Experimental evidence for the rapid evolution of behavioral canalization in natural populations.

Canalization-the evolutionary loss of the capacity of organisms to develop different phenotypes in different environments-is an evolutionary phenomenon suspected to occur widely, although examples in natural populations are elusive. Because behavior is typically a highly flexible component of an individual's phenotype, it provides fertile ground for studying the evolution of canalization. Here we report how snail populations exposed for different lengths of time to a predatory crab introduced from Europe to America exhibit different degrees of canalization of an adaptive antipredator behavior: soft tissue withdrawal, measured as angular retraction depth. Where crab-snail contact is shortest (60 years), snails showed the highest behavioral flexibility. Where crabs invaded 110 years ago, snails showed significantly less behavioral flexibility, and where the interaction is ancient (Europe), snails exhibited highly canalized behavior. Selection therefore appears to have acted rapidly to increase canalization in wild snail populations, leading ultimately to the hard-wired behavior seen in European conspecifics.