Genetic drift

(1) Chance occurrences in small populations which lead to changes in gene frequencies from generation to generation. (2) The tendency, within small interbreeding populations, for heterozygous gene pairs to become homozygous for one allele or the other by chance, rather than by selection. (3) A change in gene frequency that occurs in small populations as a result of random sampling error during reproduction. (4) The fluctuation in gene frequency occurring in an isolated population, presumably due to random variations from generation to generation. [Source: USDA National Plant Materials Manual]