Rates of protein evolution (d_N/d_S) in (A) paralogous genes in C. elegans, (B) orthologous genes between C. elegans and C. briggsae, and (C) paralogous genes in C. briggsae. Rates of regulatory evolution (d_SM) in (D) paralogous genes in C. elegans, (E) orthologous genes between C. elegans and C. briggsae, and (F) paralogous genes in C. briggsae. In comparison with orthologs, duplicate genes in both the C. briggsae and C. elegans genomes exhibit a higher rate of amino-acid substitution and proximal cis-regulatory sequence evolution for the same amount of synonymous divergence.