Deviation of genes under recent positive selection in humans. (a) Illustrated mode of evolution for a gene shared by human and chimpanzee that is under constant positive selection in the human lineage. (b) Putative effect of evolutionary mode on intensity of clustering and intensity of non-synonymous change. The rate of substitution is different under diversifying and purifying selection, however both may promote the clustering of changes along the sequence. The arrows convey the intuition: if both modes of selection promote clustering (green upward) while purifying selection yields comparatively fewer substitutions (red downward), then genes under diversifying positive selection should be evolving more rapidly than their degree of clustering predicts based on the overall genomic trend. (c) Plot of log(ω) vs. log(ρ) for human/chimpanzee orthologs. Genes annotated as being under selection in the human lineage are highlighted in orange. In black is the fitted line ; in orange is the fitted line . Both γ and β were found to be significantly larger than zero.