Selection as a force on the phenotypic variation: a study of Campeloma decisum.

Abstract

Phenotypic selection is the selective force that acts on the expression of traits. In order to determine whether phenotypic selection is occurring, traits of live and dead individuals can be examined to see what was selected. Different traits can also be selected for at different times of an individual's life according to the varying selective pressures at that time. In order to observe selection as a force on phenotypic variation, we tested traits of the freshwater snail, Campeloma decisum from Douglas Lake. Snail shells provide us with a visual record of their life history. From this we can examine traits such as shell shape, aperture shape, aperture to shell size, shell thickness, whorl tightness, and growth rate. Overall in a snail's life, we found selection favored wider aperture shapes, tighter coiled whorls, slower growth rates, and larger aperture size to shell size. Our results imply that at least for one period in a snail's life, phenotypic selection is directionally acting on thicker shells. These results further the evidence that phenotypic selection is occurring in C. decisum.