Different selective pressures shape the molecular evolution of color vision in chimpanzee and human populations.

A population genetic analysis of the long-wavelength opsin (OPN1LW, "red") color vision gene in a global sample of 236 human nucleotide sequences had previously discovered nine amino acid replacement single nucleotide polymorphisms, which were found at high frequencies in both African and non-African populations and associated with an unusual haplotype diversity. Although this pattern of nucleotide diversity is consistent with balancing selection, it has been argued that a recombination "hot spot" or gene conversion within and between X-linked color vision genes alone may explain these patterns. The current analysis investigates a closely related primate with trichromatism to determine whether color vision gene amino acid polymorphism and signatures of adaptive evolution are characteristic of humans alone. Our population sample of 56 chimpanzee (Pan troglodytes) OPN1LW sequences shows three singleton amino acid polymorphisms and no unusual recombination or linkage disequilibrium patterns across the approximately 5.5-kb region analyzed. Our comparative population genetic approach shows that the patterns of OPN1LW variation in humans and chimpanzees are consistent with positive and purifying selection within the two lineages, respectively. Although the complex role of color vision has been greatly documented in primate evolution in general, it is surprising that trichromatism has followed very different selective trajectories even between humans and our closest relatives.