The Axis of Individual Heterozygosity: Empirical Data
Indeed the evidence for greater fitness of heterozygotes is stronger than that for most other kinds of natural selection.
J. B. S. Haldane, The Biochemistry of Genetics ( 1954)
Ever since biologists began to use protein polymorphisms to study variation in populations, they have accumulated data indicating that highly heterozygous individuals enjoy fitness advantages. At first, biologists viewed these data as curiosities, suggestive of strong selection within a life cycle but inconsistent with the convenient assumption of neutrality. Such observations are numerous, have a broad systematic base ( Allendorf and Leary 1986; Mitton 1989, 1993a,b; Mitton and Grant 1984; Zouros and Foltz 1987), and appear to be consistent with predictions from recent models in population genetics ( Gillespie 1977; Gillespie and Langley 1974; Ginzburg 1979, 1983; Milkman 1978, 1982; Turelli and Ginzburg 1983; Wills 1978, 1981). A sampling of these data, biased by personal experience, follows.
The early mortality characteristic of highly fecund species is striking. Consider a single female blue mussel, depositing several million eggs in the water. If she succeeds in leaving behind two offspring that live through reproductive age, she will have attained the mean fitness in a population that is neither crowing nor shrinking. Much of the mortality in this sort of life cycle must be independent of genotype; currents carry many larvae to areas unsuitable for settlement and growth, and planktivores harvest them with little taste for their genotype. But within the life cycles of highly fecund species, even if 99% of the mortality is independent of genotype, selection has ample opportunity to modify genotypic proportions.
Mortality in the blue mussel, Mytilus edulis, modifies the genotypic proportions of the leucine aminopeptidase locus ( Koehn, Milkman, and Mitton 1976). The sizes and Lap genotypes of 1,350 mussels were examined at a site near the Bourne Bridge, in the Cape Cod Canal, to determine whether genotype varied with size. Strong tidal currents