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 ‘goose-bumps’ 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

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.

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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 in sympatry

Isolating MechanismsPre-zygotic Isolating

Mechanisms• Geographical barriers eg

mountains, deserts, water, tectonic plate shifts.

• Ecological barriers eg different niche preference.

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(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.

• 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.

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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

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–hybrid infertility due to inability to perform meiosis, no gametes.

–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.