Similarly, the PSGs associated with diet include but are not limited to taste and olfactory receptors. For example, MGAM (P = 2.4610 28 ) is essential for the small intestinal digestion of starch, giving it a critical role in human metabolism, as starches of plant origin make up two-thirds of most human diets [48] (see also [49] ). MAN2B1 (P = 1610 26 ) is involved in the cleavage of the alpha form of mannose, a sugar monomer. Defects in this gene cause lysosomal alpha-mannosidosis, a lysosomal storage disease characterized by the accumulation of unbranched oligosaccharide chains [50] . TCN1 (P = 2.9610 231 ) is a major constituent of secondary granules in neutrophils and facilitates the transport of vitamin B12 into cells, which is important for the normal functioning of the brain and nervous system, and for the formation of blood [51] . In addition, several PSGs participate in ''steroid hormone metabolism'' (P = 8.3610 24 ) including genes that metabolize xenobiotics and drugs (e.g., SULT1C3, UGT2B7, and CYP2C8). Positive selection in these and other genes is likely to Figure 2 . Hierarchical clustering of 27 over-represented GO categories identified by the Mann-Whitney U test (''biological process'' group only), based on the genes assigned to each category. This dendrogram is derived from a dissimilarity matrix defined such that any two GO categories, X and Y, have dissimilarity 0 when all genes assigned to X are also assigned to Y (or vice-versa), and dissimilarity 1 when the sets of genes assigned to X and Y do not overlap. Specifically, X and Y have dissimilarity have been influenced by changes in food preferences during mammalian evolution.