The Evolution and Physiology of Human Review Color Vision: Insights from Molecular Genetic Studies of Visual Pigments

Present-day vertebrates vary enormously in the sophistication of their color vision, the density and spatial distribution of cone classes, and the number and absorption maxima of their cone pigments At one extreme, most mammals have only three pigments: the two ancestral cone pigments and rhodopsin. Baltimore, Maryland 21205 At the other evolutionary extreme, chickens possess six Nothing in biology makes sense except in light of evo-pigments: four cone pigments, one rhodopsin, and a lution. pineal visual pigment, pinopsin. As seen in the dendro-—T. Dobzhansky gram in Figure 2, the chicken green pigment was derived from a duplication within the rhodopsin branch. Color vision is the process by which an organism ex-In this evolutionary comparison, humans and their tracts information regarding the wavelength composi-closest primate relatives represent an intermediate level tion of a visual stimulus (Figure 1). In its simplest form— of complexity. Humans have four visual pigments: a exemplified by the wavelength-dependent phototactic single member of the Ͻ500 nm family of cone pigments responses of halobacteria—color vision is based on the (the blue or shortwave pigment, with an absorption relative abundances of two isoforms of a sensory pig-maximum at ‫524ف‬ nm), two highly homologous mem-ment (Hoff et al., 1997). One isoform preferentially ab-bers of the Ͼ500 nm family (the green or middle-wave sorbs long wavelength light and mediates a photoat-pigment, and red or long-wave pigment, with absorption tractant response, and the second isoform preferentially maxima at ‫035ف‬ and ‫065ف‬ nm, respectively), and rho-absorbs short wavelength light and mediates a photore-dopsin. The presence of only a single gene encoding a pellant response. Light absorption photoconverts the Ͼ500 nm pigment in almost all New World primates, first isoform into the second, and the second into the and in all nonprimate mammals studied to date, places first. Thus, the steady-state ratio of pigment isoforms the red/green visual pigment gene duplication in the provides a measure of the spectral composition of the Old World primate lineage at ‫04–03ف‬ million years ago, ambient light. shortly after the geologic split between Africa and South The halobacterial system contains the two cardinal America (Jacobs, 1993). Current molecular genetic evi-elements of every color vision system: (1) two or more dence suggests that howler monkeys, the only known sensory pigments with different spectral sensitivities (al-species of New World primate with two Ͼ500 nm pig-though in most organisms these are distinct chromopro-ment genes, have acquired a gene duplication event teins …