COLOR-PATTERN VARIATION IN LAKE-ERIE WATER SNAKES - THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color-p attern variation in Lake Erie island water snake (Nerodia sipedon) pop ulations, rates of gene flow among island and mainland populations wer e estimated from patterns of allozymic variation detected using electr ophoresis. Rates of gene flow were high with Nm, the number of migrant s per generation, averaging 25.5 among island sites, 9.2 between the O ntario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark- recapture work and of past population size extrapolated from the exten t of shoreline habitat, values of m between island and mainland popula tions ranged from 0.0008-0.01 Synthesis of estimates of the rate of ge ne flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color-pattern variation in island populations results from a bala nce between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as dr ift may have been important in the initial stages of differentiation b etween island and mainland populations.