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The evolutionary significance of protein polymorphisms has been long debated. Exponents of the balanced theory advocate that selection operates to maintain polymorphisms, while those of the neoclassical school argue that most genetic variation is selectively neutral. In a series of laboratory and field experiments conducted over the last 5 years, we have found that the developmental rates of Fundulus heteroclitus embryos were highly correlated with genetic variation at 3 enzymatic loci: lactate dehydrogenase-B, malate dehydrogenase-A, and glucose phosphate isomerase-B. Pairwise comparisons of these loci generally indicated an averaging of developmental rates; however, a synergysm was found between the MDH-A and GPI-B loci. Our field studies indicated that selective forces may contribute to the maintenance of these polymorphisms via hatching time selection. Since feral eggs incubate aerially during the neap tides, either very rapid or very slow developmental rates may lead to deleterious stranding above the water line. In addition, very slow developmental rates may lead to increased predation pressure.
DiMichele et al. (1986) studied this question.