SNP A/S ratios and evolutionary variables. (a) Proteins with paralogs (167 proteins) are under weaker selective pressure than proteins without paralogs (12,460 proteins). The 95th percentile confidence intervals of the A/S ratio are [0.38, 0.58] for proteins with paralogs, and [0.26, 0.27] for proteins without paralogs (dark gray bars). To control for expression breadth, the subset of proteins with mRNA expression data were analyzed (65 proteins with paralogs and 10,612 without, light gray bars) and Monte Carlo samplings were performed so that the two groups had the same distribution of expression breadth. The differences in A/S ratios are significant both before (light gray bars) and after (white bars) controlling for expression. (b) Proteins that arose early in evolution are subject to stronger evolutionary constraints.