Patterns of Variation Among Loci
We conclude that those characters that exhibit greater variation in local populations show greater divergence of extremes between populations, and that such a relationship is not an artifact. . . . In providing a possible explanation for how within-population variation limits divergence, we postulate that there is an inverse relationship between the variability of a character in a local population and the effect it is likely to have on individual fitness.
A. G. Kluge and W. C. Kerfoot "The Predictability and Regularity of Character Divergence," ( 1973)
Population geneticists often assume that proteins constitute a random sample of the genome and that an electrophoretic survey of proteins can be used to estimate the level of genetic variation within a strain, a population, or a species. For some biologists, this assumption implies that genetic variation is either random or homogeneous across loci. However, the consistent patterns in the genetic variation help us understand the forces that act on proteins. This chapter summarizes consistent patterns of genetic variability observed at polymorphic enzyme loci and among groups of enzyme loci.
At most of the enzyme polymorphisms segregating more than two alleles, the electrophoretic mobility of the most common allele is intermediate, and the mobilities of rare alleles tend to be extreme. Bulmer ( 1971) first reported rare alleles to have extreme mobilities, and this observation has been extended in a series of subsequent studies that explored the charge-state model for protein polymorphism ( Brown, Marshall, and Weir 1981; Haldorson and King 1976; Richardson, Richardson, and Smouse 1975; Weir, Brown, and Marshall 1976). An extreme example of this relationship between mobility and allelic frequency is seen in the muscle esterases of the sand launce, Ammodytes, dubuis (figure 5.1). Population samples taken in successive years from the Emerald Bank, off Nova Scotia, yielded homogeneous allelic frequencies ( Mitton and Odense 1985);