Question

Heterochromia of the eye (heterochromia iridum or heterochromia iridis) is a phenomenon that can occur in dogs, cats, and humans (e.g., David Bowie, Mila Kunis).  In heterochromia, the affected eye or portion of the eye, specifically the color of the irises, contains too much or too little of melanin pigment, causing it to appear different in color then other sections of the same eye or the other eye possessed by the individual.What could be an explanation for such a differences in eye color? Present one or more potential hypotheses that suggests it is a consequence of Darwinian evolution AND one or more alternate hypotheses in which this would not necessarily be the case. Be imaginative, especially in your proposal of alternate, but potentially plausible and testable hypotheses!

Answer 1

Eye color :

Eye color is a polygenic phenotypic characterdetermined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.

The appearance of blue and green, as well as hazel eyes, results from the Tyndall scattering of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky called Rayleigh scattering.

Neither blue nor green pigments are ever present in the human iris or ocular fluid.

Eye color is thus an instance of structural color and varies depending on the lighting conditions, especially for lighter-colored eyes.

Hypothesis base on Darwinian evolution :

The earliest human ancestors are believed to have come from the continent of Africa.

As primates adapted and then branched off into many different species on the tree of life, the lineage that eventually became our modern day human beings appeared.

Since the equator cuts directly through the continent of Africa, the countries there receive almost direct sunlight all year long.

This direct sunlight, with ultraviolet rays, and the warm temperatures it brings pressured for the natural selection of dark skin color.

Pigments, like melanin in the skin, protect against these harmful rays of the sun.

This kept individuals with darker skin alive longer and they would reproduce and pass down the dark skinned genes to their offspring.

The main gene that controls eye color is relatively closely linked to the genes that cause skin color.

It is believed that the ancient human ancestors all had dark brown or nearly black colored eyes and very dark hair (which is also controlled by linked genes to eye color and skin color).

Even though brown eyes are still considered mostly dominant over all eye colors, there are several different eye colors readily seen now in the global population of human beings.

As human ancestors began to migrate to various places around the world, the pressure for selection of dark skin color was not as intense.

Particularly unnecessary to human ancestors that settled in what are now the Western European nations, selection for dark skin and dark eyes was no longer necessary for survival.

These much higher latitudes afforded different seasons and no direct sunlight like near the equator on the continent of Africa. Since the selection pressure was no longer as intense, genes were more likely to mutate.

Eye color is a bit complex when talking about genetics.

The color of human eyes is not dictated by a single gene like many of the other traits.

It is instead considered a polygenic trait, meaning there are several different genes on various chromosomes that carry information about what eye color an individual should possess.

These genes, when expressed, then blend together to make various shades of different colors.

Relaxed selection for dark eye color also allowed more mutations to take hold.

This created even more alleles available to combine together in the gene pool to create different eye colors.

Individuals who can trace their ancestors to Western European countries generally have a lighter skin color and lighter eye color than those from other parts of the world.

Some of these individuals also have shown parts of their DNA that was very similar to those of the long extinct Neanderthal lineage.

Neanderthals were thought to have lighter hair and eye colors than their Homo sapien cousins.

New eye colors could possibly continue to evolve as mutations build up over time.

Also, as individuals of various shades of eye colors breed with one another, the blending of those polygenic traits may also result in the emergence of new shades of eye color.

Sexual selection may also explain some of the different eye colors that have popped up over time.

Mating, in humans, tends to be non-random and as a species, we are able to choose our mates based on desirable characteristics.

Some individuals may find one eye color much more appealing over another and choose a mate with that color of eyes.

Then, those genes are passed down to their offspring and continue to be available in the gene pool.