Figure 4.

Role of positive selection. (A) Comparison of dN/dS ratios for tandem duplicates and retrogenes (log-scale). The non-synonymous substitutions in most duplicates are under selective constraints (dN/dS < 1); there is little difference in the dN/dS ratios between both types of duplicates (human: p = 0.607, mouse: p= 0.257, t-test) (B) Distribution of the duplicates before and after the split of human and mouse lineages (≈80 million years ago [33]). dS/2 was used to estimate the duplication age, which can be translated to the absolute time scale by using 2.5e-3 substitutions per site per million years [9]; there are significantly more tandem duplicates and retrogenes arising in the mouse-specific lineage (p < 1e-4 for both types, chi-square test)