Natural selection is one way allele frequencies change. Mutation, drift, gene flow and selection are others, and Hardy-Weinberg assumes a large population for a reason. In a small one, chance alone moves allele frequencies around. That random change is genetic drift.
Key idea
Drift is a random change in allele frequency from one generation to the next, strongest in small populations. A bottleneck (most of a population dies for reasons unrelated to its genes) or a founder effect (a few individuals start a new population) can change frequencies sharply and cut genetic variation.
Why it works
Each generation is a sample of the genes of the last one. Toss a coin 10 times and getting 7 heads is common; toss it 1,000 times and 700 heads almost never happens. In the same way, a small population's allele frequency can jump by chance each generation, and once an allele is lost (p = 0) or fixed (p = 1), drift alone cannot bring variation back; only mutation or gene flow can. Large populations drift too, but so slowly that selection usually dominates.
- Bottleneck: northern elephant seals were hunted to a very small number in the 1800s. They recovered to a large population, but today they have very little genetic variation.
- Founder effect: a few birds blown to a new island carry only some of the mainland's alleles, at frequencies that happen to differ from the mainland's.
Worked examples
Worked example: A founding group
On the mainland, allele B has frequency 0.5. Ten lizards (20 alleles) raft to an island: 5 are BB, 4 Bb, 1 bb. What is the frequency of B on the island?
- B alleles: $2(5) + 4 = 14$ of 20.
- $p = 14/20 = 0.7$.
Answer: 0.7, up from 0.5 by chance alone: a founder effect.
Worked example: Which population drifts more?
Two populations of the same plant, one of 30 and one of 30,000, start with the same allele frequencies and face no selection. Which is more likely to lose an allele in 50 generations?
- Chance changes are larger in small samples.
- The population of 30 can lose an allele in a few unlucky generations.
Answer: The population of 30.
Common mistake
Thinking drift favors helpful alleles. Drift ignores fitness: it can remove a helpful allele or fix a harmful one, especially when numbers are small. If an allele rises because its carriers survive or reproduce better, that is selection, not drift. A check: would the change happen even if the allele did nothing at all? Then it is drift.