Figure 10: Purifying selection on synonymous sites.
From: Initial sequence of the chimpanzee genome and comparison with the human genome
Mean divergence around exon boundaries at non-CpG, exonic, fourfold degenerate sites and intronic sites, relative to the closest mRNA splice junction. The divergence rate at exonic, fourfold degenerate sites is significantly lower than at nearby intronic sites (Mann–Whitney U-test; P < 10-27), suggesting that purifying selection limits the rate of synonymous codon substitutions.
