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Populations of the garter snake Thamnophis sirtalis in the Lake Erie area are polymorphic, consisting of striped and melanistic morphs. The frequency of melanistic morphs is similar between sexes but varies greatly among seven island and 15 mainland sites: from 0.00-0.59. Thermoregulatory differences thought to exist between morphs have not resulted in differences between morphs in frequency of reproduction by adult females or in snout-vent length of adults of either sex. Non-selective factors (genetic drift, founder effect) may influence morph frequencies but have not produced significantly greater variance in morph frequency among island sites than among mainland sites.
Herpetologica © 1988 Herpetologists' League