Before revision:  The PDFs of }  K_a/K_s, which is the ratio of nonsynonymous to synonymous substitution rate per site, at equilibrium confirm that T_s negatively correlates with the amino acid substitution rate.
After revision:  The PDFs of } K_a/K_s, which is the ratio of nonsynonymous to synonymous substitution rate per site, at equilibrium confirm that T_s negatively correlates with the average of K_a/K_s. Stabilizing mutations are significantly fixed by positive selection, and balance with destabilizing mutations fixed by random drift.