Genetic drift changes allele frequencies through random chance, especially in small populations.
Unlike natural selection, drift does not favor a variant because it improves survival or reproduction. A chance event may cause some individuals to leave more descendants, while other variants disappear. The effect is stronger when a population is small because random sampling has a greater influence.
Two important examples are the founder effect and the bottleneck effect. The founder effect occurs when a new population begins with a small sample of a larger population. A bottleneck occurs when a population is sharply reduced by an event such as a disaster, leaving survivors with an unrepresentative set of alleles.
Genetic drift can reduce genetic variation and can fix an allele, meaning it reaches a frequency of 100 percent. It operates alongside mutation, migration, mating patterns, and natural selection in real populations.