Forces of evolution

The essentials

In population genetics, evolution is a change in allele (gene) frequencies in a population over generations. The Hardy–Weinberg principle states that allele and genotype frequencies remain constant if there is no mutation, no selection, no migration, random mating and a very large population. When these conditions are broken, the forces of evolution act.

Key points

• Mutation: a heritable change in DNA; the ultimate source of all new variation. Mutation rates are low, so mutation alone changes frequencies slowly.
• Natural selection: individuals with traits better suited to an environment leave more offspring. Classic human example: the sickle-cell allele (HbS) is maintained in malaria regions because heterozygotes (HbA/HbS) resist malaria better (balanced polymorphism or heterozygote advantage).
• Genetic drift: random change in allele frequencies, strongest in small populations. Founder effect: a small group starts a new population carrying only part of the original variation. Bottleneck effect: a population is sharply reduced by a disaster, and survivors' alleles dominate by chance.
• Gene flow (migration): movement of genes between populations through migration and intermarriage; makes populations more similar.
• Hardy–Weinberg equations: p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies.

Exam tips

• Drift is random; selection is non-random. Gene flow reduces differences; drift increases differences between populations.
• In a quick numerical, if q² (recessive phenotype) is known, take the square root to find q.

Common mistakes

• Thinking mutations are always harmful; many are neutral and some are beneficial.
• Confusing founder effect (new small colony) with bottleneck (sudden reduction of an existing population).