A4.1 Evolution and speciationIB Biology SL: Revision notes

Evolution is change in the heritable characteristics of a population over generations. Two parts of this definition matter:

• Heritable: the change is in alleles, so it is passed on.
• Population: individuals do not evolve; populations do.

This distinguishes Darwinian evolution from Lamarckism, the idea that characteristics acquired during an organism's life (bigger muscles, a cut tail, a pruned plant) are passed to offspring. Acquired changes that are not genetic in origin are not evolution.

The theory of evolution by natural selection predicts and explains a very broad range of observations and is unlikely ever to be falsified. But no scientific theory can be formally proved true by correspondence, because a future observation could always contradict it. Evolution is therefore a pragmatic truth: it is called a theory despite all the supporting evidence.

In science, "theory" means a well-tested explanation, not a guess.

Base sequences in DNA or RNA and amino acid sequences in proteins give powerful evidence of common ancestry. Very different organisms share the same molecules (e.g. cytochrome c, ribosomal RNA) with many identical positions, which is unlikely by chance.

Mutations accumulate over time, so the fewer the differences, the more recent the common ancestor. Because the genetic code is degenerate, DNA sequences can differ even when protein sequences are identical, so DNA comparisons are more sensitive.

Selective (artificial) breeding of domesticated animals and crop plants shows how rapidly evolutionary change can occur. Humans choose parents with desired heritable traits each generation.

Examples: dog breeds from the grey wolf; cabbage, kale, broccoli, cauliflower, Brussels sprouts and kohlrabi from wild cabbage (Brassica oleracea). The variation between breeds and varieties, and between them and the wild species, arose within a few thousand years.

Homologous structures have the same basic structure because they were inherited from a common ancestor, even if their functions now differ. The pentadactyl limb (one upper bone, two lower bones, wrist bones, palm bones and five digits) is found in the arm of a human, the flipper of a whale, the wing of a bat and the leg of a frog.

Analogous structures have the same function but different evolutionary origins. They arise by convergent evolution, when similar selection pressures act on unrelated lineages. Examples: wings of insects and birds; the streamlined shape of sharks and dolphins; eyes of octopuses and vertebrates.

New species arise only by speciation: the splitting of a pre-existing species into two. Speciation increases the number of species on Earth; extinction decreases it. Gradual change within a single lineage is not speciation.

Two things are needed:

• Reproductive isolation: the populations stop interbreeding, so there is no gene flow. Geographical isolation by a barrier such as a river, mountain range or sea is one way.
• Differential selection: different environments impose different selection pressures, so allele frequencies diverge until the populations can no longer interbreed.

Example: the Congo River separates common chimpanzees (north) from bonobos (south). Neither can swim. Different conditions on each side (e.g. no gorillas competing for ground herbs in the south) are thought to have selected for different feeding and social behaviour.