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Evolution of color vision in primates
The evolution of color vision in primates is highly unusual compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. Most teleost fish, reptiles and birds are therefore tetrachromatic while most mammals are strictly dichromats, the exceptions being some primates and marsupials, who are trichromats, and many marine mammals, who are monochromats.
While color vision is dependent on many factors, discussion of the evolution of color vision is typically simplified to two factors:
In vertebrates, both of these are almost perfectly correlated to an individual's cone complement.
The retina comprises several different classes of photoreceptors, including cone cells and rod cells. Rods usually do not contribute to color vision (except in mesopic conditions[citation needed]) and have not evolved significantly in the era of primates,[citation needed] so they will not be discussed here. It is the cone cells, which are used for photopic vision, that facilitate color vision.
Each type - or class - of cones is defined by its opsin, a protein fundamental to the visual cycle that tunes the cell to certain wavelengths of light. The opsins present in cone cells are specifically called photopsin. The spectral sensitivities of the opsins are dependent on their genetic sequence. The most important (and often only important for discussions of opsin evolution) parameter of the spectral sensitivity is the peak wavelength, i.e. the wavelength of light to which they are most sensitive. For example, a typical human L-opsin has a peak wavelength of 560 nm. The cone complement defines an individual's set of cones in their retina - usually consistent with the set of opsins in their genome.
The breadth of an individual's visual spectrum is equal to the minimum and maximum wavelengths to which at least one of their cones is sensitive. In vertebrates, the dimensionality of the color gamut is usually equal to the number of cones/opsins, though this simple equivalence breaks down for invertebrates.
The cone complements exhibited by primates can be monochromatic, dichromatic or trichromatic. The catarrhines (Old World monkeys and apes) are routine trichromats, meaning both males and females possess three opsins classes. In nearly all species of platyrrhines (New World monkeys) males and homozygous females are dichromats, while heterozygous females are trichromats, a condition known as allelic or polymorphic trichromacy. Among platyrrhines, the exceptions are Alouatta (routine trichromats) and Aotus (routine monochromats).
All primates with the exception of Aotus exhibit an S-opsin (short wave sensitive) in the cone most sensitive to blue light (S-cone). This opsin is encoded by an autosomal gene on chromosome 7. The other cones differ between primates.
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Evolution of color vision in primates
The evolution of color vision in primates is highly unusual compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs. Most teleost fish, reptiles and birds are therefore tetrachromatic while most mammals are strictly dichromats, the exceptions being some primates and marsupials, who are trichromats, and many marine mammals, who are monochromats.
While color vision is dependent on many factors, discussion of the evolution of color vision is typically simplified to two factors:
In vertebrates, both of these are almost perfectly correlated to an individual's cone complement.
The retina comprises several different classes of photoreceptors, including cone cells and rod cells. Rods usually do not contribute to color vision (except in mesopic conditions[citation needed]) and have not evolved significantly in the era of primates,[citation needed] so they will not be discussed here. It is the cone cells, which are used for photopic vision, that facilitate color vision.
Each type - or class - of cones is defined by its opsin, a protein fundamental to the visual cycle that tunes the cell to certain wavelengths of light. The opsins present in cone cells are specifically called photopsin. The spectral sensitivities of the opsins are dependent on their genetic sequence. The most important (and often only important for discussions of opsin evolution) parameter of the spectral sensitivity is the peak wavelength, i.e. the wavelength of light to which they are most sensitive. For example, a typical human L-opsin has a peak wavelength of 560 nm. The cone complement defines an individual's set of cones in their retina - usually consistent with the set of opsins in their genome.
The breadth of an individual's visual spectrum is equal to the minimum and maximum wavelengths to which at least one of their cones is sensitive. In vertebrates, the dimensionality of the color gamut is usually equal to the number of cones/opsins, though this simple equivalence breaks down for invertebrates.
The cone complements exhibited by primates can be monochromatic, dichromatic or trichromatic. The catarrhines (Old World monkeys and apes) are routine trichromats, meaning both males and females possess three opsins classes. In nearly all species of platyrrhines (New World monkeys) males and homozygous females are dichromats, while heterozygous females are trichromats, a condition known as allelic or polymorphic trichromacy. Among platyrrhines, the exceptions are Alouatta (routine trichromats) and Aotus (routine monochromats).
All primates with the exception of Aotus exhibit an S-opsin (short wave sensitive) in the cone most sensitive to blue light (S-cone). This opsin is encoded by an autosomal gene on chromosome 7. The other cones differ between primates.