Genetic Drift is evolutionary change in allele frequencies caused by random sampling effects rather than by differences in the adaptive value of alleles. In every generation, the alleles passed to offspring represent a sample of the alleles present in the parental population, and random sampling can cause allele frequencies to rise, fall or reach fixation even when alleles have identical effects on fitness.
Genetic Drift is therefore fundamentally stochastic and differs from natural selection, which produces nonrandom changes in allele frequencies when heritable variation affects reproductive success. Drift occurs in populations of all sizes, but its effects are generally stronger when effective population size is small. It can reduce genetic variation within populations, increase genetic differences among isolated populations and contribute to the fixation or loss of neutral or mildly deleterious variants. Genetic Drift is one of the fundamental mechanisms of evolution in population genetics.
The strength of Genetic Drift depends more closely on effective population size than on the simple census count of individuals. Effective population size reflects the number of individuals that contribute genetically to subsequent generations under the reproductive structure of the population. Unequal reproductive success, unequal sex ratios, fluctuations in population size, overlapping generations, population subdivision and other demographic characteristics can make effective population size substantially smaller than census size.
In a small effective population, chance fluctuations in allele frequencies can be large from one generation to the next. In a very large population, random sampling effects are proportionally weaker, so selection and other deterministic processes can more readily dominate. Drift does not require environmental change, advantageous or disadvantageous traits or differential survival. It can occur even in a completely constant environment when reproduction involves finite random sampling.
Two especially important demographic manifestations of Genetic Drift are the bottleneck effect and the founder effect. A population bottleneck occurs when a population experiences a severe temporary reduction in size, leaving a small number of surviving individuals whose allelic composition may differ substantially from that of the original population. Genetic variation can consequently be lost even if the population later recovers numerically. A founder effect occurs when a new population is established by a small subset of a source population.
The founders carry only a sample of the source population's genetic variation, so allele frequencies in the new population can differ substantially by chance. Both processes can produce distinctive genetic patterns and increase the importance of stochastic evolutionary change. Neither process requires that the founding or surviving individuals possess traits that were particularly advantageous.
Genetic Drift can reduce genetic diversity because alleles can be lost randomly. Once an allele has disappeared from a population, ordinary mutation is generally required for that particular variant to reappear. Drift can also cause neutral alleles to become fixed, meaning that one allele reaches a frequency of one in the population, or lost, meaning that its frequency reaches zero. For a neutral allele, fixation probability in a diploid population is approximately equal to its initial frequency under simple assumptions.
Drift also increases genetic differentiation between isolated populations because each population follows its own random trajectory. Measures such as F statistics can be used to quantify population differentiation, although interpretation depends on demographic history and migration. Drift therefore contributes to both within population diversity loss and among population divergence.
Genetic Drift interacts continuously with natural selection, mutation and Gene Flow. Selection is stronger than drift when fitness differences are sufficiently large relative to random sampling effects, whereas mildly selected alleles can behave approximately neutrally in small populations. The nearly neutral theory of molecular evolution proposes that many molecular variants have effects sufficiently small that their evolutionary behaviour is influenced strongly by drift.
Mutation introduces new alleles, but drift determines whether newly arising neutral or weakly selected variants increase or disappear by chance. Gene Flow can counteract differentiation generated by drift by introducing alleles from other populations. The balance among these processes determines population genetic structure. Genetic Drift therefore should not be interpreted as an alternative explanation that excludes selection, because both can operate simultaneously on the same population and even on the same genomic region.
Genetic Drift has important consequences for conservation biology. Small endangered populations can lose genetic diversity through repeated drift, increasing the probability that rare alleles disappear and that populations become genetically differentiated or inbred. Reduced effective population size can therefore create evolutionary constraints, particularly when environmental conditions change and adaptive variation is limited. Conservation programmes may attempt to maintain population connectivity, increase effective population size or introduce individuals from other populations where scientifically and legally appropriate.
However, genetic management must consider local adaptation and population history because indiscriminate mixing can produce other consequences. Genetic Drift is also important in captive breeding because breeding programmes with relatively few founders can produce genetic changes that are not representative of wild populations. Population genetic monitoring can reveal changes in heterozygosity, allele frequencies and effective population size.
Genetic Drift is frequently misunderstood as meaning that evolution is completely random. The random component concerns which alleles are transmitted and how their frequencies change through sampling, not every aspect of evolutionary biology. Mutation may generate variants without regard to their future usefulness, while selection can subsequently increase variants that improve reproductive success under particular conditions. Drift can also have predictable statistical consequences even though the direction of change in a particular generation is uncertain.
For example, repeated simulations of a small population can show an expected loss of heterozygosity even though individual replicate populations follow different allele frequency trajectories. The distinction between randomness in mechanism and predictability in statistical expectation is therefore important. Drift is also not equivalent to genetic mutation, because mutation creates new sequence variants whereas drift changes their frequencies after they exist.
Genetic Drift became central to modern evolutionary theory through the development of population genetics during the early twentieth century, particularly through mathematical work by figures including Sewall Wright, Ronald Fisher and J B S Haldane. Wright emphasised the importance of random genetic processes, population subdivision and shifting balances among evolutionary forces. Modern genomic technologies allow researchers to detect historical demographic changes through patterns of genetic diversity, linkage disequilibrium and allele frequency spectra.
Ancient DNA can provide direct evidence of past population size changes and genetic turnover. Experimental evolution can also demonstrate drift under controlled conditions. Genetic Drift remains essential for understanding evolution in small populations, molecular variation, conservation genetics, founder populations and stochastic evolutionary change, and it provides one of the clearest demonstrations that evolutionary change does not always require adaptive improvement.
