Figure 1.

Hypothetical fitness reaction norms across two alternative environments, benign and harsh, for different genotypes. Each line represents the behavior of one particular genotype (a) These mutations are unconditionally deleterious. Changes are observed in the fitness rank order (the hierarchy of fitnesses among the different genotypes), together with an increase in mutational variance (the range of fitnesses seen among the different genotypes). (b) These mutations are conditionally neutral. A net increase in mutational variance is observed. (c) These mutations are conditionally beneficial. The fitness rank order changes for some mutations, although the mutational variance remains unchanged. (d) These different genotypes carry all three types of mutations: unconditionally deleterious, conditionally neutral and conditionally beneficial. A complex situation is illustrated, in which a net increase in mutational variance is accompanied by a change in fitness rank order. In all cases (a-d), the dashed line at a fitness value of 1.0 represents a neutral effect; we assume that a mutation-free genotype has a fitness of 1.0 in the benign environment.