Genetic drift (or
allelic drift) is the change in the frequency of a
gene variant (
allele) in a population due to random sampling of organisms. The alleles in the offspring are a sample of those in the parents, and
chance has a role in determining whether a given individual survives and reproduces. A population's
allele frequency is the fraction of the copies of one gene that share a particular form. Genetic drift may cause gene variants to disappear completely and thereby reduce
genetic variation.