Figure 1. Phenotypic distributions of individuals resulting from an F₂ cross when the character is equally affected by (a) 2 or (b) 26 unlinked loci, with moderate dominance (pink bars, triangles) and complete additivity (blue bars, circles). The phenotype at the midpoint of the horizontal axis (0) is exactly intermediate between the two most extreme phenotypes, at –1 and 1. Grandparental genotypes were homozygous for one of two different alleles at each locus and parental genotypes mated randomly. Allele frequencies are 0.5 with each dominant allele increasing the phenotypic effect of a locus by 50% over pure additivity. In (b) the phenotypes are graphed as having essentially continuous variation to maintain scaling for both distributions.

Figure 2. A single fitness peak defined by two phenotypic dimensions (x and y). The point A represents the phenotype having the maximal fitness, with all other phenotypes (e.g. B) having lower fitness. Single mutations can improve fitness (B > B¢), but the likelihood of improvement depends upon the size of the phenotypic effect of the mutation. To improve in fitness from B, mutations cannot have a phenotypic effect larger than d, the cross-sectional diameter of the fitness peak.

Figure 3. Adaptive landscapes with multiple fitness peaks. Single mutations may be sufficient to cause a shift from a lower fitness peak to a higher peak (a). If a single mutation is insufficient to bridge the gap between peaks and there is a complex adaptive landscape (b), chance and history could have strong effects on adaptive outcomes, trapping populations on lower fitness peaks.