Figure 7.4 A fitness surface for two loci that each have two alleles where gene action exhibits epistasis. When recombination is a weak force (r = 0.05), equilibrium allele frequencies are dictated by natural selection. Equilibrium allele frequencies depend on initial gamete frequencies since the two highest mean fitness points are separated by a fitness valley (a). When recombination is strong (r = 0.5), allele frequencies change such that mean fitness actually decreases for a time before increasing again to eventually reach the lower of the two mean fitness peaks (b). The two initial gamete frequencies in the upper right of the surface reach an equilibrium point where fitness is not maximized and there is gametic disequilibrium (D = 0.041). Relative fitness values are wAABB = 0.61, wAABb = 0.58, wAAbb = 0.50, wAaBB = 1.0,

WAaBb = °-77- WAabb = °-50- WaaBB = °'64' WaaBb = 0.62, and waabb = 0.92. The seven initial allele frequency points, shown as open circles, are identical for the two surfaces.

plex topography as dominance and epistasis increase and additive gene action decreases. For this reason, natural selection is sometimes described as shortsighted or myopic because it operates based on mean fitness each generation rather than on some plan that accounts for the entire mean fitness surface. The result is that the equilibrium produced by natural selection when mean fitness surfaces possess multiple maxima depends strongly on initial genotype frequencies.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Frequency of A allele for locus 1

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