Genetic Drift

Genetic drift (also allelic drift, or simply drift) is a change in the allele frequency of a population as a result of random, undirected processes. For instance, the random sampling of parental alleles in an offspring generation is genetic drift. It is one of the key driving forces in evolution, along with natural selection and gene flow.

An example of genetic drift is to consider a population of beetles: in a population of beetles any individual could be stamped on and killed by a human foot and have its alleles effectively 'removed' from the gene pool by random chance - regardless of biological 'fitness'. The other beetles will remain and be able to pass on their alleles purely as a result of luck. There is no directed selection going on this process; just blind statistical chance. It is also a question of 'luck' as to which parental alleles are passed down to offspring, so the allele frequency of an offspring population, as compared to the parental organisms, is also largely a consequence of genetic drift.

Because genetic drift is 'blind' in terms of environmental or adaptive pressures, it may cause mutations to persist in organisms that are either beneficial, harmful or neutral in their phenotypic effect.

New alleles may, rarely, drift to fixation. This means that a new allele appears in the population by mutation, and is passed down so many generations that it becomes a permanent substitute for the original allele(s) in the population. The rate of allelic substitution by genetic drift is inversely proportional to population size: in a larger population, less fixation can occur purely as a result of drift simply because there is a lot more competition in the gene pool.

The neutral theory of molecular evolution asserts that genetic drift is the key driving force of evolution. According to this theory, most mutations are neutral in their effect on the organism and they are either lost, or rarely fixed, in the population by genetic drift. Although neutralists accept that selective pressures do occur, they are considered marginal against the impact of genetic drift.