What is genetic drift?

Genetic drift is the random change in allele frequency from generation to generation caused by chance sampling. In a small population, a few individuals may by luck fail to reproduce, so their alleles are lost even if they were not harmful. Drift is stronger in smaller populations.

What is the Wright-Fisher model?

The Wright-Fisher model assumes each new generation of 2N gene copies is formed by sampling independently and with replacement from the previous generation. Each copy independently has probability p (current allele frequency) of being the focal allele. This gives a binomial sampling process.

What does fixation mean?

Fixation occurs when an allele reaches a frequency of 1.0 (100%), meaning every individual in the population carries it. Once fixed, the allele cannot be lost without new mutation. The probability of fixation for a neutral allele starting at frequency p is exactly p.

Why do different runs end differently?

Each run uses independent random sampling, so outcomes differ by chance. This captures the stochastic nature of drift. Running many replicates (e.g., 20) shows the full range of possible trajectories from the same starting conditions.

How does population size affect drift?

Larger populations experience weaker drift because random sampling error averages out across more individuals. Smaller populations drift faster, reaching fixation or loss in fewer generations. Try N=10 versus N=500 to see the difference.

Is mutation or selection modeled here?

No. This simulator models pure neutral drift with no selection, mutation, migration, or non-random mating. It isolates the effect of finite population size on allele frequency change.

Genetic Drift Simulator

Simulate how random sampling causes allele frequencies to drift over generations in a finite population, showing fixation and loss events.

Simulation Parameters

Population size (N)

Number of diploid individuals (2–2000)

Initial allele frequency (p)

Starting frequency of allele A (0.01 – 0.99)

Number of generations

Simulation length (10 – 500)

Number of runs

Independent simulation lineages (1 – 30)

Quick Presets

Allele Frequency Over Generations

Each line is an independent simulation run. Dashed lines mark fixation (p=1) and loss (p=0).

Set parameters and click Run Simulation.

Summary

Genetic drift is the random change in allele frequencies from one generation to the next due to chance sampling in a finite diploid population. This simulator implements the Wright-Fisher model: each generation is formed by drawing 2N allele copies at random from the previous generation. You can run multiple independent lineages simultaneously to see how drift outcomes vary, and observe whether the allele reaches fixation (100%) or is lost (0%) before the final generation.

How it works

Set population size (N), initial allele frequency (p), number of generations, and number of simulation runs.
Click Run Simulation to start the Wright-Fisher random sampling process.
Each generation, the new allele count is drawn from a binomial distribution with n = 2N and probability = current frequency.
Results are plotted as a line chart showing allele frequency over time for each run.
Fixed (100%) and lost (0%) alleles are highlighted so you can see drift outcomes at a glance.
Summary statistics show how many runs ended in fixation, loss, or polymorphism.

Use cases

Demonstrate genetic drift and the Wright-Fisher model in genetics or evolution courses.
Explore how small population size accelerates drift compared with large populations.
Visualize how an initially rare allele can be lost or fixed purely by chance.
Compare drift intensity across different starting allele frequencies.
Illustrate the founder effect and population bottlenecks in introductory biology.
Understand why genetic diversity is harder to maintain in small populations.
Support self-study of neutral theory of molecular evolution.
Generate quick simulation data for classroom discussions or lab reports.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu