Summarizing, we might say that the usual explanation that polymorphisms are the consequences of heterozygote superiority is basically wrong. . . . What we should say is that natural selection results in a subset of alleles with the average heterozygote superiority so that polymorphism and heterosis are a joint consequence of natural selection rather than the cause of each other.
L. R. Ginzburg, "Why Are Heterozygotes Often Superior in Fitness?" ( 1979)
Electrophoretic surveys of genetic variation of proteins provide estimates of the levels of genetic variability in a large number of plants ( Brown 1979; Hamrick and Godt 1990; Hamrick, Linhart, and Mitton 1979) and animals ( Nevo 1978; Nevo, Beiles, and Ben- Shlomo 1984; Powell 1975; Selander 1976). The percentage of loci polymorphic ranges from zero in elephant seals ( Bonnell and Selander 1974) and cheetahs ( O'Brien et al. 1983, 1985) to 92% in quaking aspen ( Cheliak and Dancik 1982) and 100% in the mussel Modiolus auriculatus ( Nevo et al. 1984). One of the most enduring objectives of electrophoretic studies is to understand the forces that produce differences among species in genetic variability. This chapter briefly summarizes several theories and some data relevant to this objective.
The niche-variation hypothesis was presented ( Dobzhansky 1970) as an explanation for the pattern of chromosome inversion frequency seen in Drosophila willistoni ( Da Cunha et al. 1959) and Drosophila robusta ( Carson 1959, 1965; Carson and Heed 1964). Populations of Drososphila willistoni near the center of their geographic distribution contain 14 to 18 inversions, but geographically peripheral populations contain as few as 1 to 4.
Dobzhansky perceived an axis of environmental heterogeneity parallel to the continuum in inversion heterozygosity between central and marginal populations. Geographically central populations had much more heterogeneous environments than peripheral populations, and Dobzhansky proposed that the inversion multiplicity enabled central