- Genetic drift is an evolutionary mechanism in which allele frequencies change purely by chance, not because certain traits are more adaptive or selected. It occurs in all populations of finite size, but its effects are most pronounced in small or isolated populations, where random sampling errors strongly influence which alleles are passed to the next generation. Over time, drift can cause alleles to disappear completely, reducing genetic diversity, or become fixed, reaching 100% frequency in the population.
- Genetic drift contrasts with natural selection, which changes allele frequencies based on differential survival or reproduction. Drift, by comparison, is non‑directional: alleles spread or vanish regardless of whether they are beneficial, neutral, or harmful. This randomness means that even advantageous alleles can be lost, while neutral or slightly deleterious alleles may become common. Drift is especially influential when populations undergo sharp reductions in size or when small groups colonise new areas.
- Two major forms of drift illustrate how chance shapes evolution. The bottleneck effect occurs when a population is suddenly reduced by a natural disaster or similar event, leaving a small, random subset of individuals whose allele frequencies differ from the original population. The founder effect occurs when a small group breaks away to establish a new population, carrying only a fraction of the original genetic variation. Both processes can dramatically shift allele frequencies and reduce genetic diversity, sometimes contributing to long‑term vulnerability or extinction risk.
- Although genetic drift does not drive adaptation, it plays a crucial role in shaping evolutionary trajectories. In very small populations, drift can lead to inbreeding depression, reduced fitness, and increased extinction risk. At the molecular level, drift is central to the neutral theory of molecular evolution, which proposes that most genetic changes spreading through populations are neutral and fixed by drift rather than selection. Over many generations, drift can profoundly alter gene pools, even when its short‑term effects appear small.
Further reading:
- Masel J., 2011. Genetic drift. Curr Biol. 21(20), R837-8. DOI: 10.1016/j.cub.2011.08.007, PMID: 22032182, Cell (Download PDF)
- Bach J.F., 2019. Genetic drift in mammals. An Acad Bras Cienc. 91(suppl 3): e20190339. DOI: 10.1590/0001-3765201920190339. PMID: 31460595, Scielo (Download PDF)
- Lynch M., et al., 2016. Genetic drift, selection and the evolution of the mutation rate. Nat Rev Genet. 17(11):704-714. DOI: 10.1038/nrg.2016.104 . PMID: 27739533, Nature
- Lynch M., et al., 2011. The repatterning of eukaryotic genomes by random genetic drift. Annu Rev Genomics Hum Genet. 12:347-66. DOI: 10.1146/annurev-genom-082410-101412, PMID: 21756106, Annualreviews, PMCID: PMC4519033 (Download PDF)
