Genetic drift – Wright–Fisher model, bottleneck and founder effect
BiologyEvolutionAges 17–18
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Sign in to playSeveral populations of the same size N start with the same frequency p₀ of allele A; each generation, the 2N alleles of the offspring are drawn at random from the parents' gene pool, with no selection and no mutation. Graphs show the allele frequency of every population, the spread of p among populations and the mean heterozygosity against the expected curve H₀(1 − 1/2N)ᵗ. Students change N, trigger a bottleneck or found new populations from a few individuals to see alleles fixed or lost purely by chance.
Lesson: The modern synthesis: genetic drift (chance events), the bottleneck effect and the founder effect
What it shows
Genetic drift is the change in allele frequency caused by chance alone, because each generation inherits only a random sample of the parents' alleles. This simulation uses the Wright–Fisher model: in every generation the 2N alleles of the offspring are drawn at random from the gene pool, with no selection, mutation or migration. Several populations run side by side, so students see that small populations drift quickly to fixation or loss, that heterozygosity falls as predicted, and that sudden drops in numbers, such as bottlenecks and founder events, make chance even more powerful.
How to use
Press Pause, set N and p₀, then Reset and watch the lines spread until each reaches 0 or 1. Compare N = 10 with N = 500. Press Bottleneck to let only a few individuals survive one generation, or Founder effect to start new populations from a few colonists. Set the size with Survivors/founders and tap a line to highlight one population.
Parameters you can change
- Size of each population N 4–1000 individuals
- Initial frequency of allele A p₀ 0.05–0.95
- Number of populations run side by side 1–20
- Number of survivors (bottleneck) or founders 2–50 individuals
- Generations shown on the graph axis 20–1000 generations
Questions to explore
- Why do populations of 10 individuals fix or lose allele A much sooner than populations of 500?
- With p₀ = 0.2, roughly what percentage of populations should end up with allele A fixed?
- After a bottleneck the population grows back to N, so why does heterozygosity not recover?
