On the Evolution of Color Vision

During the age of the dinosaur, mammals were almost entirely nocturnal creatures.  The giant cold-blooded reptiles dominated the days, and the small rodent-like mammals the night.  These opposing environments sculpted the evolution of sight in both of these classes of animals.  Dinosaurs likely saw in great color vision, as do their progeny alive today.  Mammals, needing to gather all the light available to them in the darkness of the Cretaceous nights, had poor color perception and good night vision.  Most mammals had tapetum lucidums as well, just like they do now.

As it still is today, the balance between color vision and night vision was an evolutionary tug-of-war in times past.  This struggle was played out between the retina’s cones and rods.  Cones are the photoreceptor cells in the eye responsible for seeing in vibrant color. They come in a wide range of flavors, humans see in the familiar red, green, and blue cones.  All cones can recognize the full spectrum of visible light.  The red-green-blue distinction refers to the wavelength at which each type of cone is most responsive.  The more diversity in the types of cones, the more positions from which the brain can triangulate the actual color of the object being perceived.  Rods are the other, more sensitive, photoreceptor cells in the retina.  Rods, unlike cones, see light in only one narrow band of the visible spectrum.  The sensitivity of rods makes them critical to night vision.  Rods tend to be at much greater densities on the outside of the retina, improving low-light vision in the periphery.  Cones, on the other hand, are concentrated in the center of the retina, so that color is sharpest when it is under focus.

Most reptiles and almost all Old World primates (including Humans) see in three colors.  They are trichromatic.  Animals that see in two colors, like most other mammals (notable exceptions include Australian marsupials), are dichromatic.  An animal that sees only one, such as the owl monkey, is  monochromatic.  Some birds and turtles are tetrachromatic, and see color more sharply than we do.

New World monkeys have an entirely different story from their eastern cousins.  At the point of their evolutionary divergence, both were still dichromatic.  Functional trichromatic color vision evolved independently in each group of monkeys for unique reasons. Yet in the New World, true trichromatic vision exists only in the howler monkey.  The story behind color vision in the remaining monkeys is quite interesting.

The genes that are activated to generate the protein for green and red cones are on the X chromosome in primates, and both are extremely similar.  The gene for blue is on chromosome 7, and is distinctly different from green and red.  Both green and red are the products of recent evolutionary divergence, whereas blue is considerably more ancient.

In New World monkeys, there is only one spot per X chromosome for this gene to land, so it can be occupied by a red or green gene but not both.  Subsequently, males (XY) get either a green or red on their X chromosome, and females (XX) get a chance to have red or green on each X chromosome.  So, female New World monkeys have a 50% chance of being trichromatic, a 25% chance of being red color blind and a 25% chance of being green color blind.  Male New World monkeys have a 50% chance of being red and a 50% chance of being green, with no possibility of being trichromatic.

This is an example of polymorphism, when two or more variants on a common gene, whose expression has a significant impact on daily life, exist in parallel in an interbreeding population.  This means that neither red or green color blindness conferred a large enough advantage to cause the other gene to disappear from the group.

There are two possible causes for this. One is that in an area with both red and green foods, there may exist a see-saw like effect whereby the dominant gene in the population (let’s say green) will begin to recess as the supply of green fruit in that part of the jungle can no longer sustain the population.  During this process, the handful of red cone monkeys will enjoy greater access to red fruit, until they become the dominant genetic force in the population, at which point they will recess as well.  This sort of equilibrium doesn’t normally last for millions of years, so this hypothetical is not very likely.  The alternative cause is that the benefit of one gene (green or red) over the other was outweighed by the benefit conferred by the existence of a stable coexistence between both red and green color blindness in the population.

One suggestion is that the cooperative behavior fostered in the search for varieties of multi-colored food paid off in other elements of their lives, like by male-female pair bonding and less sexual competition between males.  Males benefited from cooperating with females, as every other female monkey was twice as good as two males at spotting colorful food.  This also would result in reduced sexual dimorphism, which is present in many African primates.  Two males missing opposing genes for color would, as a pair, be as efficient as a trichromat.  As ancient fruits ripened throughout the seasons, the change in color could render them invisible to the background foliage for some males but extremely obvious to others, encouraging continued cooperation.

Both male and female howler monkeys have red and green genes on their X chromosomes.  They came by this lucky, as the genetic evidence suggests that a green gene was duplicated by mutation in one of their ancestors, and then subsequently overwritten with a red gene in a later generation.  This stable trichromatic formation quickly spread through the existing populations as it was considerably more beneficial to each individual monkey.  As a result, sexual dimorphism and an imbalance in the gender ratio returned.

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~ by Wil Finley on January 28, 2010.

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