A4.1 Evolution and speciationIB Biology HL: Revision notes

Evolution is change in the heritable characteristics of a population. This distinguishes it from Lamarckism: characteristics acquired during life (e.g. muscles built by exercise) are not genetic and are not evolution.

NOS: evolution by natural selection explains a very broad range of observations and is unlikely ever to be falsified, but no theory can be formally proved true by correspondence. It is a pragmatic truth, still called a theory despite the evidence.

• Sequence data: base sequences of DNA/RNA and amino acid sequences of proteins give powerful evidence of common ancestry; fewer differences = more recent common ancestor.
• Selective breeding: breeds of dog from the grey wolf and crop varieties from wild cabbage show how rapidly heritable change can occur.
• Homologous structures: the pentadactyl limb has the same bones in human arms, bat wings, whale flippers and mole forelimbs, despite different functions.
• Analogous structures arise by convergent evolution: same function, different evolutionary origin, e.g. bat and butterfly wings.

New species arise only by splitting of a pre-existing species. Speciation increases the number of species; extinction reduces it. Gradual change in one lineage is not speciation.

Populations must be reproductively isolated (no gene flow), then differential selection in different environments makes allele frequencies diverge. Example: the Congo River separates bonobos (south) from common chimpanzees (north); neither can swim, and different conditions on each bank selected different behaviour.

Reproductive isolation can be:

• Geographic: a physical barrier (river, mountain, sea).
• Behavioural: different courtship or mate choice.
• Temporal: breeding at different times of day or year.

Allopatric speciation happens in populations in different places, separated by a geographical barrier. Sympatric speciation happens in populations in the same place, through behavioural or temporal isolation, or polyploidy.

Similarities: both need reproductive isolation followed by divergence of gene pools. Difference: whether a geographical barrier is involved.

Adaptive radiation is the rapid diversification of one ancestral species into many species, each adapted to a different niche. Because each species uses different resources, closely related species can coexist without competing, which increases biodiversity where there were vacant niches.

Examples: cichlid fish in the African Great Lakes (different jaws for algae, snails, fish); Darwin's finches on the Galápagos (different beaks for seeds, insects, cacti).

Alleles are kept from mixing between species in two ways:

• Barriers to hybridization stop mating or fertilisation. In animals, courtship behaviour is often species-specific, so individuals do not recognise other species as mates.
• Sterility of interspecific hybrids: if a hybrid is formed, it cannot reproduce. A mule (male donkey × female horse; 63 chromosomes) is sterile because its horse and donkey chromosomes cannot pair in meiosis.

In plants, a new species can arise in one or two generations:

1. Two species hybridize; the hybrid is sterile because its chromosomes are not homologous.
2. The chromosome number doubles (polyploidy), so every chromosome has a partner; meiosis works and the plant is fertile.
3. The new polyploid cannot produce fertile offspring with either parent (mismatched chromosome numbers), so it is instantly reproductively isolated.

This is abrupt and sympatric. The knotweed/smartweed genus Persicaria contains many species formed this way.