Evolution of color vision in primates

The first hypothesis is that the two-gene (M and L) system of the catarrhine primates evolved from a crossing-over mechanism. Unequal crossing over between the chromosomes carrying alleles for L and M variants could have resulted in a separate L and M gene located on a single X chromosome.[6] This hypothesis requires that the evolution of the polymorphic system of the platyrrhine pre-dates the separation of the Old World and New World monkeys.[9]

This hypothesis proposes that this crossing-over event occurred in a heterozygous catarrhine female sometime after the platyrrhine/catarrhine divergence.[4] Following the crossing-over, any male and female progeny receiving at least one X chromosome with both M and L genes would be trichromats. Single M or L gene X chromosomes would subsequently be lost from the catarrhine gene pool, assuring routine trichromacy.

The alternate hypothesis is that opsin polymorphism arose in platyrrhines after they diverged from catarrhines. By this hypothesis, a single X-opsin allele was duplicated in catarrhines and catarrhine M and L opsins diverged later by mutations affecting one gene duplicate but not the other. Platyrrhine M and L opsins would have evolved by a parallel process, acting on the single opsin gene present to create multiple alleles. Geneticists use the "molecular clocks" technique to determine an evolutionary sequence of events. It deduces elapsed time from a number of minor differences in DNA sequences.[10][11] Nucleotide sequencing of opsin genes suggests that the genetic divergence between New World primate opsin alleles (2.6%) is considerably smaller than the divergence between Old World primate genes (6.1%).[9] Hence, the New World primate color vision alleles are likely to have arisen after Old World gene duplication.[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.[12]

This theory encompasses the idea that this trait became favorable in the increased ability to find ripe fruit against a mature leaf background. Research has found that the spectral separation between the L and the M cones is closely proportional to the optimal detection of fruit against foliage.[13] The reflectance spectra of fruits and leaves naturally eaten by the Alouatta seniculus were analyzed and found that the sensitivity in the L and M cone pigments is optimal for detecting fruit among leaves.[14]

While the “fruit theory” holds much data to support its reasoning,[13][14][15][16] recent research has gone on to disprove this theory. Studies have suggested that the cone pigments found in dichromats can actually distinguish the color differences between fruit and the foliage surrounding it.

This theory is centered around the idea that the benefit for possessing the different M and L cone pigments are so that during times of fruit shortages, an animal's ability to identify the younger and more reddish leaves, which contain higher amounts of protein, will lead to a higher rate of survival.[7][17] This theory supports the evidence showing that trichromatic color vision originated in Africa, as figs and palms are scarce in this environment thus increasing the need for this color vision selection. However, this theory does not explain the selection for trichromacy polymorphisms seen in dichromatic species that are not from Africa.[17]

This hypothesis suggests that trichromacy has evolved to adapt to distinguishing objects from the background foliage in long distance viewing. This hypothesis is based upon the fact that there is a larger variety of background S/(L+M) and luminance values under long-distance viewing.[16]

This hypothesis suggests that trichromacy has evolved to show higher sensitivity to low spatial frequencies. Spatiochromatic properties of the red-green system of color vision may be optimized for detecting any red objects against a background of leaves at relatively small viewing distances equal to that of a typical “grasping distance."[18]

The sense of smell may have been a contributing factor in selection of color vision. One controversial study suggests that the loss of olfactory receptor genes coincided with the evolved trait of full trichromatic vision.;[19] this study has been challenged, and two of the authors retracted it.[20] 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. Research suggests that the species Alouatta does not share the same characteristics of pheromone transduction pathway pseudogenes that humans and Old World monkeys possess and leading howler monkeys to maintain both pheromone communication systems and full trichromatic vision.[21]

Therefore, trichromacy alone does not lead to the loss of pheromone communication but rather a combination of environmental factors. Nonetheless research shows a significant negative correlation between the two traits in the majority of trichromatic species.

Trichromacy may also be evolutionarily favorable in offspring health (and therefore increasing fitness) through mate choice. M and L cone pigments maximize sensitivities for discriminating blood oxygen saturation through skin reflectance.[22] Therefore, the formation of trichromatic color vision in certain primate species may have been beneficial in modulating health of others, thus increasing the likelihood for trichromatic color vision to dominate a specie’s phenotypes as the fitness of offspring increases with parental health.

There are two noteworthy genera within the New World monkeys that exhibit how different environments with different selective pressures can affect the type of vision in a population.[7] For example, the night monkeys (Aotus) have lost their S photopigments and polymorphic M/L opsin gene. Because these anthropoids are and were nocturnal, operating most often in a world where color is less important, selection pressure on color vision relaxed. On the opposite side of the spectrum, diurnal howler monkeys (Alouatta) have reinvented routine trichromacy through a relatively recent gene duplication of the M/L gene.[7] This duplication has allowed trichromacy for both sexes; its X chromosome gained two loci to house both the green allele and the red allele. The recurrence and spread of routine trichromacy in howler monkeys suggests that it provides them with an evolutionary advantage.

Howler monkeys are perhaps the most folivorous of the New World monkeys. Fruits make up a relatively small portion of their diet,[23] 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.[4][7][17]