alleles are alleles associated with malarial resistance, it may at first glance seem reasonable to expect them both to be favored by natural selection in a malarial environment and therefore to have a positive correlation with one another. However, natural selection is not survival of the fittest trait either. Obviously, malarial resistance is the fittest trait to have in a malarial environment, but selection operates only from the gamete's point of view, which includes a weighted average of the effects of that gamete in all possible genotypes and over the entire constellation of traits (including hemolytic anemia in this case) associated with these genotypes. The negative-fitness interaction between the S and C alleles when put together in SC heterozygotes ensures that these two adaptive strategies are negatively correlated from the gametic perspective. Only the gametic perspective determines the pattern observed in the actual populations (Figure 11.6). Thus, two alleles associated with malarial resistance are negatively correlated in a malarial environment!
The contrast in the gene pools of the upper Volta populations versus the Bantu populations in the remaining malarial regions of sub-Saharan Africa illustrates that evolution induced by natural selection is very much an historical process; two populations can (and apparently did in this case) adapt to the same environment in completely different fashions depending upon their initial historical states. The course of evolution is always constrained by what genetic variation is available for natural selection to work upon and by the initial state of the gene pool.
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