Fig. 1.

A visualization of the relationship between the genotype–phenotype map and phenotype-fitness map. The phenotype-fitness map is shown as a black parabola. The genotype–phenotype map is shown as four haploid genotypes situated at their respective positions in phenotype and fitness space. The dashed curves connect genotypes differing by a single mutational step, so evolution by single steps would have to follow those lines. A genetic constraint is evident in that a large population starting with genotype ab could not evolve to AB by single, adaptive mutational steps, even though AB has higher fitness than ab. Furthermore, if these were the only alleles available, the genotypes only sparsely cover the phenotype space. Figure developed and provided by Ben Kerr.