The term for the random change in allele frequencies in a population due to chance is genetic drift. It is an evolutionary mechanism in which chance events alter how common existing genetic variants become, independently of whether those variants improve survival or reproduction.
Genetic drift occurs because offspring inherit a random sample of their parents’ alleles. The effect is strongest in small populations, where random events can shift frequencies substantially from one generation to the next. An allele may become more common, disappear entirely, or eventually become fixed at a frequency of 100%.
Two well-known examples are the bottleneck effect and the founder effect. A bottleneck occurs when a population is sharply reduced by an event such as a disaster. A founder effect occurs when a small group establishes a new population and carries only a limited sample of the original gene pool.
Genetic drift differs from natural selection, which produces non-random changes associated with differences in fitness. Mutation creates new alleles, while gene flow moves alleles between populations; drift changes the frequencies of alleles already present.