Genetic Drift

Core Idea

Genetic drift is the random change in allele frequencies caused by chance sampling events, particularly powerful in small populations. Unlike natural selection, drift is not driven by fitness — neutral or even slightly deleterious alleles can become fixed by chance. The bottleneck effect (population crash) and founder effect (new colony from few individuals) are important special cases. Drift reduces genetic diversity over time.

How It's Best Learned

Simulate allele frequency changes in small vs. large populations using coin-flip models or software. Compare outcomes of drift vs. selection across many replicate populations. Pay attention to how effective population size (Ne) differs from census size.

Common Misconceptions

• Genetic drift is not random mutation — it is random sampling of existing alleles.
• Drift can fix maladaptive alleles; evolution does not always move toward 'improvement'.
• Large populations are not immune to drift, but its effects are minor compared to small populations.

Explainer

You already know from population genetics that a population's allele frequencies change over time through mechanisms like mutation, selection, and migration. Genetic drift is a fourth mechanism — and unlike natural selection, it has nothing to do with fitness. It is pure statistical noise.

Imagine a small island population of 10 beetles, 5 carrying a red allele and 5 carrying a brown allele. If a storm randomly kills 3 red-allele carriers but no brown ones, the red allele frequency drops — not because brown is more adaptive, but because of chance. In the next generation, that skewed starting point compounds the effect. Drift is essentially sampling error in allele transmission: each generation is a random draw from the previous one, and small samples are noisier than large ones.

The expected *direction* of allele frequency change from drift is zero — it does not push alleles toward fixation or elimination in any predictable direction. But the *variance* is large in small populations and small in large ones. This is why the bottleneck effect (a population crash leaving only a few survivors) and founder effect (a few individuals colonizing a new area) are such powerful evolutionary forces: they create tiny effective population sizes (Ne), turning up the volume on drift. An allele present at 10% frequency in a large population might be entirely absent — or the only allele remaining — after a bottleneck.

Given enough generations, drift will eventually fix one allele and eliminate all others at a given locus. For a neutral allele, the probability that it reaches fixation equals its current frequency. So a rare allele has only a small chance of fixing, but if it does, it happened by chance rather than fitness advantage. This is a key insight of the neutral theory of molecular evolution: many amino acid substitutions observed between species appear to be neutral changes that were fixed by drift, not selected for by the environment.

A critical distinction to keep straight: genetic drift reshuffles the frequencies of *existing alleles* — it does not create new variants. Mutation is what introduces new alleles into a population; drift determines whether those alleles spread, persist, or disappear. Confusing these two processes is one of the most common errors when reasoning about evolutionary change.