Genetic drift is a mechanism of evolution that acts in concert with natural selection to change the characteristics of species over time. It is a stochastic effect that arises from the role of random sampling in the production of offspring. Like selection, it acts on populations, altering the frequency of alleles and the predominance of traits amongst members of a population, and changing the diversity of the group. Drift is observed most strongly in small populations and results in changes that need not be adaptive.
From the perspective of population genetics, drift is a "sampling effect". To illustrate: on average, coins turn up heads or tails equally. Yet just a few tosses in a row are unlikely to produce heads and tails in equal number. The numbers are no more likely to be exactly equal for a large number of tosses in a row, but the inequality can be very small in percentage terms. As an example, ten tosses turn up 70% heads about once in every six tries, but the chance of a hundred tosses in a row producing 70% heads is only about one in 25,000.
Similarly, in a breeding population, if an allele has a frequency of p, probability theory dictates that (if natural selection is not acting) in the following generation, a fraction p of the population will inherit that particular allele. However, as with the coin toss above, allele frequencies in real populations are not probability distributions; rather, they are a random sample, and are thus subject to the same statistical fluctuations (sampling error ).
When the alleles of a gene do not differ with regard to fitness, on average the number of carriers in one generation is proportional to the number of carriers in the last. But the average is never tallied, because each generation parents the next one only once. Therefore the frequency of an allele among the offspring often differs from its frequency in the parent generation. In the offspring generation, the allele might therefore have a frequency slightly different from p (p'). In this situation, the allele frequencies are said to have drifted. Note that the frequency of the allele in the following generations will now be determined by the new frequency p'.
As in the coin toss example above, the size of the breeding population (the effective population size) governs the strength of the drift effect. When the effective population size is small, genetic drift will be stronger.
Drifting alleles usually have a finite lifetime. As the frequency of an allele drifts up and down over successive generations, eventually it drifts till fixation - that is, it either reaches a frequency of zero, and disappears from the population, or it reaches a frequency of 1 and becomes the only allele in the population. Subsequent to the latter event, the allele frequency can only change by the introduction of a new allele by a new mutation.
The lifetime of an allele is governed by the effective population size. In a very small population, only a few generations might be required for genetic drift to result in fixation. In a large population, it would take many more generations. On average, an allele will be fixed in 4Ne generations, where Ne is the effective population size.
Genetic drift and natural selection rarely occur in isolation of each other; both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstance.
In a large population, where genetic drift occurs very slowly, even weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.
Drift can have profound and often bizarre effects on the evolutionary history of a population. These effects may be at odds with the survival of the population.
In a population bottleneck, where the population suddenly contracts to a small size and then grows again to a large population (believed to have occurred in the history of human evolution), genetic drift can result in sudden and dramatic changes in allele frequency that occur independently of selection. In such instances, many beneficial adaptations may be eliminated.
Similarly, migrating populations may see founder's effect, where a few individuals with a rare allele in the originating generation can produce a population that has allele frequencies that seem to be at odds with natural selection. Founder's effects are sometimes held to be responsible for high frequencies of some genetic diseases.