Genetic variation in marine fishes as a test of the niche-variation hypothesis

Abstract

SINCE the discovery of large and variable amounts of protein polymorphism in natural populations¹, biologists have attempted to determine what, if any, biological significance is associated with this particular form of phenotypic variation. Many population geneticists and ecologists^2–5 have adopted a selectionist point of view which states that allelic diversity is a strategy for increasing population fitness in a temporally and spatially heterogeneous habitat. This hypothesis, known as the “nichevariation hypothesis”, predicts a positive correlation between the protein polymorphism and morphological variation in a population and the relative heterogeneity of the most relevant physical (for example, temperature) and biological (for example, food and competition) variables of the habitat. The basic assumption underlying this hypothesis is that the presence of more than one variant of a given protein broadens the tolerance limits or the optimal functioning range of the structures and/or reactions associated with the protein. For example, the presence of multiple forms of an enzyme, each with a different thermal optimum, may broaden the thermal tolerance range of an ectothermic (poikilothermic; cold blooded) organism⁶.