What is the function of human eye color ?
I’m temped to say it’s so everyone can easily spot genetically superior light-eyed nordic types like myself, but I’m afraid the answer is that eye colour doesn’t have a function.
Light eyes (blue/green/grey) evolved alongside fair skin, as an adaptation to higher latitudes and less direct sunlight. But light eyes are a by-product of the fair-skin mutation, they work in exactly the same way as dark eyes, there’s no functional difference.
Brown eyes serve the function of protecting the eye from damage from UV light.
Other eye colours serve no purpose at all that anybody can tell. They are the result of the loss of the brown pigment, nothing more.
As people moved into higher latitudes excess pigmentation in the skin became a risk for various diseases, while the absence of strong UV light meant there was no liabiity to losing the pigment. So the brown pigment was lost. In some cases the loss of the brown pigment extended as far as loss of pigment from the eyes. That wasn’t because it served any purpose, it’s just that the loss of general pigmentation extended to the eyes as well.
Best evidence is that eye colours aside from brown occur simply because there is no detriment to them. There’s a theory that, because human eyes get darker with age, females with lighter coloured eyes had some sexual advantage because they appear younger. But there’s no evidence that this is the case.
That’s not entirely true. Brown eye colour serves a purpose. The other eye colours exist solely because they don’t cause any great harm at high latitudes and so can’t be selected against.
As Blake says, there seems to be no direct physiological advantage for eye colors other than brown. However, the frequency of other colors such as blue could be maintained and increased due to sexual selection, if potential partners found it attractive. (Even if the preference was purely cultural, it still could have a selective effect.)
I have read that brown eyes are more likely to be nearsighted and more likely to get cataracts because the darker color draws UV rays towards the darker iris.
No, there is no color that “sucks in” any wavelength of light.
People with darker irises might find bright sunlight more tolerable than those with lighter colors, and might spend more time in the sun and thus have more UV exposure, which is a risk for cataracts, but if that’s the case it’s a matter of behavior and not a direct result of the color.
Wikipedia reports a study by Peter Frost:
http://findarticles.com/p/articles/mi_hb6366/is_2_15/ai_n29000212/
Dark eyes do suck in the sun’s rays more than blue eyes. Have you ever worn a black shirt on a hot day? Is a white T shirt or a black one hotter? Dark colors inherently draw more of the suns rays…to say that it’s a behavior is ridiculous.
Dark colors absorb more light and convert it to heat. That’s not the same as drwing in more ultraviolet; the same amount of UV is going to fall upon the iris no matter which wavelengths it absorbs.
Sucking implies a mechanical action that requires a difference in pressures in two different places to work. An example is a baby sucking on your finger, where the cheek muscles, tongue muscles, and muscles of the throat provide the force to produce the pressure differential.
Absorbing is a physical action. From Wikipedia…"In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is often called attenuation. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in optics), the medium changes its transparency dependently on the intensity of waves going through, and the saturable absorption (or nonlinear absorption) occurs.
Absorb does not equal “suck”.
I always thought that blue eyes were better in situations like sun glare from snow, something about being more reflective instead of absorbing. Was anyone else taught this?
Of course, there tends to be a correlation between pigmentation in eyes, skin, and hair. People with darker skin also tend to have darker eyes and hair, and so on. So it could be that the primary adaptation is that skin is darker in sunnier climates, and the fact that the body has so much melanin lying around happens to make eyes and hair darker, too.
And also of course, as with almost everything else in evolution, sexual selection is relevant, too.
That may be true of hair, but in the case of eye colour, pigmentation serves the same *purpose *as skin pigmentation, it isn’t just a side effect of it. As such it is very highly selected for.