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ENVIRONMENTAL CHANGE AND THE EVOLUTIONARY HISTORY OF THE PERIWINKLE (LITTORINA LITTOREA) IN NORTH AMERICA

How does a sexually reproducing species respond evolutionarily to irreversible changes in its environment? Does it remain morphologically unchanged for long periods of time, as believed by those who hold that innovations in form evolve only in small isolated populations at or just after the time of speciation (Eldredge, 1971; Eldredge and Gould, 1972; Carson, 1975; Vermeij, 1978; Stanley, 1979), or can change be brought about in large well-established populations? To answer these questions, the following four points must be established: (1) there was an environmental change, which can be recognized independently from any phenotypic alteration in the species being studied; (2) the environmental change can be perceived by the organism; (3) adaptations to the change can be identified and measured; and (4) specimens spanning the interval of time over which the environmental change took place must be available in order to assess the phenotypic (or genotypic) response. Several examples of historical change in phenotype (and presumably in genotype) are known in gastropods (Berry and Crothers, 1974; McLeod and Moore, 1978; Murray and Clarke, 1978; Wall et al., 1980). The interpretation that these changes were caused by natural selection is complicated by the fact that the selective agents which are believed to have been responsible were inferred a posteriori and could not be identified independently from the phenotypic data. In the present study, I attempted to discover whether the arrival of the predatory green crab (Carcinus maenas) on American shores north of Cape Cod was experienced by the common periwinkle (Littorina littorea) as an increase in the incidence of predator-induced sublethal shell damage, and whether the periwinkle responded to this change phenotypically with an increase in shell thickness.