Evolution of color vision in primates
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.
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 catarrhines (Old World monkeys and apes) are routine trichromats, meaning both males and females possess three opsins classes.
Today, most other vertebrate classes have retained their 4 cones and exhibit tetrachromacy, including birds, reptiles, teleosts (fish) and amphibians.
[9] Incomplete trajectories, or evolutionary pathways, are shown to be caused by T52F mutations occurring first because T52F does not have a peak for the absorption of light within the entire visible region.
[9] Studies using in vitro assays have shown that epistatic evolution took place in ancestral Boreoeutherian species with the 7 mutations on genetically reconstructed Boreoetherian short wavelength sensitive opsins.
[9] Further analysis has shown that 4008 out of the 5040 possible trajectories were terminated prematurely due to nonfunctional pigments that were dehydrated.
[9] In other animals that possess UV vision such as birds, ultraviolet sensitivity can be advantageous for courtship and reproductive success.
To identify the path from which short wavelength opsins evolved, increases in absolute max values were used by researchers with a limitation of approximately |Δλmax|<25 nm per step.
[4] It is also proposed that the polymorphism in the opsin gene might have arisen independently through point mutation on one or more occasions,[4] and that the spectral tuning similarities are due to convergent evolution.
Despite the homogenization of genes in the New World monkeys, there has been a preservation of trichromacy in the heterozygous females suggesting that the critical amino acid that define these alleles have been maintained.
[14] There exist several theories for the main evolutionary pressure that caused primates to evolve trichromatic color vision, namely the red-green opponent channel.
This theory postulates that trichromacy became favorable due to the increased ability to find ripe fruit against a foliage background.
Research has found that the spectral separation between the L and the M cones is closely proportional to the optimal for detection of many colors of fruit (red) against foliage (green).
[20] Those findings were based upon the fact that there is a larger variety of background S/(L+M) and luminance values under long-distance viewing.
[7][22] In addition, a prominent visual discriminant between young and mature leaves is their red-green color channel, which is only discernible to trichromats.
[25] The theory is that as sense of smell deteriorated, selective pressures increased for the evolution of trichromacy for foraging.
In addition, the mutation of trichromacy could have made the need for pheremone communication redundant and thus prompted the loss of this function.
Overall, research has not shown that the concentration of olfactory receptors is directly related to color vision acquisition.
Nonetheless research shows a significant negative correlation between the two traits in the majority of trichromatic species.
Similarly, other causes of skin tone change such as blushing or rump-reddening convey important information between potential sexual partners.
Fruits make up a relatively small portion of their diet,[30] and the type of leaves they consume (young, nutritive, digestible, often reddish in color), are best detected by a red-green signal.
Field work exploring the dietary preferences of howler monkeys suggest that routine trichromacy was environmentally selected for as a benefit to folivore foraging.
