## 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.