Population Genetics of Marine Pelecypods. I. Ecological Heterogeneity and Evolutionary Strategy at an Enzyme Locus

The hypothesis is tested that some isoenzyme-synthesizing loci are responding to natural selection by physical factors of the environment and that consequently correlation in strategy as well as concomitant changes in genetic composition with a changing environment should be detectable among homologous loci of organisms living in very similar habitats. Polymorphisms at the leucine aminopeptidase (Lap) locus of the intertidal mussels Mytilus edulis and Modiolus demissus were studied and may be characterized as follows: (1) both species are polymorphic for three common and two very rare alleles, but the alleles in the two species are not electrophoretically alike; (2) the rank order of relative abundance of alleles of particular electrophoretic mobility is nearly identical for both species at all locations; (3) there is significant heterogeneity in zygotic distributions among localities in both species, but the zygotic distributions compared for the two at any one locality do not differ; and (4) deviations from expected zygotic proportions are observed in all samples, and both species deviate in the same way for all genotypes at any one locality Three additional marine pelecypods were examined, and all were polymorphic for at least three Lap alleles. Both Mytilus and Modiolus are polymorphic for two alleles at one Mdh locus and the same allelic frequencies are observed at all localities. The existence of correlated strategies among such different organisms living under nearly identical ecological conditions suggests: (1) natural selection is effecting a particular evolutionary strategy at the Lap locus. (2) organisms adapt to certain patterns of environmental heterogeneity in the same way, and (3) such loci cannot provide meaningful information about subpopulational structuring but can be used to estimate patterns of environmental heterogeneity.