In a two-dimensional stepping-stone model of finite size, if a pair of
alleles happen to segregate in the whole population, marked local differ- entiation of gene frequencies can occur only if migration between colonies
is sufficiently rare so that Nm < 1, where N is the effective size of each
colony and m is the rate at which each colony exchanges individuals with
four surrounding colonies each generation. On the other hand, if Nm ^ 4, the whole population behaves as if it were panmictic and the allelic fre- quencies become uniform over the entire distribution range unless muta- tion is unusually high. Tendency toward local differentiation is much
weaker in two-dimensional than in one-dimensional habitats.