Genetic drift is the random change in allele frequencies in a population due to chance sampling events during reproduction, as opposed to natural selection which is deterministic. In each generation, only a subset of individuals reproduce, and by chance their gametes may carry a different frequency of alleles than the full population. This sampling error is strongest in small populations, where random fluctuations can dramatically alter allele frequencies—and even fix or eliminate alleles entirely—within just a few generations.
The Wright-Fisher model is the standard mathematical description: in a population of N diploid individuals (2N gene copies), each generation is modeled as drawing 2N alleles with replacement from the previous generation's allele pool. The variance in allele frequency change per generation is p(1-p)/(2N), where p is current allele frequency. Smaller N means larger variance. Important special cases include the founder effect (new population established by a small number of individuals, with allele frequencies reflecting that sample) and the bottleneck effect (drastic population reduction followed by expansion, with reduced genetic diversity).
This simulator runs multiple Wright-Fisher drift simulations simultaneously, letting you observe the spread of trajectories over time. With large N, allele frequencies barely drift; with small N, trajectories quickly diverge and many alleles reach fixation (p=1) or loss (p=0). You can set starting allele frequency, population size, and number of independent replicates to explore how long fixation takes, how drift interacts with mutation, and how bottlenecks affect genetic diversity.
How is genetic drift different from natural selection?
Natural selection changes allele frequencies systematically based on differential reproductive fitness—advantageous alleles tend to increase in frequency across generations in a predictable direction. Genetic drift changes allele frequencies randomly due to chance sampling during reproduction—even a disadvantageous allele can increase in frequency or even fix by luck, especially in small populations. In large populations, selection dominates; in small populations, drift can overpower selection, allowing deleterious mutations to accumulate (genetic erosion) and beneficial mutations to be lost by chance.
What is allele fixation and how quickly does it happen?
Fixation occurs when one allele reaches a frequency of 1 (100%)—it is now the only allele at that locus in the entire population, and all other alleles have been lost. In a neutral model (no selection), the expected time to fixation starting from frequency p is -4N(p·ln(p) + (1-p)·ln(1-p)) generations—roughly 4N generations for a new mutation (p=1/2N). In a population of 100, fixation takes ~400 generations; in a population of 10,000, ~40,000 generations. Once fixed, the allele can only be changed by new mutation.
What is the effective population size (Ne)?
The effective population size Ne is the size of an idealized Wright-Fisher population that would undergo the same rate of genetic drift as the actual population. Real populations deviate from the ideal due to unequal sex ratios, variance in reproductive success, overlapping generations, and population size fluctuations. Ne is almost always smaller than the census population size N—often dramatically so. A bottleneck of even a few generations can reduce Ne for many generations afterward, explaining why species with large current census sizes can still show low genetic diversity if they recently passed through a bottleneck (e.g., cheetahs, northern elephant seals).
The founder effect occurs when a new population is established by a small number of individuals (founders) from a larger ancestral population. The founders carry only a subset of the original genetic diversity, and rare alleles in the founding group may become common while previously common alleles may be absent. Classic examples: Amish populations (high frequency of Ellis-van Creveld syndrome due to a founding couple carrying the allele), Finnish disease heritage (distinctive spectrum of rare diseases elevated in Finland's genetically isolated founding population), and Tristan da Cunha islanders (descended from 15 founders, showing elevated frequencies of specific genetic variants).
Mutation introduces new alleles at a rate μ per locus per generation, countering drift's tendency to eliminate diversity. At drift-mutation equilibrium in a neutral model, expected heterozygosity H = 4Neμ/(1+4Neμ)—larger Ne or higher mutation rates maintain more diversity. Migration between populations (gene flow) counters drift's tendency to differentiate populations. The migration-drift equilibrium predicts population differentiation FST ≈ 1/(1+4Nem), where m is migration rate. High migration homogenizes populations; low migration allows drift to cause divergence, eventually leading to genetic differentiation that contributes to speciation.