Genetic Drift

Genetic drift is the random change in allele frequencies that arises because each generation is a finite sample of the gametes of the previous one. By chance alone, some alleles increase and others decrease in frequency, and over time alleles may be lost or driven to fixation even in the absence of selection.

Definition

Genetic drift is the change in allele frequencies between generations that results from random sampling of which gametes contribute to the next generation in a finite population, independent of any fitness differences.

Scope

The entry covers the sampling origin of drift, its dependence on effective population size, its characteristic effects — random fluctuation, loss of variation, and eventual fixation — and the special cases of founder effects and bottlenecks. It is a conceptual and methodological topic within population genetics, not clinical guidance.

Core questions

• Why do allele frequencies change at random in finite populations?
• How does effective population size control the strength of drift?
• What are the long-term consequences of drift for genetic variation?

Key concepts

• Random sampling of gametes
• Effective population size
• Fixation and loss of alleles
• Founder effect
• Population bottleneck
• Loss of genetic variation

Key theories

Drift and effective population size

Sewall Wright showed that the magnitude of random allele-frequency change per generation scales inversely with effective population size, so drift dominates in small populations and is weak in large ones.

Mechanisms

Because a finite number of offspring inherit alleles sampled from the parental gene pool, the realised allele frequency in each generation departs randomly from the parental frequency, and these deviations accumulate over generations as an unbiased random walk. The expected size of the per-generation change increases as effective population size falls, so small populations drift quickly. Left to run, drift eventually fixes one allele and removes the others, reducing heterozygosity; founder effects and bottlenecks are abrupt forms of drift in which a small subset of individuals founds or survives in a population, sharply altering and impoverishing its allele frequencies.

Clinical relevance

Drift, especially through founder effects and bottlenecks, explains why certain disease alleles reach unusually high frequencies in particular populations or isolates, which informs the interpretation of population-specific carrier frequencies. It describes how variation is distributed across populations and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Founder effects and historical bottlenecks have left some human populations with elevated frequencies of specific recessive disease alleles relative to the global average, producing the population-specific patterns seen in carrier-screening data.

History

The role of chance in changing allele frequencies was formalised by Sewall Wright in the 1930s, who quantified its dependence on effective population size and made it one of the four principal evolutionary forces. The concept later became central to Motoo Kimura's neutral theory, which proposed that much molecular variation is governed by drift rather than selection.

Debates

How much of molecular variation is shaped by drift versus selection?

The neutral theory holds that most molecular variants are effectively neutral and governed by drift, while selectionist views emphasise pervasive selection; the balance, mediated by effective population size, remains an active question in molecular population genetics.

Key figures

• Sewall Wright
• Motoo Kimura
• Brian Charlesworth

Seminal works

• wright-1931

Frequently asked questions

How is genetic drift different from natural selection?

Drift changes allele frequencies at random because populations are finite, regardless of any fitness effect, whereas selection changes frequencies systematically in favour of alleles that improve survival or reproduction.

Why does genetic drift matter more in small populations?

Because the random sampling effect is larger when fewer individuals contribute to the next generation; the expected per-generation change in allele frequency scales inversely with effective population size.