Allozymic variation in natural populations of plants, animals, and humans based on studies published prior to early 1976 and involving 243 species, in which 14 or more loci were tested, is herein reviewed. Explanatory models are compared and contrasted in view of the evidence to see which theories best explain genetic variation in natural populations. The analysis suggests that the amounts of genetic polymorphism and heterozygosity vary nonrandomly between loci, populations, species, habitats, and life zones, and are correlated with ecological heterogeneity. Natural selection, in some form, may often be the major determinant of genetic population structure and differentiation. Yet precise critical experiments must be designed to test possible alternative hypotheses, to establish direct cause-effect relationships between ecological and genetic profiles, and to assay the contribution of single and multilocus structures to fitness.