Phenotypic evolution in a poorly dispersing snail after arrival of a predator

Abstract

The phenotypic stability of many species in the face of changing conditions suggests that adaptive evolution can occur only under limited circumstances. One of the necessary conditions may be the lack of genetic mixing between dispersed populations inhabiting different environments. Intertidal molluscs on the east coast of North America between Cape Cod and Nova Scotia were exposed to an increase in the abundance of shell breaking predators when the green crab Carcinus maenas spread gradually northward from Cape Cod in the first half of the twentieth century¹. The periwinkle Littorina littorea, which produces larvae that become widely dispersed, did not show an increase in shell thickness as an adaptation to shell-breaking predation¹. However, I show here that the dog whelk Nucella lapillus, which is poorly dispersed in the bottom-dwelling juvenile phase, did adapt phenotypically after establishment of the green crab. This suggests that phenotypic stasis and gradual change are alternative responses depending on the degree of genetic mixing between populations.