Theridion grallator is a highly polymorphic spider endemic to the wet and mesic forests of the Hawaiian Islands. The frequencies of the two major morph classes, patterned and unpatterned abdomen, did not show significant spatial or temporal variation within undisturbed or disturbed areas on Maui. Similarly, morph frequencies did not vary significantly between undisturbed areas on three different islands. Estimates of spider migration suggest that gene flow is sufficient to account for the similarity in frequency within and between areas on Maui, but it is probably not sufficient to explain the similarity among islands. Fecundity did not differ between morphs. A significant inverse relationship between morph frequency and residence time in certain cases suggests that frequency-dependent selection, perhaps mediated by bird predation, may play a role in maintaining the polymorphism.
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