Fig. 5.

Fitness and molecular effects of mutations under mutation–selection balance. (A) Mapping of ΔΔG to the selection coefficient among fixed mutations. Dots are colored similar to figure 4. (B) Arising random mutations are predominantly deleterious (fig. 4A, C). The deleterious and beneficial DFE are both characteristically leptokurtic and fits a gamma distribution. Stricly lethal mutations (Ns = −10³) are excluded in the fit to a gamma distribution. (C) The consequence of mutation–selection balance is a bimodal distribution of s and, in particular, equal number of fixed beneficial and deleterious mutations. Strict neutrality (s = 0) is not a stable fix point because mutational drift drives proteins to destabilization (fig. 4F). (D) Bimodal and equal fraction of fixed beneficial and deleterious s (panel C) maps into a symmetric distribution ΔΔG.