Non-synonymous (Ka) versus synonymous (Ks) substitution rates for the human-mouse-rat orthologous protein sets. (a) Average Ks and Ka for the human-mouse and human-rat pairwise comparisons. (b) Ks and Ka for the mouse-rat pairwise comparisons. Thick diagonal line, Ks = Ka; thin horizontal line, average Ka. All values are shown with circles and the values corresponding to the functionally diversified proteins are indicated by larger squares.