Polymorphism, Selection, and Multi-Locus Heterozygosity in the Plaice, Pleuronectes Platessa L.

Abstract

The central problem of evolutionary genetics concerning the nature of the forces primarily responsible for the generally abundant polymorphism demonstrated by electrophoretic methods in both plant and animal species forms the basis of a vigorous and continuing debate. Despite the considerable body of data now available on gene frequencies at many enzyme loci in many populations, critically unequivocal evidence that natural selection discriminates between genotypes at such loci is still sparse. There are a number of studies, both of natural populations and of laboratory populations, which indicate, frequently indirectly, that this is probable (for example, Koehn 1969; Bryant 1974; McDonald and Ayala 1974; Nevo 1976), and in the laboratory several workers have shown that strong selection is operative either on the loci themselves or on tightly linked complexes which, if they exist, must themselves display considerable polymorphism (Gibson 1970; Huang, Singh and Kojima 1971; Birley and Beardmore 1972a; van Delden, Kam-ping and van Dijk 1975). The best evidence of this sort is drawn from Drosophila, but it must be stated that some carefully controlled experiments failed to produce any evidence of selection operating upon a polymorphic esterase locus in D. pseudoobscura (Yamasaki 1971).