Correlation between protein evolution (d_N) and regulatory evolution (d_SM) in (A) paralogous genes in C. elegans (r_s = 0.24), (B) orthologous genes (r_s = 0.16), and (C) paralogous genes in C. briggsae (r_s = 0.21); P ≪ 10^–4 for all tests. Multiple regressions that included d_S revealed that the correlation between d_N and d_SM in paralogs is primarily a function of d_S (i.e., duplicate age). No such effect was found in orthologs. Note that for some orthologs and duplicates, regulatory and protein evolution appear to be completely uncoupled.