Migration, extinction, and alary morphism in water-striders (Gerris Fabr.).

A migration-extinction model has been developed for the analysis of alary polymorphism and its adaptive significance in univoltine populations of water-striders (Gerris). It is assumed that all actual and potential population sites are identical with respect to all relevant variables, i.e. that populations do not live in a 'harlequin' environment. The model can be applied to other groups with alary polymorphism. The effects of isolation, environmental stability, and environmental productivity are discussed, with numerical examples. Isolation and stability enhance the optimality of short-wingedness, while temporariness of population sites selects for long-wingedness. Optima cannot always be realized. The models reveal several critical variables which are of interest in the ecological genetics of Gerris and which should be measured in natural populations: (a) extinction probabilities (a distinction being drawn between extinction due to drying up of the population site and extinction due to other causes), (b) extent of passive dispersal, and (c) the proportion of colonized population sites in a large geographical region.