What is the process where allele frequencies change due to chance?

The process in which allele frequencies change due to chance is genetic drift. It results from random sampling in finite populations, so an allele can become more common, rarer, or disappear even when it provides no survival advantage.

A simple example is a small population containing equal numbers of red and blue alleles. If only a few individuals reproduce by chance, the next generation may contain many more copies of one allele than the other. Repeating this process can eventually eliminate one allele or make the other fixed at a frequency of 100 percent.

Genetic drift is strongest in small populations, where random events produce larger percentage changes. Two important examples are the bottleneck effect, when a population is sharply reduced, and the founder effect, when a new population begins from a small group of colonists. Drift differs from natural selection because it has no consistent direction toward improved adaptation. Mutation creates new alleles, and gene flow moves alleles between populations; genetic drift changes the frequencies of alleles already present through chance.