Genetic drift describes random changes in allele frequencies caused by chance, especially in small populations.
Unlike natural selection, genetic drift does not require an allele to improve survival or reproduction. Sampling effects can make some variants more common and others disappear, even when the variants have no fitness difference.
Two classic examples are the founder effect and the bottleneck effect. The founder effect occurs when a new population begins from a small group, while a bottleneck occurs when an event sharply reduces population size. Both can reduce genetic variation and make rare alleles unusually common.
Drift is strongest in small populations and can operate alongside mutation, migration, and selection. A common mistake is to describe every evolutionary change as adaptation; drift can change frequencies without producing a trait that is better suited to the environment.