In addition to being useful in a bulk statistical analysis of all PSGs, the Bayesian framework can be used to identify the single most likely selection history for each gene. In some cases, these histories are consistent with known functional differences between species, and help to shed light on the evolutionary basis of these differences. For example, the sweet receptor TAS1R2 has been shown in knock-out experiments to be responsible for differences between species in preferences for sweet tastes [55] . (Humans can taste several natural and artificial sweeteners that mice cannot, such as monellin, thaumatin, aspartame, and neohesperidin dihydrochalcone.) This gene is predicted to have experienced selection on the primate clade and on the branches leading to the primate and rodent clades (posterior probability [PP] = 0.20), suggesting that positive selection on TAS1R2 in both primates and rodents could have contributed to differences in sweet taste preferences. Another example is the integral membrane glycoprotein GYPC, which plays an important role in regulating the mechanical stability of red blood cells. In humans, GYPC has been associated with malaria susceptibility, and predicted to have undergone recent positive selection [56] . However, we find evidence that GYPC has experienced positive selection on all branches of the primate clade (PP = 0.66), suggesting longer-term selective pressure that have also affected nonhuman primates. A third example is CGA, which encodes the alpha subunit of the four human glycoprotein hormones (chorionic gonadotropin, luteiniz-ing hormone, follicle stimulating hormone, and thyroid stimulating hormone). This gene shows strong evidence of positive selection specific to the primate clade (PP = 0.82), consistent with the proposal that relatively recent adaptations in pregnancy and development have played a critical role in the evolution of the human endocrine system [57] . Interestingly, the closely related genes CGB1 and CGB2 (which encode two of the six beta subunits of chorionic gonadotropin) are thought to have originated by gene duplication in the common ancestor of humans and great apes [58] , and these events could have contributed to positive selection on CGA. Finally, the complement components C7 and C8B, which encode proteases in the membrane attack complex, are predicted with high probability to be under selection in rodents only (C7: PP = 0.98 for selection in mouse; C8B: PP = 0.93 for selection in mouse and rat). Differences in complement proteases are thought to explain certain differences in the immune responses of humans and rodents [59] .