SYMPATRIC CONVERGENCE AND ENVIRONMENTAL CORRELATION BETWEEN 2 LAND-SNAIL SPECIES

In the southern Appalachian region of North America, the phylogenetica lly convergent shells of the polygyrid snails Triodopsinae Neohelix ma jor (Binney) and Polygyrinae Mesodon normalis (Pilsbry) are even more convergent in size and shape in sympatry (7 sites) than in allopatry ( 23 and 10 sites). Environmental correlations account for 34% and 30% o f size and shape variations in N. major (larger, taller, and more loos ely coiled at northern, high-altitude, sheltered sites), but for only 14% and 9% in M. normalis (larger, flatter, and more loosely coiled at south-facing, exposed sites). The statistical significance of the sym patric convergence dropped out when these correlations were removed. T his phenomenon helps account for the many cases in eastern North Ameri ca of nearly identical land-snail shells in sympatry and questions the importance of competitive character displacement in the evolution of land-snail shell morphology. This apparently nonmimetic case of sympat ric convergence provides an unusually precise and well-delimited, natu rally replicated experiment in evolutionary morphology, which is analy zed for controlling factors in a follow-up paper.