In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). The effect may cause an allele and the biological trait that it confers to become more common or more rare over successive generations. Ultimately, the drift may either remove the allele from the gene pool or remove all other alleles. Whereas natural selection is the tendency of beneficial alleles to become more common over time (and detrimental ones less common), genetic drift is the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone, so long as it does not comprise all or none of the distribution. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ... Template:Otherusescccc A graph of a bell curve in a normal distribution showing statistics used in educational assessment, comparing various grading methods. ... In genetics, an allele (pronounced al-eel or al-e-ul) is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. ... For other meanings of this term, see gene (disambiguation). ... In biology, a trait or character is a feature of an organism. ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... The Galápagos Islands hold 13 species of finches that are closely related and differ most markedly in the shape of their beaks. ...

Chance affects the commonality or rarity of an allele, because no trait guarantees survival or a given number of offspring. This is because survival depends on non-genetic factors (such as the possibility of being in the wrong place at the wrong time). In other words, even when individuals face the same odds, they will differ in their success. A rare succession of chance events — rather than natural selection — can thus bring a trait to predominance, causing a population or species to evolve. The Galápagos Islands hold 13 species of finches that are closely related and differ most markedly in the shape of their beaks. ...

An important aspect of genetic drift is that its rate is expected to depend strongly on population size. This is a consequence of the law of large numbers. When many individuals carry a particular allele, and all face equal odds, the number of offspring they collectively produce will rarely differ from the expected value, which is the expected average per individual times the number of individuals. But with a small number of individuals, a lucky break for one or two causes a disproportionately greater deviation from the expected result. Therefore small populations drift more rapidly than large ones. This is the basis for the founder's effect, a proposed mechanism of speciation. The law of large numbers is a fundamental concept in statistics and probability that describes how the average of a randomly selected sample from a large population is likely to be close to the average of the whole population. ... The founder effect is an evolutionary phenomenon. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ...

By definition, genetic drift has no preferred direction. A neutral allele may be expected to increase or decrease in any given generation with equal probability. Given sufficiently long time, however, the mathematics of genetic drift (cf. random walk) predict the allele will either die out or be present in 100% of the population, after which time there is no random variation in the associated gene. Thus genetic drift tends to sweep gene variants out of a population over time, such that all members of a species would eventually be homozygous for this gene. In this regard, genetic drift opposes genetic mutation which introduces novel variants into the population according to its own random processes. In mathematics, computer science, and physics, a random walk, sometimes called a drunkards walk, is a formalisation of the intuitive idea of taking successive steps, each in a random direction. ... For other meanings of this term, see gene (disambiguation). ... Homozygote cells are diploid or polyploid and have the same alleles at a locus (position) on homologous chromosomes. ... This article is about mutation in biology, for other meanings see: mutation (disambiguation). ...

Allele frequencies

From the perspective of population genetics, drift is a "sampling effect." To illustrate: on average, coins turn up heads or tails with equal probability. 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 discrepancy in number can be very small (in percentage terms). As an example, ten tosses turn up at least 70% heads about once in every six tries, but the chance of a hundred tosses in a row producing at least 70% heads is only about one in 25,000. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ...

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). In statistics, when analyzing collected data, the samples observed differ in such things as means and standard deviations from the population from which the sample is taken. ...

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 previous generation. 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 p', slightly different from p. In this situation, the allele frequencies are said to have drifted. Note that the frequency of the allele in subsequent generations will now be determined by the new frequency p', meaning that drift is a memoryless process and may be modeled as a Markov process. In probability theory, a Markov process is a stochastic process characterized as follows: The state at time is one of a finite number in the range . ...

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. The effective population size (Ne) is defined as the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration (Sewall Wright). ...

Drifting alleles usually have a finite lifetime. As the frequency of an allele drifts up and down over successive generations, eventually it drifts until fixation - that is, it either reaches a frequency of zero, and disappears from the population, or it reaches a frequency of 100% 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. In biology, mutations are changes to the base pair sequence of genetic material (either DNA or RNA). ...

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 4N_e generations, where N_e is the effective population size.

According to the Hardy-Weinberg Principle, which holds that allele frequencies in a gene pool will not change over time, a population must be sufficiently large enough to prevent genetic drift from changing allele frequencies over time. This is why the law is unstable in a small population. Hardy–Weinberg principle for two alleles: the horizontal axis shows the two allele frequencies p and q, the vertical axis shows the genotype frequencies and the three possible genotypes are represented by the different glyphs In population genetics, the Hardy–Weinberg principle (HWP) (also Hardy–Weinberg equilibrium (HWE), or Hardy...

Drift versus selection

Genetic drift and natural selection rarely occur in isolation from 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. The Galápagos Islands hold 13 species of finches that are closely related and differ most markedly in the shape of their beaks. ...

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.

Genetic drift in populations

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 (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 even if population later grows large again. A population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing, and the population is reduced by 50% or more, often by several orders of magnitude. ...

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. The founder effect is an evolutionary phenomenon. ...

See also

• Small population size
• Antigenic drift