genetic drift

Meaning: In each generation only a sample of alleles passes to the next generation. By chance, that sample may not match the parent allele frequencies, just as a small number of coin flips may stray from 50% heads. The change has no direction toward better adaptation, so it is not driven by fitness. Over time, drift tends to lower heterozygosity within a population, and an allele can drift to loss (frequency 0) or fixation (frequency 1). Larger populations buffer against it.

Two classic examples are the bottleneck effect and the founder effect. In a bottleneck, a population is sharply reduced by a chance event (such as a natural disaster), and the survivors' allele frequencies differ from the original population. In the founder effect, a small group leaves to start a new population and carries only a subset of the original alleles.

On the MCAT: Passages describe a small isolated population, a catastrophe or a colonizing group, and ask you to predict allele frequency change, loss of variation or an effect on Hardy-Weinberg conditions. Drift violates the assumption of a very large population.

Common confusion: Students treat drift as natural selection. Selection is nonrandom, favors alleles that raise fitness and produces adaptation. Drift is random and can raise the frequency of a neutral or even harmful allele. Drift also differs from gene flow, which is movement of alleles between populations.