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Evolution
Contents
Definition
Darwin & Wallace
Natural selection - the
observations
Natural selection - the
conclusions
Evidence to support
evolution
 Anatomy
 Embryology
 Biochemistry
 Cytology
 Physiology
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Evolution
is about how modern day plants and animals
have gradually developed from pre-existing
forms, over long periods of time i.e.
changes in species by natural selection in
response to environmental changes i.e.
survival of the fittest.
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Darwin & Wallace
Many scientists had stated that species must
have evolved from one another.
Charles Darwin (1809 – 1882) – first to
suggest a mechanism or theory for it to
happen – did not publish his theory.
Sixteen years later Alfred Wallace came up
with the same theory.
The mechanism is called natural selection.
Based on four observations and two
conclusions.
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Natural selection – the
observations
1.
2.
3.
4.
Individual members of a species are
different from one another.
Offspring resemble their parents (variation
is inheritable).
More offspring are produced than can
survive and reproduce.
There is a struggle for existence and some
individuals have variations that make
them better suited to survival than others. 5
Natural selection – the
conclusions
favourable variations may make individuals
more adaptable
– these variations are passed on genetically
and will eventually become the norm.
=> survival of fittest.
 successive variations accumulate and
eventually interbreeding between this and
the original organism is no longer possible.
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=>new species formed.

Evidence to support evolution
(1/2)
The evidence shows that there was a common
descent between different groups of organisms
or that different organisms share a common
ancestor.
The evidence comes from a number of sources:  Fossils – Palaeontology
 Geographical distribution
 Taxonomy and
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Evidence to support evolution
(2/2)
from the comparative studies of:
- Anatomy
- Embryology
- Biochemistry
- Cytology
- Physiology
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Comparative anatomy
The forelimbs of terrestrial vertebrates are
similar in structure
-
the basic structure is known as the
pentadactyl limb.
The forelimbs share the same basic structure
but differ in their functions:
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Structures that differ in function
human
rabbit
whale fin
ant eater
bat wing
monkey
mole
horse
- lifting and grasping
- leaping
- swimming
- tearing
- flying
- grasping
- digging
- speed
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The
pentadactyl
forelimb in a
number of
vertebrates
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Vestigial structures
Some structures have been lost
– they no longer serve a
function.
e.g. horse stands on its third
toe.
Toes two and four are present
in a vestigial form. One and
five are no longer present.
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Back to Evidence to support evolution
Comparative embryology
The study of developing embryos of
vertebrates shows a similarity between
certain structures
e.g. the position of the brain, eyes, gill slits
and tail of the fish, tortoise, rabbit, bird,
chick and human.
This suggests that organisms descended from
common ancestors.
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e.g. gill slits
If all vertebrates came from a common
ancestor who had gills,
it would be likely that all vertebrates would
develop gills
and then modify them into other structures.
This is exactly what happens.
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A comparison of four
vertebrate embryos
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Back to Evidence to support evolution
Comparative biochemistry
The similarities between the respiratory
pathways in all living cells suggest a
common ancestor.
All living things use DNA, ATP and many of
their protein structures are very similar.
The greater the similarity in protein structures,
the more closely related they are,
and the smaller will be the difference in their
DNA structure.
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An
evolutionary
tree based
on
similarities
in fibrinogen
molecules
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Back to Evidence to support evolution
Comparative cytology
The presence of organelles such as
mitochondria, endoplasmic reticulum and
ribosomes in most cells and substances such
as DNA, RNA and ATP suggests a common
ancestor.
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Back to Evidence to support evolution
Comparative physiology
A study of the similarities between blood
pigments in the animal kingdom suggests a
common ancestor.
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END
Go to PPT on ‘The Evidence for Evolution’
Contains some very good pictures
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