Fst and genetic drift in small populations
Key to genotype colors:
- AA (yellow)
- Aa (orange)
- aa (red)
The frequency of allele A used at the start is p = 0.5 for all populations. Refresh your browser to start over. Scroll down for more details.
The following keys do something:
- ‘n’ advances to the next generation
- ’s’ toggles between 4 and 16 diploid individuals per population
- ‘h’ hides population boundaries
The status text shows:
- g = the number of generations of random mating within populations
- pmean = mean frequency of the A allele (p) over all populations
- Ho = observed heterozygosity = fraction of heterozygotes over all populations
- He = expected heterozygosity = expected fraction of heterozygotes = 2 pmean (1-pmean)
- Fst = variance of p across populations divided by pmean (1 - pmean), which is the maximum possible variance of p across populations
The blue histogram at the top shows the distribution of p over the 12 populations (p = 0 at far left, p = 1 at far right).
With each new generation, drift increases the variance of p across populations and thus increases Fst (indicated by the fraction of the status bar filled with orange) until, eventually, all populations are fixed for either the A or the a allele, the variance of p is as large as it can get, and Fst equals 1.