Figure 1
The model of the fitness of genotypes, including adaptive epistatic gene combinations. When A2 and B2 alleles occur together in a genotype they form an adaptive epistatic gene combination. Fitness is a function of the frequencies of A2 and B2 alleles in a genotype. Four surfaces are presented that encapsulate the range of possible fitness surfaces. (a) The fitness surface for the epistatic gene combination is concave down. Here a genotype has 50% of maximum fitness when the frequencies of the A2 and B2 alleles are both 1/4 in the genotype. Parameter values are α=2.5, β=5.73. (b) A more weakly concave-down surface. Here a genotype has 50% of maximum fitness when the frequencies of the A2 and B2 alleles are both 1/2 in the genotype. Parameter values are α=1.55, β=2.85. (c) An S-shaped surface in which a genotype has 50% of maximum fitness when the frequencies of the A2 and B2 alleles are both 1/2 in the genotype. Parameter values are α=20, β=0.00678. (d) A concave-up surface in which a genotype has 50% of maximum fitness when the frequencies of the A2 and B2 alleles are both 3/4 in the genotype. Parameter values are α=1.25, β=0.177. For all surfaces, parameters determining the strength of selection are κ1=κ2=0.001, λ1=λ2=0.5, ξ1=ξ2 =−0.01 and γ=0.1.
