According to Billy R. Hammond Jr, Kenneth Fuld and Max D. Snodderly from the Department of Ophthalmology at Harvard Medical School, both melanin and carotenoids are the reason for iris coloration. They believe that iris coloration has a protective purpose to limit the amount of retinal damage in an individual. They also suggest that similar environments increase the frequency of this particular phenotype. This environmental influence suggests that iris color maybe related to the amount of light in an environment. A species gene pool is influenced by its environment and the members that cohabitate in that particular ecosystem. Hammond and his colleagues believe that the accumulation of melanin and carotenoids may dwindle “due to the tendency for eyes with light irises to transmit more light than eyes with dark iris” (Hammond, Flud and Snodderly 1995).

Then again, others like Steve Conno,r a science editor for an Independent, online news agency, believe that eye color was a mutation that occurred in humans some 10,000 yrs ago. Since most primates, our closest ancestors have a brown pigmentation, it is believed that a mutation occurred in humans late in human evolution. He states that 99.5% of people with blue eyes share a common gene that expresses their eye color (Connor, 2008). This leads people like Hans Eiberg and Colleageues from the University of Copenhagen to believe that a single mutation in humans caused all of the blue eyes present in people today (Connor, 2008).

And, here's another one for you. Richard A. Sturm, Principal Research Fellow at the Institute for Molecular Bioscience at the University of Queensland in Brisbane, Australia, says, that there is no doubt that that the genes controlling eye color also influence skin color, and contribute to the lightning of skin in European peoples. But his personal viewpoint is that the eye-color genes responsible for switching on blue or brown eye color have a much greater impact on the color of a person's eyes than on the color of the skin. Sturm has absolutely no evidence supporting his view yet, but he's working on it. "We need a more mechanistic understanding of OCA2 (eye-color) gene regulation before we can go much further on what may have been the selection pressure." (Eye colour: portals into pigmentation genes and ancestry by Richard A. Sturm and Tony N. Frudakis, TRENDS in Genetics Vol.20 No.8 August 2004.) Sturm discussed the issue with his colleagues; some thought people with blue eyes "may have been able to better stand the dark, depressing days of Northern European ice-age winters than those with brown eye colour." Indeed, recent Seasonal Affective Disorder (SAD) studies support this idea. Goel et al found that among 165 depressed (bi polar disorder or major depressive disorder) people that "darker-eyed patients were significantly more depressed and fatigued [in the winter] than blue-eyed patients." He and his team concluded that lightly pigmented eyes increase the amount of light the eyes receive during the winter, which relieves depressive symptoms in vulnerable populations. Terman et al found similar results.

Your question can be decomposed into biological and evolutionary explanations.

At the biological mechanistic level, the most famous iris color-related variant occurs in a regulatory element that lies within an intron of the HERC2 gene on chromosome 15. This element regulates the expression of nearby gene OCA2.

The OCA2 regulation variant associated to blue iris color has been very strongly positively selected in Europe. This may give some indications on plausible evolutionary explanations. The fact that selection is so strong could indicate it was sexual selection (Damn I like her/his blue eyes...). Instead it could be a by-product of selection for lighter skin pigmentation, since OCA2 variants are also related to lighter skin pigmentation.

Typing OCA2 selection in Pubmed gives a number of interesting references.

(edited in 1/13 from 5/12)

I'm thinking that enhanced contrast with the scleral white might aid iris detection for defense-identification in the jungle ; that would go along with the higher melanin content of H.sapiens in tropical latitudes, but this hypothesis has holes as well.

Many species (especially predators) go a long way to disguise the eyes, but they are also often obvious when used in defensive camouflage of prey species.

BTW, the species-level number of melanocytes remains fairly consistent, with differences mostly in sheer amount of intracellular melanin content in vesicles. IOW, its not that darker species or variations have more pigment cells, necessarily, rather more pigment content in each cell.

A capacity to sense light direction (of sunlight) is a possible draw for evolution, but structurally-unimportant in the non-photo-sensitive iris; IOW, the retina senses light (CranialNerveII) and the iris reacts (via CranialNerveIII).

Under 40x magnification, the blue iris appears a cloud-like white. Color in the anterior iris is in layers: limited to the stroma in blue eyes, with some superficial granules coloring the "hazel-green" range. Blue and green irides are mostly due to the extent of Rayleigh scattering of light in the collagen-containing fibroblasts surrounding the vessels in the iris, which is generally blue in color.

This "blue" mixes with other pigments (reflecting from the stroma and over it) to produce an overall color, other-than brown: blue, hazel, green, etc.

[*Note: Refraction in the anterior media (tearfilm-cornea-aqueous humor) produce much of the "darks" seen around the periphery (nearest the limbus) of lighter eyes. This was earlier considered to be mainly due to a shadow-effect of the limbal sclera, (which still plays a role closest to the primary light source). IOW, the darker ring around the blue eye is not *only from pigment]. Likewise, internal-scattering of light into the sclera on the side opposite a light source.

All irides (excepting the albino) are backed (posteriorly) by an opaque pigment epithelium of neuroepithelial origin. This is covered (closer to eye center) by the non-pigmented epithelium of neural crest origin (that behind the iris gains pigment after birth, remains non-pigmented over the ciliary body, etc., reportedly continous with the retinal internal limiting membrane). The opacity of this melanin-pigmented layer is essential to isolating light to entering only at the pupil. [As OCT demonstrates, however, this opacity is imperfect at various frequencies.]

There is another element to play a part in the overall color of the eye: a blue-hue ring, seen through the limbal sclera in all eyes not otherwise obscured by pigment (dark brown sclera often exhibits coverage here). [This is a surgical landmark in humans.] Possibly representing refraction from the aqueous drainage canal (of Schlemm), could this be a "structural color" reflected from the laminar corneal-to-sclera transition? [Structural color is that which is reflected (by physical, lightwave harmonics of lamellae at the nanometer level) in butterflies/beetles. A laminar "cup" structure at 90nm produces a brilliant blue in "breath figures."]

Additionally, internal-reflection of light within the cornea produces a warm scattering (from hemoglobin) in the sclera opposite a light source.

Evolutionary REASONING for these differences, however, remains elusive, beyond social identification promoting bonding, thus offspring-protection, etc..

Let us know what you find....

Thank You....we're gettin' close. I do know that the higher the latitude (further from the direct angle of the sun), the lighter the skin color. This is, as you know, Nature's way of having us gobble whatever VitD the sun is offering. I suspect that the lighter iris is a similar adaptation....

Overall pigmentation may or may not be expressed in similar gene regions: follow David Enard's advice above.

[For all (I) know, THAT is a pineal adaptation related to off-axis correction of an equitorial rutting season.]

Also, there are non-imaging photosensitive pigments [called cryptophores (search "ganglion-layer pigment cells" and "extrapyramidal projection fields")] that may be more useful than iris color for geolocation of latitude.

IOW: There ARE brown irises in Finland, just (much) fewer per 100k individuals.

Also note: Polar bears do not have blue irides (and they actually cover their eyes from prey using their paws).

To Sum: *I don't know, but am working on it. :' o

Thank You! The Inuits in Alaska have dark skin and dark eyes---they get their VitD from their diet....as is the case with the Polar bears. I will certainly check in on David's advice. Once again...thanks! P.S. What a great site this is

BTW, I recently saw a quiz answer that stated that lighter irides (pl of "iris") pass more light to the retina. This is true, but only b/c the crypts of the blue iris may have circumferential furrows on the POSTERIOR iris pigment that, over time, may shed pigment, and thus leak some off-pupil light. HOWEVER, the posterior iris pigment epithelium is uncommonly uniform in its opacity, and this is a course of aging. Any lightness seen in specimens of the posterior iris pigmentation and that over the ciliary bodies and processes are reflecting post-mortem hydration changes; they are exquisitely dark in life.

According to Billy R. Hammond Jr, Kenneth Fuld and Max D. Snodderly from the Department of Ophthalmology at Harvard Medical School, both melanin and carotenoids are the reason for iris coloration. They believe that iris coloration has a protective purpose to limit the amount of retinal damage in an individual. They also suggest that similar environments increase the frequency of this particular phenotype. This environmental influence suggests that iris color maybe related to the amount of light in an environment. A species gene pool is influenced by its environment and the members that cohabitate in that particular ecosystem. Hammond and his colleagues believe that the accumulation of melanin and carotenoids may dwindle “due to the tendency for eyes with light irises to transmit more light than eyes with dark iris” (Hammond, Flud and Snodderly 1995).

Then again, others like Steve Conno,r a science editor for an Independent, online news agency, believe that eye color was a mutation that occurred in humans some 10,000 yrs ago. Since most primates, our closest ancestors have a brown pigmentation, it is believed that a mutation occurred in humans late in human evolution. He states that 99.5% of people with blue eyes share a common gene that expresses their eye color (Connor, 2008). This leads people like Hans Eiberg and Colleageues from the University of Copenhagen to believe that a single mutation in humans caused all of the blue eyes present in people today (Connor, 2008).

And, here's another one for you. Richard A. Sturm, Principal Research Fellow at the Institute for Molecular Bioscience at the University of Queensland in Brisbane, Australia, says, that there is no doubt that that the genes controlling eye color also influence skin color, and contribute to the lightning of skin in European peoples. But his personal viewpoint is that the eye-color genes responsible for switching on blue or brown eye color have a much greater impact on the color of a person's eyes than on the color of the skin. Sturm has absolutely no evidence supporting his view yet, but he's working on it. "We need a more mechanistic understanding of OCA2 (eye-color) gene regulation before we can go much further on what may have been the selection pressure." (Eye colour: portals into pigmentation genes and ancestry by Richard A. Sturm and Tony N. Frudakis, TRENDS in Genetics Vol.20 No.8 August 2004.) Sturm discussed the issue with his colleagues; some thought people with blue eyes "may have been able to better stand the dark, depressing days of Northern European ice-age winters than those with brown eye colour." Indeed, recent Seasonal Affective Disorder (SAD) studies support this idea. Goel et al found that among 165 depressed (bi polar disorder or major depressive disorder) people that "darker-eyed patients were significantly more depressed and fatigued [in the winter] than blue-eyed patients." He and his team concluded that lightly pigmented eyes increase the amount of light the eyes receive during the winter, which relieves depressive symptoms in vulnerable populations. Terman et al found similar results.

Wow, thanks for the detailed revision. I will look for the references you mention and would appreciate a list or a URL. Can we propose that light-colored irides may open the pupil wider in the lower light levels? Or is there light-transmission data THROUGH the irides involved? Again, thanks for your contributions.