A study of selection of phenotypic traits in Pyganodon grandis, Lampsilis siliquoidea, Ligumia nasuta, Elliptio dilatatus, and Elliptio complanatus in Northern Michigan.

Abstract

The adaptationist paradigm leads many scientists to assume natural selection is occurring in all populations without experimental support to back up this asumption. In this study we measured the frequency of phenotypic traits in dead and live clam populations found throughout Cheboygan County, Michigan to look for evidence of phyenotypic selection. Pyganodon grandis, Lampsilis siliquoidea, and Ligumia nasuta were the species found in Douglas Lake. Black River contained Elliptio dilatatus and Elliptio complanatus. We examined three phenotypic traits: length/height ratio (relative length), length/breadth ratio (slenderness) and annual growth rate. Annual growth rate measured growth between the first and fifth year. Ratios were used to avoid an age bias in relative length and slenderness measurements. We looked for evidence of directional, stabilizing, disruptive, or relaxed selection on the phenotypic traits measured and in the absence of those we concluded selection had not occurred. Directional selection favors both extremes of the phenotypic range, enhancing variability in the live population. Relaxed selection favors all traits equally and is signified by a significantly larger variance in the living population. In the study of relative length, slenderness, and growth rate, evidence of all five selection types were seen. The most common selection type appeared to be directional selection favoring a phenotypic extreme of a population. More slender clams were observed in the live Maple Bay L. siliquoidea (t-test: p=.03386), Hook's Pointe L. siliquoidea (t-test; p=.0094), Hook's Pointe P. grandis (t-test: p=.00427) and Hook's Pointe L. nasuta (t-test: p=.00399), and Black River E. complanatus (t-test: p=.00845), and Black River E. dilatatus (t-test: p = 7.6E-8). Shorter relative lengths were observed in the living Maple Bay L. siliquoidea (t-test: p=.00217), Hook's Pointe L. nasuta (t-test:p=.00489), and Black River E. complanatus (t-test: p=.00086). Smaller juvenile growth rates were observed in the living Maple Bay L. siliquoidea (t-test: p=7.55E-6), Maple Bay L. nasuta (t-test: p=.00253), Black River E. complanatus (t-test: p=9.77E-10), and Black River Elliptio dilatatus (t-test: p=3.0E-8). In a comparison between clams sampled from rocky and mucky substrate, the populations from rocky regions displayed no evidence of selection for the growth rate or relative length, while those from the mucky region displayed evidence of favoring the mean growth rate and relative length. Some proposed explanations for the presence or absence of evidence of different forms of selection found are predator size preferences, environmental size constraints of refugia, as well as other abiotic and biotic factors. Genetic drift and the founder effect have not been ruled out as possible explanations of the phenotypic differences observed.