Selection, ecology and evolutionary adjustment within bivalve mollusc populations

Abstract

Distributional and experimental studies of protein polymorphisms of bivalve populations permit considerations of the roles of migration and selection in evolution. Minimal variation in west coast North American Mytilus californianus populations, relative to greater geographic differentiation in east coast M. edulis , correlates with the relatively steep latitudinal thermal gradient of the east coast. Among-locality differences are probably due to local selection and differences in the genetic composition of larval immigrants. Ecological influences are indicated by correlations of genetic structure with intertidal height, hydrographic structure, and greater amonglocality differentiation of the eurytopic M. edulis relative to the stenotopic M. californianus in the same region. Problems in determining experimentally the relative contributions of migration and selection are highlighted by a genetic difference in M. edulis within and outside Long Island Sound, U.S.A. A dramatic cline in allele frequency occurs over a distance easily traversed by the planktotrophic larval stage. The large amount of selection indicated, however, is not confirmed through shockmortality experiments in the laboratory or among-genotype measures of physiological response such as growth. The within-sound populations may be more isolated than we now suppose, either through the estuarine flow of the sound or by the existence of physiological races of ecotypes. The establishment of such isolation would permit genetic difference to accumulate slowly.