a1 National Institute of Genetics, Mishima, Japan
In a two-dimensional stepping-stone model of finite size, if a pair of alleles happen to segregate in the whole population, marked local differentiation 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 frequencies become uniform over the entire distribution range unless mutation is unusually high. Tendency toward local differentiation is much weaker in two-dimensional than in one-dimensional habitats.
(Received December 02 1970)
* Contribution no. 805 from the National Institute of Genetics, Mishima, Shizuoka-ken, Japan. Aided in part by a Grant-in-Aid from the Ministry of Education, Japan.