Genetic drift

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Genetic drift is the random change in the frequency of alleles in a population due to chance events causing unequal participation of individuals in producing succeeding generations. Genetic drift can be a principal force in the evolution of some populations; it is especially associated with populations of small size or where polymorphisms in a gene do not have very different fitnesses.

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Genetic Drift as it Relates to Allele Frequencies

Different forms of a gene are called alleles. Individual members of a population have different alleles. Together, all the alleles for all the genes in a population constitute the "gene pool" of the population. Through reproduction, individuals pass their genes on to the next generation. Considering only the effect of genetic drift, the larger the population is, the more stable the frequency of different alleles in the gene pool will be over time; this is because random chance will play less of a role in determining who breeds. In small populations, allele frequencies are likely to change rapidly and dramatically over very few generations, or "drift," because of chance events. This rapid change can occur in small populations because each individual's alleles represent a large fraction of the gene pool, and if an individual did not reproduce it could have a much larger effect than in the case of an individual in a large population not reproducing. Also, alleles that are found infrequently are more likely to be lost due to random chance.

After many generations, if only genetic drift is operating, populations (even large populations) will eventually contain only one allele of a particular gene, becoming "monomorphic," or fixed for this allele. This can be seen as a loss of information in the population, though in real populations, this process is prevented by mutations. As a general rule, the chances of an allele becoming fixed in the population are equal to its frequency in that population. For example, if a population has a frequency of 0.8 for allele A, then A has an 80 percent chance of being fixed.

Random Events that Affect Genetic Drift

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 randomly 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 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. One consequence of this process is inbreeding, and when inbreeding occurs, deleterious alleles can "drift" to high frequency in the population. This can ultimately lower the total fitness of a population, although these effects can be mitigated by gene flow (introducing new variation by breeding with other populations) and selection acting on new variation.

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.

Genetic drift and evolution

Along with selection, mutations, and gene flow, genetic drift is one of the major mechanisms that can cause populations to change over time. In the neutral theory of molecular evolution, genetic drift is seen as the most important element causing evolutionary change at the DNA sequence level: Since most changes at the molecular level are neither good (beneficial) nor bad (deleterious), but neutral, these changes must "drift" to fixation.

Further Reading

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.
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