5.1 Genetic drift

In all populations, genetic drift acts to change allele frequencies over time. Drift refers to random changes in an allele’s frequency between generations. These random changes occur because individuals carrying different alleles will have different numbers of offspring due to chance.

Fig. 1. An allele’s frequency “drifts” between generations due to random chance.

Drift differs from selection, which is a deterministic (non-random) change in an allele’s frequency. If an allele is under selection, it’s more likely to increase or decrease in frequency depending on whether it is beneficial or deleterious. Genetic drift, on the other hand, cannot consistently cause an allele’s frequency to increase or decrease.