Figure 1. Adaptive distance plot for the human sickle cell polymorphism, drawn from the data in Table 3. The adaptive distance plot assumes that selection favouring heterozygotes is acting directly on the locus, and tests whether allelic frequencies are at an evolutionary equilibrium defined by the genotypic fitnesses. The fitnesses are highest in the heterozygotes, intermediate in the common homozygotes, and lowest in the rare homozygotes. The relationship between equilibrium allelic frequencies and adaptive distances is linear when selection is weak, but it may depart from linearity when selection is very strong, as is the case for SS homozygotes, which commonly perish from damage caused by blood clots.