This figure is obtained by determining the alleles at a sample of loci (as many as possible) for a sample of individuals (as many as possible) in a sample of populations (as many as possible)

Each allele occurs in a proportion of the total number of populations sampled (i, the horizontal axis of the graph).   An allele that occurs in only one population is called a private allele; one that occurs in only a small proportion of populations is a restricted allele.

i = proportion of the populations sampled in which an allele occurs

p(i) = the frequency of alleles in the populations in which they occur (averaged over all alleles with the same i)

The plot shows p(i) as a function of i.

When restricted alleles (i near 0) reach high frequencies in the populations in which they occur (p(i) near 1), then gene flow (gene migration, natal dispersal) between populations must be low.   Restricted alleles cannot reach high frequencies on average when gene flow between populations is high.