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1. LIFE SCIENCES
Topic: Evolution (By natural selection)
Presenters: Nonhlanhla Mtshali & Vedika Maharaj
11 August 2022

2. EVOLUTION EXAM GUIDELINE: EVOLUTION (54 MARKS)
CONTENT ELABORATION
Origin of an idea about
origins
Ideas on evolution in the order of their origin are as follows:
•Lamarckism, Darwinism, Punctuated Equilibrium
Lamarckism (Jean Baptiste
de Lamarck –1744–1829)
Lamarck used two 'laws' to explain evolution:
•'Law' of use and disuse , 'Law' of the inheritance of acquired characteristics , Reasons for Lamarck's theory being rejected
Darwinism (Charles Darwin
– 1809–1882)
Darwin's theory of evolution by natural selection:
•There is a great deal of variation amongst the offspring.
• Some have favourable characteristics and some do not.
•When there is a change in the environmental conditions or if there is competition,
•then organisms with characteristics, which make them more suited, survive
•whilst organisms with unfavourable characteristics, which make them less suited, die.
•The organisms that survive, reproduce
•and thus, pass on the allele for the favourable characteristic to their offspring.
•The next generation will therefore have a higher proportion of individuals with the favourable characteristic.
Punctuated Equilibrium
(Eldredge and Gould –
1972
Punctuated Equilibrium explains the speed at which evolution takes place:
• Evolution involves long periods of time where species do not change/change gradually through natural selection (known as
equilibrium).
•This alternates with (is punctuated by) short periods of time where rapid changes occur through
•natural selection during which new species may form in a short period of time

3. THEORIES OF EVOLUTION – LAMARCK
Jean Baptiste de Lamarck (early 1800’s)
He explained evolution using the following
two laws:
1. The law of use and disuse:
As an organism uses a structure or organ more
regularly, it becomes better developed or enlarged.
If an organism does not use a structure or organ
frequently, it becomes less developed or reduced
in size and may disappear altogether.
2. The inheritance of acquired characteristics:
Characteristics developed during the life of an
individual (acquired characteristics) can be passed
on to their offspring.

4. THEORIES OF EVOLUTION – LAMARCK
Lamarck’s approach using the giraffe as an
example:
•All giraffes had short necks originally
•Giraffes frequently stretched/used their necks
to reach for leaves of tall trees
• Causing their necks to become longer
•The characteristics of long necks acquired in
this way was then passed on to the next
generation/inherited
•forming offspring with longer necks than the
generation before

5. THEORIES OF EVOLUTION – LAMARCK
1. NAME Lamarck’s laws. (2)
- Law of use and disuse 
- Law of inheritance of acquired characteristics 
The red-bellied black snake (Pseudechis porphyriacus)
and the green tree snake (Denderelaphis
punctulatus)are predators that sometimes feed on cane
toads (Bufo marinus) that contain a toxin that may kill
them.
The snakes consume the toads by swallowing them
whole. A decrease in the average jaw size of the
snakes has been observed over a period of 70 years.
With this change it was also noted that the snakes
could no longer swallow large cane toads. This has
resulted in an increase in the survival of the snakes.
2.How would Lamarck have explained
the development of a small jaw size in
the snakes? (4)
- Since the snakes' jaws were used
less /not used
- the snakes developed smaller jaws 
- This characteristic (of a smaller jaw)
was inherited by the offspring 
- the jaw of the snake became
smaller

6. THEORIES OF EVOLUTION – LAMARCK
When answering questions based on Lamarck’s theory, remember the following:
•An inherited characteristic is a characteristic that an offspring is born with, having
been inherited from one of the parents; a characteristic controlled by a gene.
•An acquired characteristic is a characteristic that an offspring is not born with but
which develops/is acquired through the course of its lifetime; a characteristic not controlled
by a gene.
Guiding questions when Lamarck’s theory is applied to a new situation:
• What was the original characteristic?
• What was the challenge?
• What did the organism do/what characteristic was then acquired?
• What was the result?
• What happened to this acquired characteristic?
• What was the result of this?

7. THEORIES OF EVOLUTION – LAMARCK
Mutations in genes allow Tibetans to survive
Is it possible to cope with low oxygen content at high altitudes.
One way is for the body to produce more red blood cells in response to
an increase in altitude. Another way of coping has developed in Tibetans
as a result of gene mutation that they inherited from their ancestors. The
mutant gene helps them to use the low amount of oxygen present more
efficiently. The mutant gene was found in more than 87 % of Tibetan
population but only 9% of the Han population that lived at lower altitudes
than Tibetans.
Describe how Lamarck would have explained the survival of Tibetans at
high altitudes (5)

8. THEORIES OF EVOLUTION – LAMARCK
•Originally the amount of red blood cells was similar in all
humans✓/the Tibetans did not produce a large number of red
blood cells
•As a result of the low oxygen content at high altitudes✓ the red blood
cells tried to increase the amount of oxygen absorbed✓
•As a result, ancestral Tibetans produced more red blood cells✓/
developed ways of using oxygen more efficiently
•To increase the availability of oxygen to the body✓
•This acquired characteristic✓ was then passed to their offspring✓
•All Tibetans now produce more red blood cells✓/use oxygen more
efficiently to survive at high altitudes

9. Reasons why Lamarck’s theory was rejected:
•There is no evidence that acquired characteristics are inherited
by offspring.
•Organisms did not evolve because they were determined to
change but changes took place randomly due to mutations
•An organism cannot change its DNA
•According to Mendel, it is the genotype that determines
the phenotype, not the other way around.
REJECTION OF LAMARCK’S THEORY

10. THEORIES OF EVOLUTION – DARWIN
CHARLES DARWIN (Book published 1859)
“On the Origin of Species by Means of Natural
Selection”
His theory of natural selection has two main points:
1.Species were not created in their present form but evolved
from ancestral species i.e. whale evolution
2.Proposed a mechanism for evolution – Natural selection

11. NATURAL SELECTION

12. NATURAL SELECTION
Darwin's theory of evolution by natural selection:
•There is a great deal of variation amongst the offspring.
•Some have favourable characteristics and some do
not.
•When there is a change in the environmental conditions or
if there is competition,
•then organisms with characteristics, which make them
more suited, survive
•whilst organisms with unfavourable characteristics,
which make them less suited, die.
•The organisms that survive, reproduce
•and thus, pass on the allele for the favourable
characteristic to their offspring.
•The next generation will therefore have a higher proportion
of individuals with the favourable characteristic.

13. THEORIES OF EVOLUTION – DARWIN
A population of lizards on an island, Island A, were well
suited to feed mainly on insects. Scientists moved five
adult pairs of the lizard species to a neighbouring
island, Island B. here they reproduced, and a new
population formed. Island B has a larger supply of
plants with tough fibrous leaves and fewer insects.
Exposure to this new environment may have caused
the lizards to undergo evolution.
Thirty-six years later, scientists returned to Island B to
conduct further investigations on the lizard population
there. They observed that the jaw size of the lizards
had increased. Scientists also analysed the stomach
content of the lizards and found that it was mainly
plant-based. They also confirmed that the two
populations still belong to the same species.
Use Darwin’s theory of natural selection to
explain the evolution of lizards with larger
jaws
•There is variation in the size of the lizards’ jaws✓
•Some have small jaws and others have large jaws ✓
•Due to the larger supply of fibrous plants ✓/fewer
insects
•Those with smaller jaws will be unable to feed ✓
•and die ✓
•The lizards with the larger jaws will have more food ✓
•and survive ✓
•to reproduce ✓
•The allele for larger jaws will be passed on to the
offspring ✓
•The next generation will
have a higher proportion
of lizards with larger
jaws ✓

14. THEORIES OF EVOLUTION – DARWIN
A population of frogs live in a pond. The frogs may be
darker or lighter in colour. A population of herons (long-
legged water birds), who feed on the frogs, live in the
same habitat. Over a period of time, the water in the
pond became darker in colour. This caused the frog
population to change over a period of 4 years as shown
in the diagram below.
Use Darwin’s theory of evolution to explain how the frog
population changed over the period of 4 years. (6)
- There was variation in the frog population.
-Some of the frogs were light in colour and some of the
frogs were dark in colour.
-Those that were light in colour were not camouflaged/
were more visible and were eaten by the heron  /did
not survive.
-Those that were dark in colour, were camouflaged/were
less visible and survived/ were not eaten by the
heron.
-The frogs that survived, reproduced
-and passed on the allele for dark colour to their
offspring, 
-leading to the population of dark coloured frogs  after
four years.

15. PUNCTUATED EQUILIBRIUM
Punctuated Equilibrium (Eldredge and Gould –
1972)
•Punctuated Equilibrium explains the speed at which
evolution takes place:
•Evolution involves long periods of time where
species undergo little change or do not change
(known as equilibrium)
•This alternates with (is punctuated by) short
periods of time where rapid changes occur
through natural selection during which new
species may form in a short period of time

16. PUNCTUATED EQUILIBRIUM VS GRADUALISM
GRADUALISM PUNCTUATED EQUILIBRIUM
slow and gradual changes in
species occurring as a result
of a series of small changes.
rapid change occurs and is
followed by long periods of
no change

17. THEORIES OF EVOLUTION
The diagrams below represent three
ideas proposed to explain evolution
1.What does MYA stand for? (1)
Million years ago
2.Give the letter of the diagram (A, B or C)
that would best represent:
a)Punctuated equilibrium (1)
C
b)Lamarck’s explanation of evolution (1)
A
c)Darwin’s explanation of evolution (1)
B

18. Exam guideline

19. Terminology
•Species: organisms that have the same
characteristics, capable of random interbreeding
and producing fertile offspring
•Population: individuals of the same species living
in the same area that can randomly interbreed
•Speciation: is theevolutionaryprocess by which
populations evolve to become distinctspecies.
•Geographical isolation and reproductive
isolation mechanisms, isolate the gene pool of a
species resulting with formation of new species.

20. Speciation
New distinct
species
No gene flow
between two
populations
Formation of
geographical
barrier
Original
population

21. Mechanism of speciation
•If a population of a single species becomes separated
by a geographical barrier (sea, river, mountain, lake)
•then the population splits into two.
•There is now no gene flow between the two
populations.
•Since each population may be exposed to different
environmental conditions/the selection pressure may
be different
•natural selection occurs independently in each of the
two populations
•such that the individuals of the two populations become
very different from each other
•genotypically and phenotypically.
•Even if the two populations were to mix again
•they will not be able to interbreed.
•The two populations are now different species.

22. Applying the recipe
DBE/November 2020(2)
Pottos and lemurs are small mammals.
Scientists believe that pottos and lemurs share a common ancestor that existed in Africa. Presently
pottos only occur in Africa while lemurs are only found in Madagascar.
Madagascar is an island off the East coast of Africa as shown in the diagram below.

23. Applying the recipe
Exam guideline Application to the question
•If a population of a single species
becomes separated by a
geographical barrier (sea, river,
mountain, lake)
•Then the population splits into
two.
•The common ancestor became
separated into two groups by the
ocean*✓
•There is now no gene flow
between the two populations
•There was no gene flow between
the two groups✓
•Since each population may be
exposed to different
environmental conditions/the
selection pressure may be
different
•Each group experienced different
environmental conditions✓
Describe the speciation of the pottos and lemurs to become different species. (6)

24. Applying the recipe
Exam guideline Application to the question
•Natural selection occurs
independently in each of the two
populations
•And underwent natural selection
independently✓
•Such that the individuals of the
two populations become very
different from each other
•The individuals in each group
become different✓
•Genotypically and phenotypically•Genotypically and phenotypically✓
•The two populations are now
different species
•Even if the two species were to
mix again
•They will not be able to interbreed
•To form the pottos and lemurs*✓
•Eventually if the two groups are
mixed again, they cannot
interbreed/produce fertile offspring

25. RECAP - Biogeography
The effect of continental drift on biogeography is that it provides
evidence for the evolution of ancient species. For example, flightless
birds in the southern hemisphere. Biogeography therefore provides
evidence for evolution.

26. DBE (Nov 2021)
The present-day distribution of three closely related species of the dog
family, the coyote, jackal and dingo, is shown on the world map below.

27. DBE (Nov 2021)
1. What type of evidence for evolution is represented here? (1)
‒Biogeography ✓
2. Describe how these three species could have evolved from a common
ancestor. (7)
‒The original population /common ancestor once lived on a large
continent ✓
‒and became separated by continental drift ✓ /oceans
‒There was no gene flow amongst the three populations ✓ *
‒Each population experienced different environmental conditions ✓
‒and underwent natural selection independently ✓
‒The individuals in each population became different✓
‒genotypically and phenotypically ✓
‒Even if the (three) populations are mixed again ✓ they would not be able
to interbreed ✓ /produce fertile offspring - forming the different species,
the coyote, jackal and dingo ✓ *
2 compulsory* + any 5

28. Speciation examples

29. Examples of speciation due to continental drift
Pangolins/
Anteaters
Madagascar
Baobab trees
Australian
Baobab
trees
South African
Baobab trees

30. Examples of speciation due to continental drift
Proteas
AUSTRALIA SOUTH AFRICA

31. Mechanisms of reproductive isolation
•Breeding at different times of the year
•Species-specific courtship behaviour
•Plant adaptation to different pollinators
•Infertile offspring
•Prevention of fertilization

32. Question
REPRODUCTIVE ISOLATING MECHANISM (RIM)
When scientists mated a male lion with a female tiger, the resulting offspring was called a liger. In
nature, interbreeding between these different species is prevented by the prevention of the fertilization
of the egg, differences in behaviour and by non-biological factors such as differences in range.
Usually, male ligers and many female ligers that arise by accident do not develop functional sex cells.
Such hybrid sterility prevents the formation of viable or fertile offspring.
This RIM prevents the formation of hybrids between members of different populations and reduces the
viability or fertility of hybrids
.
1.Name THREE reproductive isolating mechanisms in the passage above (3)
- Prevention of fertilization
- Differences in behaviour
- Non-biological factors/differences in range
- Infertile offspring

33. THANK YOU