Figure 1.

Substitution rate as a function of the effective strength of selection. The bold solid curve represents the slightly deleterious divergence rate between species. Remaining curves represent adaptive divergence under three scenarios: open triangles when the beneficial mutation rate (u_b) is 1% of the deleterious mutation rate (u_d); open circles when u_b is 5% of u_d; 'X-marks' when u_b is 10% of u_d. The strength of selection against deleterious mutations (|N_es_d|) is shown on the x-axis, where s_d is the average strength of purifying selection. A. The average strength of positive selection (s_b) is equal to the average strength of purifying selection (s_d); B. s_b is twice as strong as s_d; C. s_d is twice as strong as s_b. When selection is weak, mildly-deleterious substitutions outnumber adaptive substitutions; divergence in weakly-selected genes is therefore predicted to be negatively correlated with local recombination rate. As the strength of selection increases, adaptive substitutions predominate; divergence in strongly-selected genes is therefore predicted to be positively correlated with recombination rate. The adaptive divergence rate is u_b(1 - e^-4Nsp)/(1 - e^-4Ns), where s is the average benefit conferred by each mutation, and p is the initial frequency of each mutation (results for p = 0.0001 are shown). The slightly deleterious rate is u_d(1 - e^-4Nsp)/(1 - e^-4Ns), where u_d is the deleterious mutation rate, and s is the average cost of each mutation (equations modified from [30]; N_e = N). The assumption that the beneficial mutation rate is much smaller than the deleterious mutation rate is supported by theory and mutation accumulation experiments [38,55-57; but see 58,59].