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Evolutionary Considerations

The following hypotheses explore the role of evolution in dichromatism.

• Adaptation: During the Second World War, the U.S. Army discovered that colorblind soldiers could distinguish camouflaged targets better than their counterparts with color vision could. Further studies have shown that dichromats are better at detecting camouflaged targets in which the object’s color accounts for differences in texture between the object and its surroundings, have sharper vision, and may be less subject to the effects of “chromatic noise.” Other studies suggest a dichromat advantage in mesopic vision and scotopic vision. There is also a hypothesis that X-linked color deficiency leads to better discrimination against blue backgrounds, conferring and advantage to dichromats in fishing. As a result, dichromats may have an advantage over trichromats in detecting some kinds of prey, which could explain higher rate of dichromatism in relation to other defects.

• Evolutionary Legacy: Another hypothesis posits that the high frequency of dichromatism in humans is due to a relaxation of pressure for trichromats in societies that have been traditionally pastoral and agricultural. Because color vision is less important to survival in these societies, positive selection for trichromatism would be relaxed. Because the only genetic difference between a dichromat and a trichromat is in the opsin genes, in agricultural-pastoral societies the ancestral dichromat phenotype not being a reproductive hindrance (and therefore not being subject to negative selection)—but rather the newer trichromat phenotype merely being more advantageous in pre-agricultural societies (subject to positive selection)—accounts for the relatively high frequency of dichromatism in these societies.