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Evolution

The process by which populations of living organisms change in their heritable characteristics over successive generations. Charles Darwin and Alfred Russel Wallace proposed natural selection as its principal mechanism: individuals with heritable traits better suited to their environment tend to survive and reproduce more successfully, so those traits become more common in the population over time.

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Population Genetics: Quantifying Allele-Frequency Change

Evolution can be defined precisely as a change in allele frequencies within a population's gene pool across generations. For a gene with two alleles at frequencies p and q (p + q = 1), the Hardy-Weinberg principle predicts the genotype frequencies a population would show if it were NOT evolving.

p+q=1p + q = 1

Allele frequencies for a two-allele gene must sum to 1.

p2+2pq+q2=1p^2 + 2pq + q^2 = 1

Hardy-Weinberg genotype frequencies (homozygous, heterozygous, homozygous) expected under no evolution.

Hardy-Weinberg equilibrium assumes no mutation, no migration, random mating, an infinitely large population and no selection. A real population's departure from these predicted proportions is evidence that one or more of these assumptions is being violated — for example, selection changing an allele's frequency, as in antibiotic resistance spreading through a bacterial population.

Info: Hardy-Weinberg equilibrium is a null model, not a claim that real populations stay unchanging — it is the reference point against which real allele-frequency change (evolution) is measured.

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Sources and methodology

  • Charles Darwin and Alfred Russel Wallace proposed natural selection as the principal mechanism of evolution, with Darwin's account published in 'On the Origin of Species' in 1859.
    • On the Origin of Species by Means of Natural Selection
  • Natural selection requires heritable variation within a population, a link between that variation and differential survival or reproduction, and successive generations for the resulting differences to shift the population's trait composition over time.
    • On the Origin of Species by Means of Natural Selection
  • Evolution acts on populations, not individuals: a single organism's heritable traits do not change during its own lifetime — only a population's trait frequencies shift across generations.
    • Evolutionary Analysis
  • The twentieth-century Modern Synthesis integrated Mendelian and population genetics with Darwinian natural selection into a single, unified quantitative framework of evolutionary biology.
    • Evolutionary Analysis

Content status: published, last reviewed 30 September 2026.