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EVOLUTION -
Evolution is the accumulation of inheritable changes within
populations over time.
Darwin and evolution
Evolution as a Scientific fact
 If evolution has occurred, there are a variety of signs we should be able to detect.
 No test indicating that life has evolved has failed.
 However there is still considerable debate about the actual mechanisms of evolution.
Evidence for descent with modification
 Biogeography
 Functional morphology
 Paleontology (fossils)
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Comparative embryology
Animal and plant breeding
Molecular evidence
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Evidence from Biogeography
Darwin observed that organisms on isolated islands appear closest to organisms on the nearest
mainland. During the voyage of the HMS Beagle 1831-36, Darwin studied the Armadillos and
fossil Glyptodonts. He found:
 Both organisms were found in the same locations in South American. If these had
been created separately, why would both living and extinct forms be restricted to the
same area.
 Darwin thought it made more sense to assume the armadillo evolved from the
glyptodon.
Most evidence comes from the geographical distribution of species. Ostriches are found in
Africa. They have small wings and are flightless. In Australia, the emu is very similar in
appearance and body plan. In South America, there are rheas and in Madagascar there is the
elephant bird. The giant moa lived in New Zealand.
Each Galapagos island has a slightly different species of finch, all of which appear close to a
single species found on the South American mainland.
Evidence from Functional Morphology
Vestigial Organs: those structures that functioned in an ancestral organisms, but that are
reduced (in structure and function) in the descendant
 Humans have muscles to move their ears.
 Humans have a vestigial tailbone.
 Vestigial toes in the horse.
 Vestigial limbs in whales and snakes
 Nictating membrane of eye
 Caudal vertebrae of coccyx
 Appendix
Vestigial toes in the horse
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Vestigial pelvis and femur in whales
Vestigial pelvis and femur in some snakes
By comparing the anatomy of species, gradual progressions in structural characteristics can be
observed.
For example, comparison of skulls of gorillas, chimpanzees, oranutangs, humans and many
now extinct anthropoids has made it possible to suggest pathways of evolutionary progression
from common primate ancestor to the existing species.
When comparisons are made of the anatomy of structures which seem to have evolved very
different functions, it can be seen that the process of natural selection has led to these
differences. Homologous structures such as the forelimbs of a variety of mammals (e.g.
human, cat, whale and bat) can be shown to possess the same skeletal elements, suggesting that
a common ancestral forelimb has been modified for many different functions. (see Divergent
Evolution)
Homologous structures such as those described above should not be confused with analogous
structures (e.g. bat wing and insect wing), where evolution has given rise to anatomically very
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different structures which have similar function (aid in flying) where this function provides a
survival advantage in a particular environment. (see Convergent Evolution)
Comparing structures of various organisms leads to system of classification that indicates
evolutionary relationships.
Evidence from animal breeding
 Darwin was greatly impressed by how much domesticated animals had been changed
by artificial selection.
 Breeds of dogs, cattle, pigeons
Fossils: The Hard Evidence for evolution (Paleontology)
Fossils can take many forms:
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Hard parts of animals, teeth and bones, are often fossilized
Thin tissues such as leaves may be preserved as films
Entire organisms may be preserved e.g. frozen or in amber
Plant tissue is sometimes found "petrified"
Imprints such as footprints of dinosaurs occur, but rarely.
Fossils are most often found in sedimentary rocks such as sandstone, mudstone, shale and
limestone. The layers of silt or other debris which accumulate over time exert pressure on plant
and animal remains, or settle into the imprints left by passing animals, minerals are exchanged,
and fossils are formed.
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Fossils show a succession from very simple morphological forms early in the fossil
record to much more complex forms that appear much later in the fossil record.
Fossils show multiple examples of transitional forms.
Direct evidence for descent with modification
Classic example: Evolution of the hoof
Evidence from comparative embryology
Closely related organisms go through similar stages in their embryonic development.
In fact, the similarities between early embryos of fish, frogs, snakes, birds, cats and humans are
much more evident than are the subtle differences.
By following the progression of development of embryos, and studying the paths of
differentiation of tissues in different species, many similarities between species, which are
difficult to detect in adults, become increasingly clear.
Certain embryonic structures are shared by all chordates, but show interesting structural and
functional changes during development,
e.g.
1. notochord
2. gill slits
3. aortic arches
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Early embryos of very different organisms closely resemble each other.
More recent evidence
 Explosion of data in all of the above fields in the 100 years since Darwin has
confirmed and expanded upon Darwin’s observations.
 Molecular data
Evidence from biochemistry
With recent technological advances in the analysis of the molecular composition of biological
molecules, some of the most compelling evidence for evolution has been discovered.
Since the hereditary information of an individual is located in its genes, we would expect that
closely related individuals (e.g. siblings) would have a large amount of very similar DNA and
proteins. This is found to be true. For the same reason, we would expect that members of the
same species would also possess much common DNA and many common amino acid
sequences in their proteins. This is also found to be true.
Examination of nucleotide sequences in DNA and amino acid sequences in similar proteins
from a large number of species has shown conclusively that closely related species have much
in common biochemically. BUT, evolutionary distance (and time for accumulation of
mutations) leads to wider and wider differences between phenotypes, genotypes, proteins and
DNA.
The first 35 amino acids of the protein, cytochrome c, from the human, chimpanzee and rhesus
monkey are identical. The horse, which is less closely related, shows 3 differences in the
sequence. The more distantly related the organisms are to man, the more their sequence
differs. The silkmoth has 10 differences, the wheat plant has 13 and the yeast has 14
differences.
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Paths of Evolution
The agents of evolution include:
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natural selection,
mutation,
genetic drift,
migration,
nonrandom mating.
Of these, only natural selection leads to adaptations in populations within their changing
environments. The others merely produce new opportunities for natural selection to work.
However, without mutations to provide the raw material on which natural selection can work,
the variability in populations would be strictly limited.
With this in mind, it is possible to trace the evolutionary pathways taken from ancient ancestral
species to the present day species. Since the biochemical thread of evidence is only possible to
trace in current species, the combined information of the ancient fossil evidence and our
understanding of relationships between present species together provides powerful support for
the modern theory of evolution.
DIVERGENT EVOLUTION - One ancestral stock evolves into two species, which continue
to evolve and become less and less alike over time.
The ancestral mammal probably existed about 190 million
years ago. It was around at the time of the dinosaurs, but
with the extinction of the dinosaurs, the mammals were
able to undergo extensive adaptive radiation to fill a
wide range of ecological niches.
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PARALLEL EVOLUTION - In parallel evolution two related species arise from a common
ancestor. The two species then evolve in much the same way over time, probably in response to
similar environmental selection pressures."
In this example, both the woolly mammoth, which occupied parts of North America, and the
elephant, still found in Asia and Africa are presumed to have evolved from a common ancestor.
Their geographical isolation and environmental selection pressures caused further evolution of
the species, but each, in its own location, occupies(d) a similar niche.
CONVERGENT EVOLUTION - convergent evolution occurs when two or more groups that
are not closely related come to resemble each other more and more as time passes. This is
usually the result of occupation of similar habitats and the adoption of similar environmental
roles.."
From three very different ancestral sources, three flying animals evolved.
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Gradualism
During speciation, a new species diverges from its parent species as a small isolated
population. According to the gradualist model, species descended from a common ancestor
diverge more and more in morphology as they acquire unique adaptations.
Diagram A illustrates one view of the rate of evolution. It shows a branching of an
evolutionary path, from a common ancestor, and the gradual emergence of one or more new,
different species. The gradual slope of the branches indicates that many intermediate forms of
the organism are believed to have existed although evidence of them (usually by fossils) is
absent or incomplete.
Punctuated equilibrium
Diagram B reflects the theory that evolution has not occurred at a constant rate, rather it has
occurred in short bursts with long periods of stability between. This theory explains the absence
of the infinite range of intermediate fossils in branches of evolution such as the vertebrates.
Eldredge and Gould proposed that most major morphological change occurs (relatively)
quickly in small peripheral population at the time of speciation. New forms will then invade the
range of their ancestral species. Thus, at most locations that fossils are found, transition from
one species to another will be abrupt.
Diagram A
Diagram B
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