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In a world I am building, a race of humans exists on a tropical archipelago with a very hot climate. Their skin, naturally, is very dark, but I want to have their eyes [most, if not all of their eyes] be light grey. Is this possible?

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  • $\begingroup$ Heat is not the cause of dark skin. Strength & harshness of light, how much U.V. it has, etc., are things that factor into the evolution of eyes. $\endgroup$ – Tim Jan 26 '17 at 0:26
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    $\begingroup$ @Tim The place I speak of is pretty much directly at the equator of my world. $\endgroup$ – CHEESE Jan 26 '17 at 0:48
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    $\begingroup$ Perhaps they evolve a protective pigment for the iris that’s reflective rather than dark, but still keeps light from damaging the tissue. $\endgroup$ – JDługosz Jan 26 '17 at 21:08
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Yes. Blue eyes are actually made by a lack of melanin in the iris and other colours a variation on this. (Blue is caused by the refraction of light around the iris, same reason the sky is blue).

We do, in fact get black people with blue eyes in humans. They are a minority but still do exist.

So yes, grey eyes are also possible for your humans.

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  • $\begingroup$ This may be a misunderstanding, but I am asking about the entire race, or at least a majority, having light eyes, not just a few. $\endgroup$ – CHEESE Jan 26 '17 at 0:07
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    $\begingroup$ If it occurs in a minority then it is possible in an entire race. It is the same for any variation. Take ginger hair as an example, I would suppose that only occurs in a minority in most places but it isn't so unusual in Ireland. If you start off with a group of people with the necessary genes then all their offspring will have it too. $\endgroup$ – Lio Elbammalf Jan 26 '17 at 0:16
  • $\begingroup$ The link you have explains the phenomenon by a disease and interbreeding (which is not possible here). $\endgroup$ – CHEESE Jan 26 '17 at 0:22
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    $\begingroup$ My point was merely that the two do not contradict each other and so yes, it is possible. Your race could have been a result of interbreeding thousands of years ago, they made the trek across a frozen sea during an ice-age to their new home land. The others could have all been wiped out in a flood when the ice-age ended if you don't wish there to be any other races. Whatever you wish: In your story you can make up the reason. You only asked if it was possible. $\endgroup$ – Lio Elbammalf Jan 26 '17 at 0:32
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    $\begingroup$ @Karl all that's needed is for a "distinct advantage" is (a) some subpopulation happening to have that genetic variation, (b) a coincidence so that this subpopulation manages to gain excessive political power at some point of time (a decade would be sufficient), and (c) an internal conflict with ethnic cleansing. In many times of our history, there have been strong "selective pressures" for or against a random feature that just happens to mostly correlate with belonging to a certain "tribe". $\endgroup$ – Peteris Jan 26 '17 at 23:39
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Very dark skin with very light eyes is maybe genetically possible but extremely unlikely.

Brown eyes have dark melanin. Blue-eyed people lack this melanin and their eye tissue is colorless (the blue is a result of light scattering, same reason the sky looks blue)…. It's not a coincidence that people who lack melanin in their eyes would also lack it in their skin, hence blue eyes are most common in Northern Europe among very light skinned people.

Grey eyes occur in North Africa, the Middle East, and Asia. According to this website the exact genes are unknown but probably a similar combo to the blue-eye genes. Again these are not "very dark" people. Think of the famous "Afghan Girl" from the magazine cover Her eyes are green but very light:

enter image description here

To get around this, you'd need a genetic combo that codes for lots of melanin, but with a defect so the melanin is blocked locally in the cells of the eyes. It's possible, but then you'd need to replicate that gene defect through the entire island population.

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  • $\begingroup$ But what would cause the melanin to be blocked from the eyes? Just a genetic mutation, or something having to do with habitat/lifestyle? $\endgroup$ – CHEESE Jan 26 '17 at 0:49
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One way to achieve this is by selective breeding. It could be that evolution resulted in your people having dark eyes, but then one small group had a mutated gene which gave them grey eyes. For some reason this gene tends to be dominant.

Over time (thousands of years), people preferred grey eyes, and tended to choose grey-eyed partners. This is selective breeding, in which male and female are selected (or select each other) in order to produce offspring with certain traits. We do this with animals today.

So just make it that grey eyes are more desirable, and as a result of years of selective breeding, the majority now has grey eyes. Those with dark eyes still occur, but are the minority.

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  • $\begingroup$ Doesn't work, unless the "grey eye" gene was bound to the Y cromosome. And then how would it affect females? Women get pregnant, regardless of eye colour. Not speaking of the chances of this "grey eye" fashion surviving for thousands of years. $\endgroup$ – Karl Jan 26 '17 at 1:07
  • $\begingroup$ @Karl - it's sci-fi/fantasy, right? Could be like, the force, or magic, when people see grey eyes they get all horny. And maybe their genes work differently to ours, because science. Besides, we're not talking overnight change here. We're talking selection over the course of thousands of years, where a mutation (or combination of) can come to take affect. If dark-eyed people breed less, then eventually grey-eyed would be the norm. $\endgroup$ – Tim Jan 26 '17 at 1:47
  • $\begingroup$ The OP said "humans", didn't mention sci-fi or magic. Human procreation is limited by the rate at which females bear children, not what makes individuals horny. I've never heard that beautiful women have more kids on average. $\endgroup$ – Karl Jan 26 '17 at 4:06
  • $\begingroup$ @Karl If you are trying to argue that sexual selection doesn't play a role in humans I don't think most biologists would agree with you. See wikipedia for a general overview, and a paper specifically regarding the origin of eye color $\endgroup$ – Mike Nichols Jan 26 '17 at 20:31
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No, or at least unlikely.

There is no evolutionary advantage in having blue, green or grey eyes. Pale skin, yes, in the north, but eye and hair colour are only a secondary effect of that.

If your islanders are really human, it's improbable. No melanin production in the eye would be the result of a genetic defect. That would usually only proliferate thoughout a whole population if it had some secondary advantage, for example resistance to an endemic infection, even if it was otherwise completely harmless. A dominant genetic variation occuring spontaneously is something rather rare, i believe.

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In a manner of speaking, yes.

Recently, we have found evidence that the blue eye gene stemmed from the random mutation of one individual. Random mutation is the key to the bulk of ethnic geodiversity here. Contrary to popular belief, for example, it was agriculture, not latitude, that created the Caucasian and Mongoloid paleness. If it were latitude, the evidence would have shown us a case of human whitewashing far older than eight thousand years. The reason for the more likely connection between whitewashing and agriculture is that plant foods don't have much of the vitamin D that a dark skin needs.

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