The rate of nonsynonymous evolution correlates negatively with the proportion of boundary-proximal sequence. K
is plotted as a function of the proportion of coding sequence located within 70 bp of an exon-intron boundary for (a) D. melanogaster-D. pseudoobscura orthologous genes (rho = -0.26, P = 2.2E-16, N = 4,132) and (b) C. elegans-C. briggsae orthologous genes (rho = -0.08, P = 6.18E-09, N = 5,248). The data have been divided into bins along regular decimal intervals (0.1, 0.2, and so on) and the mean K
within each bin plotted against the mean proportion of sequence near the boundary. The last (a) and first (b) three bins, respectively, have been pooled to obtain approximately equal bin sizes. Negative trends are present for both sets of aligned genes, but a departure from the general trend is evident for nematode genes with a low proportion of boundary-proximal sequence.