Introduction

Compared with techniques for analysing genetic variation at the DNA level, the older established technique of protein electrophoresis has made by far the largest contribution towards our understanding of population and evolutionary genetics. Protein electrophoresis measures biochemical genetic polymorphism as allozymes, that may be defined as the product of different alleles at a single locus. Numerous studies of allozyme variation have established that genetic diversity is very high within animal species (for a recent review, see van Delden, 1994). This suggests that the consequences of biodiversity, which is often only assessed in terms of species, should also be considered both within and between populations of the same species. An emerging body of evidence indicates that genetic variation does not simply result from the neutral processes of mutation, genetic drift and migration. Positive relations have been predicted on theoretical grounds between fitness and multiple locus heterozygosity, assessed for each individual as the proportion of measured polymorphic loci that are heterozygous (for an example, see Charlesworth, 1991). Such associations have frequently been confirmed in many species of animal (for reviews, see Mitton & Grant, 1984; Zouros & Foltz, 1987). Further investigations have established that genetic differences in the catalytic efficiency of specific enzymes have profound effects on physiological performance, and which may represent the mechanistic basis for long-term adaptation resulting from the functional consequences of natural selection (for reviews, see Nevo, 1983; van Delden, 1994).