Genetic drift: chance has a vote
Hardy–Weinberg assumes an *infinitely* large population. Real populations are finite, and finiteness alone causes change. Genetic drift is the random fluctuation of allele frequencies from one generation to the next, caused purely by sampling — only some of the gametes in the gene pool actually become offspring, and which ones is partly luck.
The key rule: drift is stronger in small populations. Flip a fair coin 4 times and three heads wouldn't surprise you; flip it 4,000 times and you'll land very near half. Likewise, in a population of 20, an allele can lurch from 0.5 to 0.3 by chance in one generation; in a population of 20,000, it barely budges.
Drift is directionless — it doesn't favor “better” alleles — but it is not harmless. Run it long enough and an allele eventually wanders all the way to frequency 1 or 0. Reaching 1 is fixation (the allele is now the only one left); reaching 0 is loss. Either way, the polymorphism is gone and variation is permanently erased unless mutation or migration reintroduces it.
Gene flow: populations exchange alleles
Drift pushes populations apart. Gene flow — the movement of alleles between populations through migration and interbreeding — pulls them back together. When migrants join a new population and reproduce, they carry their home alleles into the local gene pool, nudging both populations' frequencies toward each other.
One generation of migration, locus with allele A
Local population: freq(A) = 0.20
Migrant source: freq(A) = 0.80
Migrants make up m = 10% of the breeding pool this generation
New freq(A) = (1 - m) * local + m * migrant
= 0.90 * 0.20 + 0.10 * 0.80
= 0.18 + 0.08
= 0.26
Even modest migration shifts frequencies toward the source
and erodes differences between populations.Effective population size & inbreeding
How small is “small”? The headcount can mislead. The effective population size (Ne) is the size of an ideal population that would drift at the same rate as the real one. Skewed sex ratios, a few individuals doing most of the breeding, or boom-and-bust cycles all make Ne much smaller than the census count — so drift bites harder than you'd guess.
Small Ne also raises inbreeding — mating between relatives, which is hard to avoid when few individuals are around. Inbreeding increases the chance that an individual inherits two copies of the same allele, raising homozygosity and exposing rare recessive alleles. Together, small Ne, drift, and inbreeding are the central worries in conservation genetics.