How evolution works

Darwin's variations were small and built up gradually, in a direction. Hugo de Vries, working on the evening primrose, proposed instead that large, sudden changes, which he called mutations, cause new species to arise in a single step: saltation. Mutations are random and have no direction. Today we know both kinds of change matter.

Hardy-Weinberg principle. In a population, each gene has alleles at certain frequencies. If nothing disturbs the population, these frequencies stay constant from generation to generation: the population is in genetic equilibrium. For a gene with two alleles, A (frequency p) and a (frequency q), where p + q = 1, the genotype frequencies are:

p² (AA) + 2pq (Aa) + q² (aa) = 1

For example, if q = 0.3, then p = 0.7, and the fraction of heterozygotes is 2pq = 2 × 0.7 × 0.3 = 0.42.

When the measured frequencies differ from these values, the population is evolving. Five factors disturb the equilibrium:

• Gene migration (gene flow): individuals move in or out, adding or removing alleles.
• Genetic drift: in a small population, allele frequencies change by chance. Sometimes a small group drifts away and starts a new population; its allele frequencies may be very different, and it can become a new species: the founder effect.
• Mutation: new alleles appear.
• Genetic recombination during gamete formation creates new combinations.
• Natural selection.

Natural selection can work in three ways on a trait such as body size:

• Stabilising: individuals near the average are favoured, and the extremes decline.
• Directional: individuals at one extreme are favoured, and the average shifts that way.
• Disruptive: individuals at both extremes are favoured, and more appear at both ends of the curve.