Variation in Genotype Frequencies During the Life History of the Bivalve, Dreissena Polymorpha
Populations of marine benthic invertebrates often show geographic and temporal genetic heterogeneity, even though these species have widely dispersing planktonic larvae that tend to homogenize genetic structure of populations. Much of this heterogeneity is attributed to localized selection or immigration of genetically different larvae (Koehn et al. 1980; Gartner-Kepkay et al. 1983; Johnson and Black 1984; Hilbish 1985). However, these studies have been handicapped by an inability to examine genetic variation in the larvae themselves. Selection may be most intense on larvae and newly settled individuals (Johnson and Black 1984; Hilbish 1985; Hilbish and Koehn 1985; Gosling and McGrath 1990) because extremely high mortality may occur during the pelagic stage and in the transition from planktonic larvae to sedentary juveniles (Lewandowski 1982a; Powers et al. 1991). Therefore, selective mortality of larvae may largely determine the genetic structure of adult populations, and selection on adults may be relatively unimportant in producing and maintaining local heterogeneity (Ayre 1990).