When a small population reproduces, the subsequent generation is
derived from a sample of parental gametes. Each offspring receives one
allele, selected at random, from each parent. Just by chance, some
alleles, especially rare ones, may not be passed on to the offspring and
may be lost. The frequencies of alleles that are transmitted to the following
generation are likely to differ from those in the parents (Fig. 8.1).
Over multiple generations allele frequencies change, or drift, from one
generation to the next, a process termed random genetic drift.