Speciation, types & role of ecology in speciation

Evolutionary biology, Types of
Evolution

1. Speciation Types of Speciations

2. Speciation : Evolution of a new species which are reproductively isolated.   Rate of speciation depends on generation time, environmental conditions, etc Can be caused by a change in just 1 gene or a set of genes causing reproductive isolation Reproductive isolation leads to reinforcement of the distinction between species through natural selection and sexual selection. Most important criteria for speciation is inhibition of gene flow among the population which will lead to reproductive barrier/reproductive isolation development.

3. Types of Speciation: (primarily based on the fact that how the population is developing mechanism to become reproductively isolated)   Allopatric Speciation — gene flow is interrupted when a population is divided into geographically isolated subpopulations Parapatric Speciation — occurs when part of a population enters a new habitat bordering the range of the parent species – Some gene flow may occur between populations in border zone Sympatric Speciation —occurs in populations that live in the same geographic area – Less common than allopatric speciation – Happens when gene flow is diminished by: • Polyploidy • Habitat differentiation • Sexual selection

4. Allopatric speciation: Biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier breaks down, individuals of the population can no longer interbreed.  Example: Charles Darwin’s Galápagos Finches. > Members of population become geographically isolated due to a geographical barrier (river, mountain, volcano etc.) or due to emigration/dispersal > Separated populations undergo divergence in genotype and phenotype due to different selection pressure >Natural selection cause the genetic drift in the separated populations > Separate populations may develop morphologically distinct features due to adaption to their new environment. > Difference become very prominent & ultimately lead to reproductive isolation occurs, preventing the inbreeding of populations and thus forming new species

5. Allopatric speciation in Mexican spotted owl and the Northern spotted owl being separated by rivers , and therefore isolated from each other by geographical barrier resulted in allopatric speciation. 

6. Example: Divergent populations of finches inhabiting the Galapagos Islands, and known as ‘Da rwin finches

7. Experimental Evidence of Allopatric speciation

8. Founder Principle Few individuals or small group of individuals from larger population invade/migrate to a new or isolated geographical region (Founder Group) Founder population are exposed to chance events amplifying some traits and diminishing others. Change in allele frequency from the original population. Explain many instances of rapid speciation

9. Significance: Isolated population differ from source population in the frequencies of their alleles. Gene / Chromosomes arrangement carried by the founders have a chance of becoming established in new populations. Presence of high frequency of certain inherited disorders among isolated human populations occur due to founder effect and inbreeding. Example: Founder population of Green Iguanas on Anguilla islands in Caribbean. Iguan population transferred by hurricane to the island and then slowly after multiple generation, the population allele frequency will become different from original population. 2. Picture winged drosophila more than 100 native species on Hawaiian Islands. Result of founder events. e.g. 40 species unique

10. Bottle Neck Effect:

11. Genetic Drift: Non directional, random fluctuations in the allele frequencies in the gene pool of small populations, occurring by CHANCE & NOT Adaptive Random genetic drift, allelic drift or the Wright effect Genetic Drift causes allele frequencies to change in population Alleles are lost more rapidly in small populations It is a result of the influence of chance. When population size is small, chance events more likely to have a strong effects. Sampling error is higher in smaller samples

12. Theory for Genetic Drift: (Sewall Wright & Motoo Kimura) In small population, effects of sampling error is explained by Sewall Wright effect. Refers to “Random fluctuations” in allele frequencies in a small population from one generation to the next. Salient features of genetic drift Genetic drift as sampling error (Fluctuations of allele frequencies due to variable sampling from gene pool, e.g. founder effect) Random fluctuations in Allele Frequencies (Change in allele frequencies at random over time, bottle neck effect) Genetic drift tend to reduce genetic variability (small population, changed allele frequency, changed conditions lead to lost of some and fixation of few alleles) Genetic drift & Non-adaptive Traits ( tend to preserve or eliminate alleles without distinction, harmful alleles may be fixed by chance) Genetic drift and divergence between population (Isolated populations or demes come to possess some unusual characteristics not present in the large parental population) Genetic drift and fixation of new mutations (because of random fixation or elimination of any allele, there is 50% probability of new mutation to be lost)

13. 1. SIBLING SPECIES 2. POLYMORPHIC SPECIES 3. POLYTYPIC SPECIES

14. Sibling Species: They are true sympatric species that are morphologically identical or nearly so but are reproductively isolated. For example, Drosophila pseudo-obscura and D. persimilis are sibling species. The vulnerability of a purely morphological species concept in sexually reproducing species can be demonstrated primarily by two lines of evidence: The presence of conspicuous morphological differences among conspecific individuals and populations ( intraspecific variation ). The virtual absence of morphological differences among certain sympatric populations ("sibling species") that otherwise have all the characteristics of good species ( genetic difference and reproductive isolation ). The naturalist occasionally encounters sympatric populations that are morphologically exceedingly similar, if not identical, but are reproductively isolated. Adherents of a purely morphological species concept will not classify them as species because for them, as formulated by Sturtevant (1942), "distinct species must be separable on the basis of ordinary preserved material."

15. Natural populations that are not readily distinguishable but are nevertheless reproductively isolated have caused considerable difficulties in the biological and taxonomic literature. Such populations have sometimes been called "biological races," Most "biological races" are indistinguishable from other valid species, except by the slightness of the morphological difference. For such exceedingly similar species, the term "sibling species" was introduced (Mayr 1942) Sibling species are of threefold importance in biology: they permit us to test the validity of the biological versus the morphological species concept; ( b) they are of great practical importance in applied biology , in agricultural pest control, and in medical entomology; (c) they are of historical importance in the study of speciation Sibling species are common among the tyrant flycatchers, particularly the genera Empidonax, Elaenia , and Myiarchus .

16. Two song birds commonly found in the eastern US. Even in the hand they are difficult to distinguish, but their songs are distinct! gray tree frog and Cope’s gray tree frog. In addition to the difference in their calls, these species differ in the number of chromosomes

17. Polymorphic species: Species exist as multiple populations and each accumulate variability. Existence of two or more forms (morphs) of a individuals in the same species within the population at the same time and same place. Differences in forms may be morphological, physiological or biochemical and are genetically determined. As per E.B. Ford, polymorphism is occurrence of two or more distinct forms of individuals in the same population or species and in same locality. Features: Caused due to existence of more than 2 types of genotype Different forms are adapted to different types of environment All mendelian populations are polymorphic Increases efficiency in resource utilization Help survival of species in variety of environment Result of evolution, traits are inheritable and naturally selected Presence of different forms of genes c/as alleles

18. Types of Polymorphism Transient Polymorphism: Due to strong selection pressure, one form or morph is being strongly favored while other is getting eliminated. Crucial is the transient / temporary condition. A new mutation becomes advantageous in changed environment and is selected by nature while the wild type becomes rare due to negative selection. e.g. in case of industrial melanism in black peppered moth ( Biston betularia ). Selection of black melanic form ( carbonaria ) in sooty environment of industrialized Manchester, resulted in reduction of non melanic form. From 1845 to 1895, 98% population becomes dominated by melanic form.

19. Balanced Polymorphism: 2 or more forms co-exist in same population of a species in stable environment and show almost constant ratio. Genotypic frequencies of various forms occur at equilibrium . It is also called as persistent polymorphism. Balance of selective forces Each form has selective advantage of equal intensity Usually heterozygotes have selective advantage and is called as heterzygote superiority against homozygotes. Different alleles of a gene are maintained in the population because each is favoured by separate environmental force. Examples: ABO blood group: Human population has existence of A, B, AB and O blood groups. None of the allele has selective advantage. Drosophila polymorpha , 3 types of colouration on their abdomen. The light, dark and intermediate colour. Light color (aa), dark color (AA) and intermediate (Aa). Heterozygotes are more abundant.

20. A polymorphic species, the Neotropical strawberry frog ( Oophaga pumilio ). Since each individual and the clone to which it gives rise remain permanently independent of all related clones, one would expect, owing to mutation, a steady genetic divergence between clones, ultimately resulting in high variability in the parthenogenetic "species." Some of the most highly polymorphic species of British sawflies , Mesoneura opaca and Eutomostethus ephippion , are completely parthenogenetic (Benson 1950).

21. Polymorphic species: Throat colours of orange (left), blue (middle) and yellow (right) homozygous males from a trimorphic population of Uta stansburiana . (B) The three female colour morphs of Ischnura elegans : infuscans (top left), male-mimicking androchrome (top right) and infuscans-obsoleta morph (bottom) in its juvenile phase ( rufescens ). (C) White (left)- and tan (right)-striped morph of Zonotrichia albicollis . (D) Blotched and plain morphs of Neochromis omnicaeruleus . Female orange-blotched (OB) morph (top left), white-blotched (WB) (bottom left), plain male (top right) and plain female (bottom right). (E) Colour morphs of Heliconius numata with the numata arcuella morph on the top left, aurora on the top right, bicoloratus (middle left), silvana (middle right), tarapotensis (bottom left) and timaeus (bottom right).

22. Polytypic species: Any species which has more than 2 subspecies are called as polytypic species. Many types. Concept was given by Beckner in 1959. It is important in development of classification of animals, because certain local species from various parts of the world were combined into groups (allopatric species). These allopatric species were more close to one another than to other species so they were put into single polytypic species. Most species are Monotypic Horned Lark ( Eremophila alpestris alpestris , E alperstris strigata , flava, longirostris ): wide distribution in many continents. Polytypic Polytypic species:

23. Importance of Polytypic species: Recognition of polytypic species in well known groups of animals such as birds, mammals, butterflies etc. helped in the simplification of classification . The reclassification into polytypic species of geographically representative forms that had originally been separately described as monotypic species led to great clarification of the system. E.g. 19,000 monotypic species of birds listed in 1910 have now been reduced to about 8600 sps . Restoration to the species category of a definite biological meaning & homogeneity. The reclassification of local assemblage into polytypic species, reveals many taxonomically & biologically interesting situations. Best evidence for allopatric speciation

24. Difficulties : Polytypic species are composed of allopatric populations that differ from each other. Can be closely related polytypic/ 2 polytypics etc. However, all populations of sexually reproducing organisms differ slightly from each other, and certain standards must be met before subspecies can be recognised. Occasionally, closely related species with similar ecological requirements replace each other geographically and yet are full species and not subspecies. A polytypic species is often a compound of several “species” originally proposed as monotypic. Isolated populations are in process of evolving into new species & are in borderline between species and subspecies.

25. Examples: 40-80% species are polytypic White Wagetail : yellow, Reddish & brown. Earlier Swedish population was named as Motacilla alba and other subpopulations were also considered as monotypic species but later suggested that they are only polytypic species.

26. P rocess, by which barriers to gene flow evolve between populations as a result of ecologically based divergent selection between environments .” S ome environmental factors affect population structure more than others and are therefore more important for the evolutionary potential of species Common species, on the other hand, show great adaptability. As a consequence, a single population may be able to occupy many different ecological niches, and each of these populations may show such great phenotypic variation that localized subspecies cannot be delimited. If each genotype in a population is somewhat specialized ecologically, the more genotypes a species contains, the more versatile and ecologically tolerant it will be. Genetic variability and ecological versatility then form a mutually reinforcing system: the more widespread and common a species is, the more genetic variability it can store, the more tolerant ecologically it can become and the wider it can spread, and so forth. Role of Ecology in speciation

27. Habitat Characteristics and Habitat Utilization. The relation between an organism and the environment in which it lives seems sufficiently important to serve as the basis of a classification of kinds of species. One has been proposed by Thoday (1953), who distinguishes three classes of species: (1) Those that live in a relatively uniform and stable environment and will therefore not be exposed to selection in favor of genetic or phenotypic flexibility; such species will be selected primarily for stability and adaptation to this uniform environment; (2) Those that live in a heterogeneous environment and will therefore be strongly selected for phenotypic flexibility, especially if the generation time is long; (3) Those that live in an unstable environment and will therefore be strongly selected for genetic flexibility, although selection for this genetic flexibility will be less, the greater the phenotypic flexibility.

28. Different environments require different solutions. when a population with certain characteristics is faced with a certain set of environments (including other species), T here are only four distinct ways by which ecological performance may increase and therefore by which reproductive success may increase: Natural selection , meaning that individuals with unfavorable characteristics have a larger probability to die and a smaller probability to reproduce. If the favored characteristics have a heritable basis, then natural selection is a great way to adapt populations to their local environments. However, natural selection cannot improve the performance of locally maladapted individuals directly. Therefore, three other ways to improve performance may evolve to help such maladapted individuals, as individually flexible responses. Phenotypic plasticity , whereby individuals change their characteristics such that they have a better match to the local environment (e.g., if the environment is too cold, fluff up your feathers or grow a denser plumage in birds).

29. Adjusting the local environment , alternatively individuals may also change their environment such that this provides a better match to their phenotype. This can be done by actively adjusting the local environment, i.e., manipulating and improving aspects of the current environment (e.g., building a warmer nest in a colder environment). Finally, individuals could select environments that have a better match to their phenotype (e.g., an individual with a thinner plumage, feeds in sunnier places or at lower latitudes or altitudes).