Genetic variation as a test of natural selection

Allozymic variation encoded by 26 loci was analyzed electrophoretically in 507 specimens representing 12 populations of green toads, Bufo viridis, in Israel and the Vis Adriatic Island. Genetic variation in Bufo viridis is higher than in any vertebrate yet studied. Mean heterozygosity per locus per individual (H) is 0.133 (range, 0.105 to 0.159). H is higher in central populations as compared with isolates, and varies among four major protein classes, being highest in transferases and hydrolases and lowest in oxidoreductases and nonenzymatic proteins. Differential gene frequencies among polymorphisms was tested as an indicator of natural selection. Significant heterogeneity between loci in their apparent inbreeding coefficients was found for all alleles and for each of the four major classes of proteins tested, which may be taken as evidence of selection. Both uniform and diversifying selection are suggested by the low and high values of the inbreeding coefficient respectively. The general pattern of high heterozygosity in Bufo viridis is best explained as an adaptive strategy in heterogeneous environments.