Evolution
Evidence of Evolution
Speciation
Isolating Mechanisms
Evolution
= a genetic change in a
population
- Due to the increase or
decrease of gene
frequencies.
The Evidence for Evolution
• Fossils
- show anatomical links
between groups eg series of leg
bones showing hoof
development in horses,
Archaeopteryx has reptilian and
bird features
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- Time of first appearance eg a
pattern exists where fish
appear first, then amphibians,
then reptiles then birds and
mammals
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Comparative Anatomy
• Homologous structures – limbs
with different functions have
similar structures
Eg the pentadactyl limb in
humans, horse, dog and mole.
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• Analogous structures - limbs
with similar functions have
different structures
• Eg swimming forelimb in a
penguin is more similar to a
pigeon’s than a whale’s.
Functionless Structures and
Processes
• Energy is used to build
useless structures eg the
muscles that cause ‘goosebumps’ or move ears.
• Vestigial organs that are no
longer needed.
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Bad Design
• The ‘inverted retina’ of
vertebrates.
• Octopus have ‘correct’ retina.
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Embryonic development
• While the adults appear very
different, their embryos are
very similar.
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Fossil Genes
• Chickens carry tooth genes
but don’t express them.
• Can be induced to grow.
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Geographic distribution
• No mammals in NZ except
two species of bats.
• Widespread fossils but
descendants only found in
one place eg Tuatara
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Observed evolution
• Bacterial and insecticide
resistance
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Species
• A species is a collection of
populations sharing a common
gene pool (within which regular
gene flow occurs) which is
reproductively isolated from
other gene pools.
A species is normally divided up
into smaller demes. Each deme
is a partly isolated, interbreeding
population.
All pop’ns of a wide-ranging sp
show graduated variation =
cline. For example human skin
pigmentation from the tropics to
the poles.
Speciation
• The formation of a new species
is called speciation.
• Speciation can occur in the
following way:
–a population of an original
species becomes isolated from
other members of the same sp.
–changes develop as a result
of natural selection
pressures.
–Chromosomal and
phenotypic changes occur
that prevent interbreeding.
Allopatric and Sympatric
Speciation
• A ‘genetic barrier’ must exist
when members of two different
species don’t interbreed.
• Allopatric speciation occurs in
different areas due to physical
barriers eg Kaka and Kea.
• Sympatric speciation occurs
when the two species are
living in the same habitat and
is due to biological barriers.
• Barriers to gene flow are
called isolating mechanisms.
Topographical map of
Lake Barombi Mbo in
Cameroon, West Africa.
Apparently nine kinds of
cichlids evolved by
sympatric speciation in
this small, isolated crater
lake from which even
microgeographic
separation is absent. There
is separation by feeding
preference but all species
breed near the lake bottom
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in sympatry
Isolating Mechanisms
Pre-zygotic Isolating
Mechanisms
• Geographical barriers eg
mountains, deserts, water,
tectonic plate shifts.
• Ecological barriers eg different
niche preference.
(a) Blue-headed Wrasse from the Atlantic side of the
Isthmus of Panama and (b) Cortez Rainbow Wrasse from
the Pacific side. Both descended from a common ancestor
that split when the Isthmus was created.
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• Reproductive barriers eg
prezygotic and postzygotic
mechanisms
• Allopolyploidy, with
amphiploidy, creates instant
speciation.
• Fertilisation may be
prevented between two
different species by:
–breeding at different times
of the year.
–having different
courtship and mating
behaviour.
–mechanical differences
in genitalia.
Courtship displays between male
and female albatross.
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• chemical mismatch
between sperm and eggs,
including the chemistry of
copulation/pollination.
• using specialised animal
pollinators.
Postzygotic Isolating Mechanisms
• Following fertilisation between
two different species , zygote
development may be prevented
by:
–hybrid inviability via
incomplete development
–hybrid infertility due to
inability to perform
meiosis, no gametes.
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–hybrid breakdown due to F2
chromosomal crossover creating
disharmonious gene complexes.
• Often several isolating
mechanisms operate
simultaneously.
–eg lions and tigers interbreed in
captivity but not in the wild.
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• Isolating mechanisms can
also be quite weak.
–eg native grey duck and
introduced mallard
hybridise freely.