Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab.

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell-thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator-mediated selection on local snail populations.

Modelling the genetics and demography of step cline formation: gastropod populations preyed on by experimentally introduced crabs.

Whether a prey population goes extinct or adapts in response to an invading predator may depend on the number of contiguous populations that experience increased predation. We created invaded snail populations by building shelters for predatory shore crabs on a rocky intertidal bench. The crabs preyed selectively on thin-shelled snails tethered next to the shelters but did not prey on those more than 2 m away. This caused strong directional selection for increased shell thickness in populations close to the shelters but did not change selection in those farther away. The field experiment was used to parameterize a new individual-based quantitative genetic model that included demography. In the model a detectable step cline in shell thickness evolved rapidly even though the region of increased predation was shorter than Slatkin's characteristic length. The cline's step size in the model was similar to that measured in the field 10 years after the experiment began.