Many types of random events that can affect the likelihood of alleles being passed to future generations can be imagined. An adult may fail to mate during mating season due to unusually adverse weather; a pregnant mother may discover a rich food source and produce unusually strong or numerous off-spring; all the offspring of one parent may be consumed by predators. Many other scenarios are possible.
To see how such events affect allele frequencies, imagine a population that contains four individuals of an organism that reproduces once and dies. Let us examine how allele frequencies change for a gene that has two alleles, A and a. As with other genes, each individual has two alleles, one inherited from each parent. Imagine that three of the individuals are aa genotype, and one is Aa genotype. Thus, of the population's eight copies of the gene, one is A, and seven are a. Now imagine that because of random chance, the Aa individual does not reproduce. Therefore, only aa offspring are produced and the A allele is lost to the population. The A allele goes from a frequency of one-eighth to zero through the process of genetic drift.
A large reduction in population size can lead to a situation known as a genetic bottleneck. After a genetic bottleneck the population is likely to have different allele frequencies. When only a very small number of individuals are left after a population decline, the population will have only the alleles present in these few individuals. This is known as the "founder effect." The founder effect can be viewed as an extreme case of a genetic bottleneck. If a population decline affects all individuals in the population without respect to the alleles they carry, genetic drift will have an effect on all genes.
Genetic drift has important implications for evolution and the process of speciation. When a small group of individuals becomes isolated from the majority of individuals of a species, the small group will genetically drift from the rest of the species. Because genetic drift is random and the smaller group will drift more rapidly than the larger group, it is possible that, given enough time, the small group will become different enough from the large group to become a different species.
The fact that small populations are more subject to genetic drift has important implications for conservation. If the number of individuals of a species becomes small, it becomes increasingly influenced by genetic drift, which may result in the loss of valuable genetic diversity. Conservation biologists seek to maintain populations at sufficient numbers to counteract genetic drift.
R. John Nelson
Avise, John C. Molecular Markers, Natural History and Evolution. New York: Chapman and Hall, 1994.
Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, 1998.
Mayr, Ernst. Evolution and the Diversity of Life: Selected Essays. Cambridge, MA: Belknap Press, 1976.
Weaver, Robert F., and Philip W. Hedrick. Genetics, 2nd ed. Dubuque, IA: William C. Brown, 1992.