Figure 10.

Alternative models for mutations of large effect and lethals. (A) The fitness function is truncated for extreme trait values: any mutation with effect larger than some fitness threshold has a much stronger deleterious effect than with a smooth landscape model. As a consequence, the average heterozygous effect of mutations of weak homozygous effect (pink) is necessarily smaller than that of mutations of large homozygous (blue). (B) Mutations of large effects and lethals result from genetic incompatibilities unrelated to the trait values. For the sake of illustration, these incompatibilities are illustrated as small random holes on the fitness surface, but these holes are not necessarily a fixed feature of the fitness surface, they may differ across genetic background or environments. In this case the average heterozygous effect of mild and strongly deleterious mutations (including lethal) is equal on average.