Is there a purpose for our eye color? I know I’ve read questions relating to how some people might see differently with different eye colors but is there any reason we have colored irises?
Which leads me to the next obvious question, what is the correct spelling for more than one iris?
When the topic of why our eyes have color a side topic came up; What are all of the parts of the body that have no apparent purpose.
What I want to know is why do my eyes change colors?
I used to think it was with what I wore, then I thought the light in the room, then the weather… but none of those are consistant.
Rarely they are blue, sometimes they are hazel, I LOVE it when they’re green. And sometimes they’re gray, and sometimes brown. And sometimes a mix of any of those colors…
Why? Is that possible? Am I an alien?
Plural is “iris” is “irides”. (It’s Latin.) The plural of the flower is usually “irises.”
I believe Dave Feldman covered the genetic basis of eye color in one of his books. What I have read about human genetics confirms what I recall that he wrote. What your high school biology teacher taught you (brown eyes are a dominant trait; blue eyes are recessive) is a radical oversimplification. It doesn’t account for green, hazel, of violet eyes, for one thing.
The color of the iris is blue before it’s pigmented with melanin. (Why it should be blue, I’ve never been able to learn. Is this the natural color, or is there another pigment involved?) There are many different gene loci (places on the chromosomes) where melanin can be produced, and in addition there are slightly different genes that produce different colors of melanin (brown, yellowish-brown, and reddish-brown, IIRC). The number of loci that actually produce melanin (these are the dominant genes), the color of the melanin, and the interaction between these genes and regulatory genes (which tell the dominant genes how much melanin to make), all help explain how much melanin is in the iris, and what kind, and hence the color.
violet iris: a blue iris with some red showing from blood vessels.
blue iris: an iris with no melanin, or very little
hazel iris: a blue iris pigmented with a small amount of melanin
green iris: this is supposed to be produced by more melanin than that which produces the hazel iris. I’m not sure this makes a lot of sense to me.
brown eyes: quite a lot of melanin
black eyes: a great deal of melanin.
As for why people have different color eyes, and what evolutionary benefit this might give them, I seem to recall, very vaguely, that people with blue eyes are less likely to suffer snow blindness. Snow blindness is temporary blindness caused by too much light reaching the eye, usually when the sun is reflected off the snow. Why many northern Europeans have blue eyes, and Eskimos don’t, I can’t say. Most newborn babies have blue eyes, and their eyes are more sensitive to sunlight than adults’.
Work is the curse of the drinking classes. (Oscar Wilde)
Waneman: According to a book called Racial Adaptation different coloured eyes have different abilitities to distinguish between colours that are almost the same and to see large distances. I don’t remember which eye colour gave which advantage.
I doubt this advantage is big enough that northern peoples evolved with lighter eyes to make use of one of those abilities over another.
My guess is that light eyes are more likely to be damaged by the sun so they mostly got evolved out. People in hot places don’t have 'em because of the strong sunlight and mongoloids don’t have 'em because they spent a lot of time around snow which reflects a lot of sunlight.
Me, too. My eye color depends on what I’m wearing and a bunch of other things, but I can’t carry a mirror around long enough to discover.
When I’m asked for eye color I have to take one one of my existing IDs and see what I put down before…
“Am I an alien?” I donno, What does the following mean to you?
“Never land alone.” J.Winters.
Brown!? you get brown? no fair. I never get brown. I do sometimes get blue with a yellow ring around the pupil. That’s kinda nice.
One of my favorite things to do if there is a lull in a conversation with a cute girl is to look her in the eyes and ask what color my eyes are. They seem to be most striking when they are green.
Also, what bibliophage said.
I’ve yet to find a good book or article that explains eye color, hair color or skin color since they are all under the control of several or at least more than one gene.
Are you driving with your eyes open or are you using The Force? - A. Foley
Dang! No.TheNerd, never brown. Sorry. Speed reading strikes again.
Are you driving with your eyes open or are you using The Force? - A. Foley
I’m back with more information. Most of the information I gave above comes from my memory of having read Textbook of Human Genetics, third edition, by Max Levitan (copyright 1971, 1977, 1988). He says,
I find this explanation untenable. It’s hard to imagine that such a thin layer of tissue could scatter light so much. Such scattering is appreciable in the sky only when sunlight travels through hundreds or thousands of miles of the atmosphere. A similar scattering occurs in water (making lakes appear blue), but only when the light passes through several meters of water. The blueness of a glass of water, which is much thicker than your iris and cornea combined (and about the same density), is barely perceptible. Furthermore, if an unpigmented iris is supposed to be blue, why does an albino’s iris appear pink and not blue? (Unless perhaps the pinkness of the albino’s iris is related to the lack of melanin in the retina behind it.) A more believable explanation comes from The Physiological Basis of Medical Practice 4th edition, by Charles Herbert Best (copyright 1945):
I haven’t been able to discover exactly what the pigment in the posterior epithelial layer is. Anatomy books call the layer “highly pigmented” or “deeply pigmented” but they annoyingly fail to mention what it’s pigmented with. I suppose it could be melanin that appears to be blue only because the light passes through the stroma where the red end of the spectrum might be absorbed. Or it could be a blue pigment. Does anybody out there know?
When I said most babies had blue eyes, I’m afraid I was being ethnocentric. Dark pigment doesn’t appear in the stroma until a few weeks after birth in white babies. In other races, it is present at birth. It’s interesting to note that in many northern dog breeds (Samoyed, Malamute, Husky), blue eyes are common. The hypothesis about blue eyes being protective against snow blindness is repeated at [Foundation"]http://lucy.ukc.ac.uk/LOCAL-ONLY/FHC/FHCL1495-6.html]Foundation]( [url="http://lucy.ukc.ac.uk/LOCAL-ONLY/FHC/FHCL1495-6.html) of Human Culture - Human Morphological Variation. (A fascinating site, I highly recommend it, though I don’t agree with everything written there). This site also repeats the light-scattering theory of blue eyes that I mentioned above. About green eyes, the site says that truly green eyes are extremely rare and are caused by the presence of the yellow pigment carotene in an iris that would otherwise be blue.
Levitan discusses how two blue-eyed parents can have a brown-eyed child, in contradiction to the oversimplified model that too many of us learned in high-school biology class.
Superfly: As for changing eye color, I don’t know about the iris, but the amount of pigment in the retina changes with changing light conditions. The same could conceivably happen in the iris.
As for the plural of iris (of the eyes, not the flower), “irises” seems to be in common use except in anatomy books.
I mucked up the link. Let’s try again.
bibliophage, There is only one pigment, good old melanin.
Also: “The absence of pigment in the outer part of the iris causes the eye to appear blue for the same reason that an uncloudy sky appears blue: as light is scattered . . .
the blue is scattered the most and reflected back to the eyes of the beholder.”
I’ve heard this more than once.
Also: “Furthermore, if an unpigmented iris is
supposed to be blue, why does an albino’s iris appear pink and not blue? (Unless perhaps the pinkness of the albino’s iris is related to the lack of melanin in the retina behind it.”
I think everyone has some pigment except for albinos.
And note that albinos have no pigment in all three: skin, hair and eyes.
Are you driving with your eyes open or are you using The Force? - A. Foley
Along with a variety of threads on the message board as well.
In the article linked to above, Cecil says, “If your eyes have a lot of [melanin], they’re brown. If they don’t, they’re blue. (Some details of this explanation are in dispute, but don’t worry about that now.” The details are in dispute? Say it isn’t so, Cece. I can’t even agree with myself on this question from minute to minute.
Collier’s Encyclopedia: “Blue eyes are those with black pigment.” I swear I’m not making this up.
Encyclopedia Britannica says that blue eyes are cause by a combination of scattering and “selective absorption.” Now selective absorption is what makes most blue things blue. This is what’s happening in your bluejeans when the indigo dye absorbs the non-blue and reflects the blue. If I didn’t know better, I would suspect the EB is hedging its bets by claiming both processes.
I looked in 4 ophthamological books and one medical text while I was at the university library this morning. Not one considered eye color to be important enough to mention. For some strange reason they seem to think that glaucoma and blindness and such are more important. I did learn from one of these books that the retina contains both melanin and non-melanin pigments, at least in frogs.
I got the answer I’m most comfortable with, not at the big university library (1 million+ volumes) but at the small city library (80k volumes.) It’s right there in Gray’s anatomy, where I should have looked in the first place.
So it seems that Gray’s Anatomy and Physiological Basis of Medical Practice agree. I still don’t know what makes the posterior surface purple. One candidate is rhodopsin, the purple pigment of the rod cells of the retina. It’s supposed to turn yellow on contact with light, though, and the iris doesn’t.
As to why there’s such variation in eye color, it’s probably mostly tied to variation in skin color, which does have known evolutionary advantages in various climates (dark-skinned people have less risk of sunburn/skin cancer, light-skinned people naturally produce more Vitamin D, etc.). The body produces more or less melanin in order to bring the skin to its optimal tone (or at least, what was optimal for one’s ancestors), and as a side effect, other body parts end up getting some of that pigmentation, too-- hence people with dark skin also tend to have dark hair and eyes, and vice versa. Of course, it’s possible for a fair-skinned blonde to have brown eyes, or for an African to have blue, so this isn’t a complete explanation, but I think it’s probably most of it.