Mechanisms of evolution

The essentials

Evolution needs heritable variation. Variation can arise from mutation, recombination during gametogenesis, and gene flow. Darwin spoke of small, directional variations. Hugo de Vries, working on the evening primrose (Oenothera lamarckiana), proposed that evolution is driven by mutation: large, sudden changes that he called saltation (single-step large mutation). Mutations are random and directionless, while Darwinian variations are small and directional.

Recombination during meiosis (crossing over and independent assortment) shuffles existing alleles into new combinations.

Gene flow (gene migration): when individuals move from one population to another, alleles are added to or lost from the populations, changing gene frequencies.

Genetic drift: when changes in gene frequency happen by chance, especially in small populations. Sometimes the change is so large in a new sample of a population that it becomes a different species; the original drifted population becomes the founder, and the effect is called the founder effect.

Natural selection may act in three ways:

• Stabilising: more individuals acquire the mean character value; the peak gets higher and narrower.
• Directional: more individuals acquire a value other than the mean; the peak shifts in one direction.
• Disruptive: more individuals acquire peripheral character values at both ends; two peaks form.

Key points

• Mutation is the ultimate source of new alleles.
• Genetic drift is random; natural selection is non-random.
• Evolution is a stochastic process based on chance events in nature and chance mutation in organisms.

Common mistakes

• Disruptive selection favours both extremes, not the average.