This graph shows ten different populations, each consisting of two
individuals.

Initially, each population has p = 0.5 and q = 0.5

Each population might take a different time to become "fixed" at one
allele or the other, but all of them eventually do.

For example, due to sampling error, two of the populations become
fixed at "a" in only one generation. Two more take two generations.

Note that the "4" on the diagram is the total number of alleles in
the population, since each individual has two.

The populations that eventually go to "0" frequency for the a allele
have become fixed for the "A" allele.