 66, 134, 302
Genetic constraints , 302, 385
Genetic correlation , 302
Genetic death, 233-234
Genetic drift, 92, 100, 217
Genetic homology, 19, 22
Genetic load, 233-234
Genetic polymorphism, 148, 232, 24 1 Genetic variability in populations, 95, 233 Genic selection, 309
Genome duplication, 347
Genotype frequency, 221, 224, 327 Geochrono logy, 497
Geographic isolation , 180, 194, 265 Geographical isolation, 98, 260
Geographical races, 165-66, 260
Geometric increase, 80, 82
George Cuvier, 9
George H.F. Nuttal, 60
Germ line mutations, 127
Germinal mutations , 125, 127
Germina l variation, 86, 108
Germplas m, 10, I I, 74
Giant land tortoises , 54
Gilmour, 2 17
Gregor, 217
Glandularia , 185
Globigerina ooze, 482
Goatsbeard , 185
Gossypium , 184
Gould, 277, 359, 382
Granite rocks, 484
Graywacke sandstone, 483

1199. Gunter Wagner, 105 Gypsum rocks, 483
:
H abitat isolation, 199- 20 I
Haldane, 89, 95, 207
Half life, 41, 498, 500
Halophiles, 432
Hampton Carson, 278
Haploidy, 97, 175, 177
Haplotype, 346
HardyWeinberg principle, 226, 228, 232 Hardy-Weinberg, 226-228
Herbert Spencer, 3, 89
Hereford cattle, 94
Heritable variation, II , 75, 82, 108 Heslop Harrison, 75
Heterologous translocation, 113
Heteroploidy, 97, 109
Heterosis, 251, 322
Heterozygote advantage , 248, 322 Heterozygote superiority, 243, 248, 322 Hidden mutations , 100
High temperature , 74, 187, 4 18
Histometabasis, 490
Homo erectus, 523, 54 1, 553
Homo erectus erectus, 546
Homo erectus pekinensis, 546, 547 Homo ergaster, 532, 545
Homo habilis, 523, 545
Homo sapiens, 478, 52 1, 525
Homologous organs, 18, 22
Homologous translocation, 11 3
Homology, 18, 20
Homology in forelimbs, 18
Homoplastic organs, 369
Homoplasty, 369
Homoplasy, 18, 24
Homoselect ion, 294
Homozygous translocations , 167
Hu go De Vries, II , 92
Human accelerated regions, 537 H. Urey, 4 17
Hybrid breakdown , 206
Hybrid inferiority, 196, 206 Hybrid inviability, 196, 205, 210 Hybrid sterility, 207, 2 10
Hybrid vigour, 322
Hybridisation, 12, 97, 120 Hypsodont, 366, 5 16
Hyracotherium, 379, 5 12
: J.B.S . Haldane, 12, 89, 232 Jack King, 349
J ames B. Herrick, 243 James Hutton, 8, 9
James V. Neel, 243
Jan Swammerdam, 7
Joyce, 351
Jukes and King, 100

1200. Julian Huxley, 12, 91
Justin Ramsey, 207
:
I gneous rocks, 480, 485, 500
Iguana, 54, 77, 290
Immigration, 220, 229, 376
Immunological distance, 350, 535 Impressions, 38, 40, 492
Inconstant species, 93
Indeterminate variation, 106-07
Induced mutations, 96, 128, 146
Industrial melanism, 89, 249, 314 Inheritance of acquired characters, 143 Insertion mutations, 116, 143,
158 Insertion, 5
Integrading organisms, 121, 177, 180 Intergeneric, 121, 177
Intergeneric hybridisation, 7
Internal teleology, 81, 120, 177
Interspecific, 120
Interspecific hybridisation, 3 11
Intrasexual selection, 195
Intrinsic isolating mechanisms, 121, 233,
273
Introgressive hybridisation, 97, 113, 160 Inversion, 131, 136, 427
Ionisation, 194, 203, 207
Isolating barriers, 194-97, 203
Isolating mechanisms, 194-97, 208 Isolation due to distance, 198
Isolation theory, 12
:
K -adaptations, 407
Kselection, 324-25
Karl Von Naegeli, II
Karpechenko, 121, 183, 275 Keightley, 145
Kettlewell, 90, 303, 402 Kimura, 100, 283, 349
Kimura's neutral theory, 100 Kin selection, 404-05
Knapp, 130
Knuckle walkers, 529
Kwang Jeon, 436
:
L. 0 0110, 32
L.S.B. Leakey, 523
Lactate dehydrogenase, 34 1 Ladder of life, 7
Lake, 125, 172, 200
Lamarck, 9, 70-75
Lamarckism, 70-75, 83, 387 Lamotte, 9, 70-75
Land pattern, 49
Langley, 350

1201. Lazzro Spallanzani, 416 Lederberg, 147, 306, 330 Leibach, 206
Leon Durfour, 203
Leonardo de Vinci, 38
Lepidosiren, 52, 58
Leslie Orgel , 414
Lethal mutations, 93, 127, 146 Lewin, 555
Lewis and John, 133
Limestone rocks , 539
Limnopithecus, 58, 255
Linnaeus, 381
Living fossils, 381
Locy, 7
Lord Brahma, 413
Louis Lerman, 429
Louis Pasteur, 4 16
Lovejoy, 529
Lynn Margulis, 435
Lysenko, 9
:
M. Wanger, 12
Macroevolution, 13-15, 91, 358
Macroevolutionary changes, 345 Macrofossils, 486
Macromutations, 379
Man-made radiations, 130
Mantis religiosa, 89
Manuyuan Long, 342
Mariner, 442
Mary Leakey, 523, 545
Mass extinction, 15, 43, 383 Mast crop, 406
Mast year, 406
Maupertius, 8
Maxon and Wilson, 350
Mayr, 4, 257, 546
McDougall, 75
Mechanical isolating barriers, 203 Mechanical rocks, 482
Megaevolution, 358, 368, 384 Meganthropus, 544
Melanic form , 90, 249, 303
Mendelian population, 95, 217, 219 Mendelian recombination, 97, 116 Mendelian recombination of genes,
97 Meristic variation, 106
Mermen and mermaid, 8
Metamorphic rocks , 419 , 480 , 485 Methanogens, 420, 432
Michael White , 264, 279
Microevolution, 14, 265, 358
Microfossils as fuel indicators, 40, 432 ,
486
Micromutations, 132

1202. Microspheres, 427
Milky way, 418
Miller-Urey, 425
Mills , 351
Mimetic colouration, 393
Mimicry, 94, 319, 395
Mineral contents, 483
Missense mutations, 133, 145
Modem synthetic theory of evolution, 12,
95
Molds and Casts, 40
Molecular basis of single gene mutations,
131
Molecular clock(s), 43, 62, 349
Molecular homology, 23, 60
Mongoloid race, 553
Monomorphic, 233
Monophyletic, 5, 256, 555
Monophyletic evolution, 360-61
Monophyletic genealogy, 5
Monophyletic species, 259
Monoploidy, 176, 177
Monosomics, III, 191
Moody, 288
Morgan, 74, 95, 125
Morphological variation, 105
Mosaic evolution, 383, 507
Motoo Kimura, 283, 349
Mountain, 3, 98, 452
Muller , 75, 124, 210
Muller's view, 210
Multiallelic genes, 125
Multiple alleles, 125
Multiple hypothesis, 555
Multiple isolating barriers, 207
Multiple translocation, 167
Mummies, 492
Mustard, 75, 131, 141
Mustard gas, 75, 96, 131
Mutagenic agents, 96, 128, 131
Mutagens, 13 1, 150,375
Mutation rate, 94, 128, 144
Mutational load, 234
Mutations, II, 55, 316
Mutations and evolution , 148
Mutations and genetic polymorphism , 148 Mutualistic coevolution, 372

1203. :
N.W. Pirie, 371
National Aeronautics and Space Administration, 442
Natural radiations, 130
Natural selection, 76, 79, 297-98
Naturalistic theory, 421
Nebulae, 417
Nebular hypothesis, 418
Nei and Roy Choudhury, 555
Neo-Darwinian theory, 12, 13, 91
Neoceratodus, 52, 58
Neopilina, 56
Neoteny, 36
Neutral mutations, 100, 128, 146
Neutral theory of evolution, 99
Nicotiana, 185
Niles Eldredge, 359, 381, 387
Noethling, 130
Non-heritable variation, 106
Non-random reproduction, 99
Nondirectional fluctuations, 235
Nonionising radiations and their effects, 130
Nonsense mutations, 133
Nonvisual, 390
Nucleotide substitution
rate, 144
Nullisomics, 112
:
Oenothera, 92 , 1215, 167 Ohta, 349
Oil soaked ground, 39
Ontogeny, II, 33, 37
Oparin-Haldane theory, 421 Orbigne,9
Oreopithecus, 539
Organismic adaptations, 404-05 Orgel,414
Origin of life, 14,413,416 Origin of man, 183, 524, 554 Origin of universe, 417
Orr, 211
Orrorin tugensis, 504-41
Orthogenesis, II, 519
Orthogenetic evolution, 519 Orthologous genes, 337-341 Osborn, 61, 362
Othniel C. Marsh, 12, 47 Otto Schindewolf, 382
:
P .L. Sclater, 51
Paedogenesis, 36
PAHs, 443
Palaeobotany, 38, 41, 432 Palaeolithic, 522, 549, 553 Palaeozoology, 38

1204. Pangenes, 9, 10, 88
Panmictic, 217, 226, 283 Paracentric, 113, 160, 164
Parallel evolution, 24 , 361, 369 Paralogous genes, 337
Paranthropus, 544
Parapatric speciation, 263, 271-72 Paraphyletic evolution, 361
Paraphyletic taxon, 361
Parasitic adaptations, 389, 404
Passive trends, 385
Pasteur, 415
Patterson, 195, 205
Pavlov, 74
Pentaploidy, 97, 109
Pericentric, 113, 161
Period of equilibria, 359, 381
Period of stasis, 359, 381
Peripatus, 14, 56
Phenocopy, 330
Pheromones, 203
Photoproducts, 131
Photosynthesisers, 433
Phylogenetic, 333-37, 346--51
Phylogenetic tree, 64, 259, 277 Phylogeny, II, 33, 334
Physical evolution, 3
Physiological, 60, 98, 204
Physiological isolation, 212
Physiological variation, 105
Pithecanthropus erectus, 545, 546 Pithecanthropus pekinensis, 546 Pleiotropism, 114
Pleiotropy, 303, 323
Pliopithecus, 538
Point mutation(s), 94, 114, 124 Polycyclic aromatic hydrocarbons, 443 Polygenes, 106, 114
Polymorphic, 100, 105, 148, 233 Polymorphism, 100, 148, 233
Polyphyletic, 361, 554
Polyphyletic evolution, 361
Polyphyletic origin, 361, 555
Polyphyletic taxon, 361
Polyploidisation, 273, 275
Polyploidy, 97, III
Polysomies, 112
Pomato,98
Population genetics, 91, 217
Postadaptation(s) , 117, 384
Postmating isolating mechanisms, 196 Postulates of Lamarckism, 71
Postzygotic barriers, 205
Postzygotic isolating mechanisms, 196, 208 Potassium-Argon method, 41, 50 I
Poulten and Carpenter, 402
Preadaptation(s) , 117
Prebiotic soup, 426, 437
Precipitated rocks, 482
Preformation theory, 7-8

1205. Premating barriers, 196
Prevost, 8
Prezygotic barriers, 205
Prezygotic isolating mechanisms, 196 Primaeval matter, 417
Primula, 186
Proconsul, 476, 539
Progressive species, 92
Propliopithecus, 538
Prospective adaptations, 406
Protective adaptations, , 388, 390
Protective mimicry, 395, 40 I
Proteinoid microspheres, 427--428
Protobionts, 428, 431
Protopterus, 52, 58
Protoribosomes, 431
Protoviruses, 431
Pseudodiversification, 507-508
Pseudoextinction, 507-508
Pseudofossils, 40, 487
Pseudogenes, 339-340, 343
Pseudomorphs, 40
Psychological variation, 40
Pteropod ooze, 482
Punctuated equilibria, 359, 381
Punctuated equilibrium, 358-359
Punctuations, 359, 381
:
R-ad aptati ons, 406-407
R-selectio n, 324-325
R.A. Fishe r, 89, 226
Race atavi sm, 31-32
Radiations , 82, 195-196, 128 Radioactive carbon method, 41, 500 Radiometric dating, 41
Radiomet ry, 497-498
Radionuclear explo sion, 4 17
Ramapithecus, 523, 539
Random mating , 99, 219
Randomness of mutations, 147 Raphanobrassica , 98, 121, 181 Rate of speciation, 277- 278
Raymond A. Dart, 522
Realms , 5 1
Recapitulation theory, 11 ,33,37 Recapitulation theory of Haeckel, II Recessive mutations, 96, 127
Recognition marks , 393
Recombinational speciation, 276 Reducing atmosphere, 419 , 426, 434 Regulatory genes , 345 , 352
Relative fitness , 236, 248, 307 Rensch , 12, 91, 359
Replacement hypothesis, 554-556 Reproductive barriers, 194, 200, 208 Reproductive isolation, 12, 98, 171
Restriction site maps, 346
Retrogressive species, 92
Retrotransposons, 342
Reverse mutation, 96

1206. Rho Coron ae Borealis, 443
Ribozyrnes, 431
Richard, 18, 86, 382 , 538
Richard Goldschmidt, 382
Richard Owen, 18, 86
Richter, 4 14
RNA worl d, 431
Robert Broom, 523, 542
Robert Nayes, 443 Rock salt rocks, 483 Ronald A. Fisher, 95 Russel Landle , 271
:
S .S. Chetverikov, 13
S.M . Williston, 494
Sahelanthropus tchadensis, 523 , 540 Saltation(s), II , 92, 107
Saltatory, 92
Sandstone rocks , 482
Scansorial adaptations, 388
Scattering of variability, 283
Scientific industrial revolution, 551 Secondary atmo sphere , 419
Secondary sexual characters, 87 Sedimentary breccia, 482
Sedimentary rocks, 8, 38, 453 Sedimentation, 9, 39, 481
Segregational advantage, 309
Segregational distortion, 309
Segregational load, 234
Selection coefficient, 307, 327 Selection for multi gene traits , 308 Selection for single gene traits , 307
Selection pressure, 210, 249 , 252 Semispecies, 261, 279
Sequential evolution, 357, 378 Seth Wright , 124
Sewall Wright effect , 99, 283 Sexual colouration, 394
Sexual reproduction, 14, 75, 97, 233 Sexual selection, 10, 86-88, 310 Shale rocks, 483
Sibley, 120
Sibling species, 257-259
Simpson, 12, 259, 406 , 505
Sinanthropus pekinensis, 546-547 Single locus heterosis, 248
Sivapithecus, 539, 540
Smallscale evolution, 14
Smocovitis, 95
Social adaptations, 404
Social context, 404-405
Solanum tuberosum, 185
Solar radiations, 418, 420
Somatic mutations, 95, 125-126 Somatic variation, 75, 108
Somatogenic variation, 106, 108 Somatoplasm, 10-11
Soretium,7
Spallanzani, 415--416
Spontaneous mutations, 128, 130, 146 Sports, 92, 125
Stabilising selection, 243, 245, 313, 316 Stabilised genes, 537
Stadler, 130
Stadler, 130

1207. Stanley, 277, 417, 424
Stanley Miller, 417, 424
Statispatric speciation, 264, 279
Stebbins, 12, 91
Stellar evolution, 4
Stephen Jay Gould, 359, 381
Stochastic mode of speciation, 280 Stratified rocks, 488
Stratigraphy, 40, 497
Stromatolites, 438
Structural genes, 343, 345
Structural homology, 18, 20
Stubbe, 130
Subfossils, 486
Substantive variation, 106
Substitution mutations, lIS
Superspecies, 261
Supraindividual structures , 405
Survival rate, 205, 244
Swinnerton, 538
Swynnertion, 402
Symbiotic adaptations, 404
Sydney F. Fox, 429
Synonymous (silent) mutations, 134 Systematic mutations, 379
:
T.H. Morgan, 74, 95
Tachytelic evolution, 359
Tautomeric shifts, 135
Tautomerisation, 135
Tautomers, 135
Tectology, 17-1 8
Teleology, 7
Test tube evolution, 351-352
Tethys ocean, 49
Tetraploidy, 97, 109
Thales,6
The core, 418
The crust, 480
The phylogenetic trees, 357
Theodor Dobzhansky, 4
Theodosius Dobzhansky, 12, 91 Theory of catastrophism, 9
Theory of eternity of life, 9
Theory of orthogenesis, II
Theory of pangenesis, 9, 10
Theory of panspermia, 414
Theory of sexual selection, 10, 86 Theory of special creation, 414 Theory of spontaneous generation, 415
Theory of uniformitariani sm, 9
Thermophile s, 405
Thomas Jukes, 349

1208. Thomson , 555
Time phyletic rate, 507-508
Tissue plasminogen activator, 342 Tool making revolution, 551
Tornaria larva, 34
Tower, 74-75
Tower, 74-75
Trace fossils, 487
Tracks, 40, 492
Trails , 492
Transient polymorphism, 249, 323 Transition(s), 14, 43
Translocation, 97, 113, 157, 163 Transposable elements, 157, 388 Transposition, 113, 156
Transposons, 342
Transversion(s), 115, 134, 141 Tree of life, 59, 277
Triplet code, 144
Triploidy, 97, 109
Trochophore larva, 34, 56
Type of fossils, 446, 493
:
Volpe, 205
Von Baer, 32, 37
:
W.F . Libby, 41, 500
Wallace, 51, 79
Walter and Luis Alvarez, 43 Warmick Kerr, 287
Warning colouration , 393 Warwick, 206, 287
Weismann, 8, 10, 73
Westoll, 506
White, 46, 74
White race, 552
WolpotT, 556
Uber, 75, 130
Unconditional mutations, 128
Uniformi tarianism , 8-9
Unusual fossils, 486
UraniumLead method, 4 1, 499 Uvarov, 402
:
Xenophanes, 6, 415
:
:
Yelm,417
Van Hel mont, 415
Variation, 7, 67
Vestigial organs, 17, 28
Viking, 412

1209. Visual, 202, 390
Visual stimuli, 202
Volcanic eruptions, 9, 81, 420
:
Ziegler , 505
Zinjanthropus, 523
Zuckerkandl and Pauling, 63 Zygotic mortality, 196
Zygotic mutations, 127

1211. LIFE SCIENCE
Organic Evolution
(Evolutionary Biology)
Evolutionary Biology is a fast developing subject to encompass animals and plants, past and present. This
book on Organic Evolution is an updated exposition of the subject to include the latest concepts, theories
and observations in a simple and lucid language.
• Text is divided into ve parts: Part-I deals with concepts, evidences and theories of evolution. Part-II peeps
into the Mechanism of Evolution. Part-Ill provides a systematic approach to the process of Speciation. Part-
IV explores the basic patterns of Evolution, and Part-V provides an insight into the origin of life, fossil-
records and history of life on Earth.
• Text is profusely illustrated with diagrams to ensure an easy comprehension of the underlying concepts.
• The book would be a useful textbook for students of biological sciences.
Dr. (Mrs.) Veer Bala Rastogi earned her master's degree in Zoology with distinction standing rst in order of
merit. She did her Ph.D. from Meerut University under the guidance of eminent zoologist, (Late) Dr. M.L.
Bhatia, Professor in Zoology (Retd.), University of Delhi as External Supervisor, and a member of Fellow of
Academy of Zoology. Dr. Rastogi was a member of academic staff of Zoology, Meerut College, Meerut
during 1961-1967, teaching undergraduate and postgraduate students. She retired as Associate Professor
from the Gargi College, University of Delhi. Dr. Rastogi is a member of Fellow of Academy of Zoology. She
was also a member of Textbook Evaluation Committee, NCERT, New Delhi. She has vast and variegated
experience of teaching for more than 45 years of various disciplines of Biological Sciences, viz., Cell
Biology, Genetics, Molecular Biology, Environmental Biology, Developmental Biology, Invertebrates and
Vertebrates. She has been writing books for over ve decades. She has authored many books in biology for
ISC, CBSE and several state boards. Her books on Cytology, Genetics, Molecular Biology, Biostatistics,
Evolutionary Biology, Developmental Biology, Animal Ecology, Environmental Biology (Ecology), etc. are
very popular at university level all over the country. She brings a new functional approach to the study of
evolution. This book will prove to be an excellent textbook on evolution at university level both in India and
abroad. Dr. Veer Bala Rastogi was conferred 'Distinguished Author Award' 2012 by the Federation of
Educational Publishers in India, Delhi.
Brief Contents:
Unit I-Concept, Evidences and Theories of Evolution: Concept of Evolution • Evidences for Evolution •
Theories of Evolution.
Unit II-Mechanisms of Evolution: Variation • Gene Mutations • Chromosomal Aberrations
• Variation in Chromosome Number (Heteroploidy) • Reproductive Isolating Barriers. Unit III-Speciation:
Population Genetics, Gene Frequencies and Hardy-Weinberg Equilibrium • Persistence of Variability within
Populations: Polymorphism • From Population to Species (Speciation) • Genetic Drift and Gene Flow •
Natural Selection in Action • Evolution of Genes and Genomes.
Unit IV-Basic Patterns of Evolution: Patterns of Evolution • Microevolution and Macroevolution •
Adaptations.
Unit V-Fossils and History of Life on Earth:Origin of Life on Earth • History of Life on Earth
• Fossils and Fossil Records • Origin and Evolution of Horse • Origin and Evolution of Man.

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9 789387 465329A Division of Scientific International
Website: www.siplind.com ISBN 978-93-87465-32-9