ARE THE OPPOSING SELECTION PRESSURES ON EXPOSED AND PROTECTED SHORES SUFFICIENT TO MAINTAIN GENETIC DIFFERENTIATION BETWEEN GASTROPOD POPULATIONS WITH HIGH INTERMIGRATION RATES

Throughout the world intertidal gastropods living on exposed rocky shores differ strikingly in a number of morphological and life history traits from those on protected shores. Where surf is heavy gastropods tend to be smaller and to have thinner and smoother shells with larger apertures than do those from sheltered areas where crab predation is more intense. These morphological differences can occur within a species and there is evidence that they can be partially genetic and partially environmental. In addition the convergence of shell features in each habitat suggests that there are consistent differences between the selective pressures on exposed shores and the selective pressures on protected shores. I constructed a simulation model for a polygenic trait that experiences different selective pressures on exposed and sheltered shores. The results show that genetic differences can be maintained between the two populations despite high intermigration rates. Replacement of a portion of the random environmental variance with adaptive environmental variance reduces the effect of selection and thus the size of the difference maintained between the two populations. Genetic differentiation between exposed and protected populations can persist for significant periods of time and may have sometimes been the first step in speciation. © 1990 Kluwer Academic Publishers.