Genetic Drift Simulator
Solve Ht = H0(1−1/2N)^t for how much genetic diversity a population is expected to lose purely by chance over generations, then watch simulated random allele-frequency trajectories for different population sizes. Two 3D diagrams compare a large population (slow drift) to a small one (fast drift), and charts trace several random walk trajectories alongside the expected heterozygosity decay curve.
Reviewed by the ToolNestr Editorial Team — July 2026
Large vs. small population drift
1. Large population
Many individuals sampled each generation — drift proceeds slowly.
2. Small population
Few individuals sampled each generation — drift proceeds quickly.
Drift charts
How it works
The core idea in one line: every generation is a random sample of who happens to survive and reproduce, and the smaller that sample is, the more its results can wander away from the previous generation's true allele frequency by chance alone — exactly like a small poll being a noisier estimate than a huge one.
Ht = H0 × (1 − 1/2N)ᵗ
expected heterozygosity after t generations — H0=starting heterozygosity (2p0q0), N=population size
Genetic drift happens because each new generation's alleles are, in effect, a random sample drawn from the previous generation's gene pool — and any random sample carries sampling error. In a small population, that sample is small, so chance fluctuations are large relative to the total population; in a large population, the same random sampling process averages out much more smoothly. Averaged across many hypothetical replicate populations, this sampling error causes expected heterozygosity to decay predictably according to Ht=H0(1−1/2N)ᵗ — but any single real population's actual trajectory is a genuinely random walk, wandering unpredictably above or below that expected trend line.
Worked example 1 — a moderately sized population
Given: A population of N=50 individuals, starting allele frequencies p0=q0=0.5 (H0=0.5), projected 50 generations forward.
Even after 50 generations, a population of 50 individuals retains a majority of its original genetic diversity — moderate population sizes drift relatively slowly.
Worked example 2 — a much smaller population
Given: A tiny population of N=5 individuals, same starting H0=0.5, projected only 10 generations forward.
This tiny population loses proportionally far more diversity in one-fifth the time — small populations lose genetic diversity to drift dramatically faster than larger ones.
How population size changes the rate of diversity loss
Starting from H0=0.5 in all cases, after 20 generations.
| Population size (N) | Heterozygosity after 20 generations |
|---|---|
| 5 (very small) | ≈0.061 |
| 20 (small) | ≈0.301 |
| 50 (moderate) ★ | ≈0.409 |
| 500 (large) | ≈0.490 |
★ Reference row. A tenfold increase in population size (50→500) dramatically slows diversity loss — this is exactly why small, isolated populations are at much greater risk of losing genetic diversity than large ones.
Where genetic drift actually matters
🦏 Endangered species conservation genetics
Conservation biologists monitor genetic drift closely in small, endangered populations, since rapid diversity loss can reduce a species' ability to adapt to future environmental challenges or disease.
🏝️ Island and isolated population biology
Species on islands or in other geographically isolated habitats often show strong genetic drift effects (and founder effects), producing genetically distinctive populations compared to their mainland relatives.
🐕 Purebred animal breeding
Breeding within a small, closed population — as in many purebred dog breeds — accelerates genetic drift and inbreeding, contributing to the reduced genetic diversity and elevated disease risk seen in some breeds.
🧬 Human population genetics history
Genetic drift signatures (like reduced diversity in populations descended from small founding groups) help population geneticists reconstruct historical migration and settlement patterns.
Common misconceptions
"Genetic drift and natural selection are just two names for the same process."
They're fundamentally different processes — natural selection is a consistent, directional force based on fitness differences, while genetic drift is purely random chance that can push allele frequency in any direction, including against an allele's own fitness advantage.
"Genetic drift only matters in very unusual, extreme circumstances."
Drift is always acting on every finite population to some degree — it's simply far more noticeable and consequential in small populations, while its effects are comparatively minor (though never fully zero) in very large ones.
"A population can only lose diversity to drift, never regain it."
Once an allele is completely lost to drift, it can only return through new mutation or migration from another population — but heterozygosity itself is a population-average measure, and while drift-driven diversity loss is directional on average, individual populations can show real fluctuations along the way.
"Larger populations are completely immune to genetic drift."
Drift technically still occurs in large populations — it just proceeds far more slowly, since the formula's (1−1/2N) term approaches 1 for large N, meaning heterozygosity is lost extremely gradually rather than not at all.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 20, "Evolution" (free, peer-reviewed). openstax.org
- • Wright (1931) — foundational population genetics drift theory, Genetics.
- • Hartl & Clark, Principles of Population Genetics — Genetic drift chapter.
Ht=H0(1−1/2N)ᵗ (Wright-Fisher model expected heterozygosity decay). Simulated trajectories use random binomial sampling each generation. Results are rounded for display.
How to use this calculator
Enter population size and starting frequency
Effective population size and the starting allele frequency.
Enter number of generations
How far forward to project expected heterozygosity.
Compare to simulated trajectories
Random walk simulations illustrate the variability drift produces around the expected trend.
Related tools
Frequently asked questions
What is genetic drift?
Genetic drift is random change in allele frequency from one generation to the next, caused purely by chance in which individuals happen to survive and reproduce — distinct from natural selection, which is driven by consistent fitness differences.
Why does population size matter so much for genetic drift?
Smaller populations sample fewer individuals each generation, so random chance has a much bigger relative effect on allele frequency — exactly like how flipping a coin 10 times gives a far less reliable estimate of "50/50" than flipping it 10,000 times.
What is expected heterozygosity, and why does it decay under drift?
Expected heterozygosity (H=2pq) measures genetic diversity at a gene. Random drift causes alleles to be lost by chance over generations (especially in small populations), so average heterozygosity across many replicate populations steadily declines according to Ht = H0(1−1/2N)^t.
Can genetic drift alone cause an allele to disappear completely?
Yes — unlike selection against a rare recessive allele (which slows dramatically as the allele becomes rare), genetic drift can fully eliminate or fix any allele by pure chance, regardless of whether it's helpful, harmful, or neutral.
What are the founder effect and bottleneck effect?
Both are extreme, real-world examples of genetic drift: a founder effect occurs when a small group establishes a new population (carrying only a random sample of the original population's genetic diversity), and a bottleneck effect occurs when an existing population's size crashes sharply, randomly losing much of its diversity in the process.