Which shell overcome? Phenotypic selection in Anguispira alternata.

Abstract

Evolution through natural selection is commonly assumed to be the cause of diversity and forms of all life. It is less common to see evolution acting, or to test whether natural selection is altering certain traits within a population. It is possible to test whether phenotypic selection is occurring, which differs from natural selection in that the heritibility of the trait is uncertain. We examined nine traits in a common land snail, Anguispira alternata. Those phenotypes disproportionately represented in the dead population are presumably less favorable than those phenotypes in the comparable live population. Four traits demonstrated no evidence of selection: height of the shell relative to its diameter, number of coloration bands in the terminal half whorl, number of bands per millimeter, and shape of periphery. Three of the shell traits appear to be undergoing directional selection, each of which was compared relative to the shell's diameter: size of the aperture; diameter of the umbilicus; and thickness of the shell wall. The height of the peripheral angle relative to the height of its whorl appears to be undergoing stabilizing selection, while aperature shape exhibits disruptive selection. The results demonstrated that phenotypic selection does occur in certain traits of terrestrial snails and suggest directions for further study.