Evolution and eye colour

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Evolution and eye colour

#1  Postby RayClarke » Jun 02, 2013 3:06 pm

One thing that has puzzled me regarding evolution is eye color and its purpose.

I might be wrong but i don't think there are any species on earth with more different eye color than those of humans.

But WHY what exact evolutionary purpose does eye color serve? Is it a left over from some of our earlier ancestors and if that is the chase what purpose does it serve?

I could imagine that it might serve as a kind of reflector for some of the sun so that our eyes might not get over exposed, but that doesn,´t explain the myriad of colors that our eye has.

So exactly what purpose does our eye color serve?
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Re: Evolution and eye colour

#2  Postby DavidMcC » Jun 02, 2013 3:19 pm

RayClarke wrote:One thing that has puzzled me regarding evolution is eye color and its purpose.

I might be wrong but i don't think there are any species on earth with more different eye color than those of humans.

But WHY what exact evolutionary purpose does eye color serve? Is it a left over from some of our earlier ancestors and if that is the chase what purpose does it serve?

I could imagine that it might serve as a kind of reflector for some of the sun so that our eyes might not get over exposed, but that doesn,´t explain the myriad of colors that our eye has.

So exactly what purpose does our eye color serve?

It may be to do with sexual selection, in humans, at least. The oldest eye colour in humans is probably brown or black (= high melanin content, for good UV protection). Reduced melanin levels were enabled when we migrated away from the sunny, tropical climes, and this probably lead to the various colours we have now. BTW, the eye colour we see is actually a finely-divided pattern of many different colours, in a complex pattern, as illustrated in the image in the Wiki link.
http://en.wikipedia.org/wiki/Eye_color
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Re: Evolution and eye colour

#3  Postby laklak » Jun 02, 2013 3:20 pm

There is some statistical evidence that those with darker eye colors are more likely to suffer from SAD (seasonal affective disorder) than those with lighter eyes. Also, there is some evidence that the genes responsible for eye color are linked in some way to those controlling skin color, but the short answer is no one really knows, yet, what selection pressures were active. I'd imagine others here have more information on it.

http://www.wonderquest.com/eye-color-evolution.htm
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Re: Evolution and eye colour

#4  Postby DavidMcC » Jun 02, 2013 3:48 pm

laklak wrote:There is some statistical evidence that those with darker eye colors are more likely to suffer from SAD (seasonal affective disorder) than those with lighter eyes. Also, there is some evidence that the genes responsible for eye color are linked in some way to those controlling skin color, but the short answer is no one really knows, yet, what selection pressures were active.

...

True, but it is not unreasonable to suppose that Africans have dark irises for the same reason that they have dark skin and dark hair - melanin absorbs UV more effectively when there is more of it, and more of it means dark brown.
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Re: Evolution and eye colour

#5  Postby Calilasseia » Jun 11, 2013 4:16 pm

Humans aren't the only animals to exhibit eye colour variation. Here's a Koala with brown eyes:

Image

Here's a blue-eyed Koala:

Image

Likewise, there are cat breeds exhibiting different eye colours. The most famous of these is the Van Cat, which frequently exhibits different eye colours in the same individual. Here's a particularly spectacular example:

Image

In the case of humans, a good explanation of eye colour can be found here.
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Re: Evolution and eye colour

#6  Postby CdesignProponentsist » Jun 11, 2013 4:42 pm

Variation in eye color is not strictly human as demonstrated here already. And I think Laklak is probably closest to the correct answer.
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Re: Evolution and eye colour

#7  Postby orpheus » Jun 11, 2013 4:53 pm

laklak wrote:There is some statistical evidence that those with darker eye colors are more likely to suffer from SAD (seasonal affective disorder) than those with lighter eyes. Also, there is some evidence that the genes responsible for eye color are linked in some way to those controlling skin color, but the short answer is no one really knows, yet, what selection pressures were active. I'd imagine others here have more information on it.

http://www.wonderquest.com/eye-color-evolution.htm


That's really interesting, lak, about SAD. So there may be some truth to the poetic cliché of "sad brown eyes".

I wonder if there is a direct cause responsible for both, or whether it's that certain lines of ancestry that tend to have brown eyes have settled in certain places and that something about those places and the life there brings about a higher incidence of SAD. (It might not even be the seasons that are the direct cause.)

It's something I'm interested in because a minor component of my bipolar depression seems to be related to seasons. Curiously for me, it's reversed: I feel much worse during summer, and I hate heat and bright sunlight. I have brown eyes; I have no idea if that means anything or not.
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Re: Evolution and eye colour

#8  Postby Delvo » Jun 12, 2013 3:48 am

The variability of human eye colors expanded a lot with one drastic event within the last 10 millennia. Although there are at least a half-dozen genes that affect eye color, the one with the biggest effect is HERC2, the lightening allele of which is single-handedly responsible for most of the blues and greens out there. (I believe it's the only one of the eye-color genes with a drastic enough effect on its own to make eyes blue/green instead of brown/black regardless of which allele is present for any of the other relevant genes, although the others will affect the exact shade and probably could add up to blue/green on their own if they occurred in the right combination of alleles.)

10 millennia is a lot more recent than most of the major human migration events that established populations in different parts of the world, so the allele is still mostly found in the descendants of the population in which it originated, decreasing in frequency the farther you get away from the Baltic Sea. Whether the "new" mutant allele could be useful in farther-away parts of the world or not, it just hasn't had the chance.

I've heard that lighter-eyed people are more irritated by bright light and prone to being blinded by bright light including afternoon sunlight, but but also have better night vision. The sources I've heard this from were not scientific.

Mine have a blue background and enough of a starburst pattern of amber specks around the pupils to make them seem green on average, at least under some light conditions (blue under others), and I hate bright sunlight and love cloudy weather and the deep shade of a thick leafy forest... but I have reason to believe my preferences are based less on the physical nature of my eyes and more on the experiences stored in my brain.
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Re: Evolution and eye colour

#9  Postby DavidMcC » Jun 12, 2013 1:12 pm

Thanks for that, Delvo.
Delvo wrote:...
I've heard that lighter-eyed people are more irritated by bright light and prone to being blinded by bright light including afternoon sunlight, but but also have better night vision. The sources I've heard this from were not scientific.

Well, it is known that albinos (who have little or no melanin in their eyes, and often little elsewhere either), suffer from a semitransparent iris, letting light through to the retina that has not been through the pupil. Perhaps this effect can occur even without total loss of melanin. It is worth noting that melanin loss is not necessarily the same in the skin and hair as in the eyes - it can be ocular only:
http://en.wikipedia.org/wiki/Albinism#Signs_and_symptoms
In humans, there are two principal types of albinism, oculocutaneous, affecting the eyes, skin and hair, and ocular affecting the eyes only.
In other words, it is possible that some lighter-eyed people are partial ocular albinos.
In humans, there are two principal types of albinism, oculocutaneous, affecting the eyes, skin and hair, and ocular affecting the eyes only.
Most oculocutaenous albinistic humans appear white or very pale as the melanin pigments responsible for brown, black, and some yellow colorations are not present. Ocular albinism results in pale blue eyes, and may require genetic testing to diagnose.
...
The human eye normally produces enough pigment to color the iris blue, green or brown and lend opacity to the eye. However, there are cases in which the eyes of an albinistic person appear red, pink or purple, depending on the amount of pigment present, due to the red of retina being visible through the iris. Lack of pigment in the eyes also results in problems with vision, both related and unrelated to photosensitivity.
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Re: Evolution and eye colour

#10  Postby MathieuT » Jul 25, 2013 2:32 am

It is hard to accept the idea that simply because there is no more selective pressure, a fair amount of a population will turn light eyed within 10 000 thousand years.

I assume night vision is the key.
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Re: Evolution and eye colour

#11  Postby Macdoc » Jul 25, 2013 2:36 am

I've heard that lighter-eyed people are more irritated by bright light and prone to being blinded by bright light including afternoon sunlight, but but also have better night vision. The sources I've heard this from were not scientific.


That is counter intuitive as Nordic people with light blue eyes and light skin dominate the north ( tho not the far north ).
I would tend to think they might be less prone to snow blindness.

That said albinos certainly have bright light and eye issues but that may be unrelated.
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Re: Evolution and eye colour

#12  Postby Oeditor » Jul 28, 2013 2:30 pm

It doesn't seem long since PZ was treating us to an account of the genetics of eye colour in... fruit flies. So it isn't even restricted to vertebrates.
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Re: Evolution and eye colour

#13  Postby DavidMcC » Jul 28, 2013 2:39 pm

Macdoc wrote:
I've heard that lighter-eyed people are more irritated by bright light and prone to being blinded by bright light including afternoon sunlight, but but also have better night vision. The sources I've heard this from were not scientific.


That is counter intuitive as Nordic people with light blue eyes and light skin dominate the north ( tho not the far north ).
I would tend to think they might be less prone to snow blindness.

That said albinos certainly have bright light and eye issues but that may be unrelated.

AFAIK, snow is not a permanent feature of most northern areas inhabited by pale-eyed people, Macdoc.
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Re: Evolution and eye colour

#14  Postby DavidMcC » Jul 28, 2013 2:52 pm

Macdoc wrote:
I've heard that lighter-eyed people are more irritated by bright light and prone to being blinded by bright light including afternoon sunlight, but but also have better night vision. The sources I've heard this from were not scientific.


That is counter intuitive as Nordic people with light blue eyes and light skin dominate the north ( tho not the far north ).
I would tend to think they might be less prone to snow blindness.

That said albinos certainly have bright light and eye issues but that may be unrelated.

You may be thinking of full albinism - pink eyes and white hair, etc. I am thinking of partial albinism, as linked before.
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Re: Evolution and eye colour

#15  Postby CdesignProponentsist » Jul 28, 2013 8:47 pm

DavidMcC wrote:
laklak wrote:There is some statistical evidence that those with darker eye colors are more likely to suffer from SAD (seasonal affective disorder) than those with lighter eyes. Also, there is some evidence that the genes responsible for eye color are linked in some way to those controlling skin color, but the short answer is no one really knows, yet, what selection pressures were active.

...

True, but it is not unreasonable to suppose that Africans have dark irises for the same reason that they have dark skin and dark hair - melanin absorbs UV more effectively when there is more of it, and more of it means dark brown.


I think skin color is a result of the evolutionary regulation of Vitamin D production, if I'm not mistaken.
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Re: Evolution and eye colour

#16  Postby hackenslash » Jul 28, 2013 11:22 pm

RayClarke wrote:One thing that has puzzled me regarding evolution is eye color and its purpose.

I might be wrong but i don't think there are any species on earth with more different eye color than those of humans.

But WHY what exact evolutionary purpose does eye color serve? Is it a left over from some of our earlier ancestors and if that is the chase what purpose does it serve?

I could imagine that it might serve as a kind of reflector for some of the sun so that our eyes might not get over exposed, but that doesn,´t explain the myriad of colors that our eye has.

So exactly what purpose does our eye color serve?


My response will be different than most, but more in line with what evolutionary theory says, in some respects.

It doesn't serve any real purpose, AFAIK, nor does it actually have to. In line with Kimura's theory of neutral evolution, and later work by Ohta, who gave us the nearly neutral theory, significantly extending Kimura's excellent work, we can just point out that eye colour is invisible, or nearly invisible, to selection. It's a case of one allele being just as good as another. In this case, it's passed on because the parent carried it. I dare say that there is some correlation between eye colour and environment, but it will be a weak correlation.

Not all things are visible to selection to the degree that an explanation is necessary.

I should also point out that 'purpose' is pretty much entirely misplaced here, as it implies teleology.
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Re: Evolution and eye colour

#17  Postby Macdoc » Jul 28, 2013 11:37 pm

AFAIK, snow is not a permanent feature of most northern areas inhabited by pale-eyed people, Macdoc.


who said anything about it being permanent as a necessary factor in selective pressure.
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Re: Evolution and eye colour

#18  Postby DavidMcC » Jul 29, 2013 12:08 pm

Macdoc wrote:
AFAIK, snow is not a permanent feature of most northern areas inhabited by pale-eyed people, Macdoc.


who said anything about it being permanent as a necessary factor in selective pressure.

I meant "permanent" enough to be an evolved trait, not "lasting forever".
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