Intense natural selection caused a rapid morphological transition in a living marine snail.

Shell shape and shell thickness of the intertidal snail Littorina obtusata changed markedly between 1871 and 1984 in northern New England. Shells collected prior to 1900 were high-spired with thin walls, whereas shells collected in 1982-84 were low-spired with thick walls. An intertidal crab (Carcinus maenas) which preys on L. obtusata expanded its range into northern New England around 1900. This suggests that the change in snail shell form was a response to predation by Carcinus. Field and laboratory experiments demonstrated that the high-spired form of L. obtusata, which can still be found in some Maine localities, is more vulnerable to predation by Carcinus than is the low-spired form of L. obtusata. Electrophoretic comparisons of high- and low-spired populations of L. obtusata confirmed that these populations represent different morphological forms of L. obtusata rather than different species [Nei's D (unbiased measure of genetic distance) = 0.003]. These data demonstrate that classical Darwinian selection can produce a rapid morphological transition without speciation.