I’ve wondered about this before, but was reminded of it after seeing Nora O’Donnell on the nightly news. For her, and many others, crystal blue eyes are a strikingly attractive feature. But do they serve another purpose?
That eye color seems so out of keeping with the rest of color schemes on the outside of the human body.
Inform me.
This could be the answer right here. It’s like asking why peacocks have such colorful tails.
In short, it attracts members of the opposite sex and contributes to reproductive success.
There’s nothing adaptive about the blue eye trait. It’s a secondary effect of genes that code for lighter skin color. Lighter skin color also isn’t adaptive to anything. It’s just what happens in climates where organisms don’t need the biological expense of producing so much pigmentation.
People have any number of reasons for being attracted to different traits, but for blue eyes, it’s just because they are rare in some places, and people like novelty. It’s not just blue. I have violet-gold eyes… it’s just a fancy type of brown, but it draws a lot of notice.
I always thought it was adaptive to making vitamin D in the far northern places. But I agree on the secondary effect idea.
That is my understanding, as well. As noted in this abstract of a scholarly article:
Blue eyes are caused by a defective gene, called the OCA2 gene. This gene controls the production of melanin, the pigment that causes brown eyes to be brown and also that causes our skin and hair to be brown.
If the OCA2 gene is completely borked, you’re an albino (well, technically one type of albino - there are several types involving several different genes, but that’s not worth going into here). With a completely defective OCA2 gene, your body won’t produce any melanin. You’ll have blue eyes (possibly appearing pink in certain lighting conditions), white hair, very white skin, and all sorts of vision problems. It turns out that melanin is important to the formation of your eyes, and without melanin the retinas and the optic nerves don’t develop properly. For reasons that I personally also don’t understand, albinism also results in problems with the eye muscles, leading to rapid uncontrolled eye movements, weak eye muscles, and both eyes not always facing the same direction, all of which contribute to even worse overall vision.
People with blue eyes don’t have an OCA2 gene that is completely whacked out, but our OCA2 gene doesn’t work the way that it should. Since this defective OCA2 doesn’t allow the body to fully produce melanin, those of us who have this blue eye defect end up with lighter skin and blue eyes. We don’t however end up with all of the vision problems that albinos have with their completely futzed version of OCA2.
Your skin color doesn’t just depend on the OCA2 gene. There are other genes involved, such as the SLC24A5 gene (that I had to look up because I can remember OCA2 but I can’t remember SLC24A5 or any of the other related genes…). But overall, lighter skin color rapidly spread through the population of northern Europe, for reasons that aren’t fully understood. The leading theory is that lighter skin allows better absorption of sunlight, leading to better vitamin D production in lighter skinned people. Or to put it more simply, us light-skinned folks end up being healthier in places like northern Europe where the weather sucks.
Blue eyes are often seen as attractive, so there may be some sexual selection to it as well, but mostly it’s thought to be the lighter skin and vitamin D that made the OCA2 defect spread so quickly through northern Europe.
The vitamin D theory also helps to explain why lighter skin developed at least twice, possibly more. Asians with lighter skin don’t have the same genetic defects as Europeans, which proves that light skin at least evolved separately in Europe and Asia. There is more diversity in the Asian genes that cause lighter skin, so lighter skin might have actually evolved more than once in northern Asia. What is important to the vitamin D theory is that light skin in Asia developed in northern Asia, so again we’re back to vitamin D making lighter skinned people healthier in places where the weather sucks.
Seems like a good theory to me.
I wish to quibble that if a trait provides an advantage, even a small one, it is arguably incorrect to call it a “defect”. Better terns would be “variant” or “allele”.
That said, blue eyes don’t have to be adaptive - they just need to cause so little harm that they are never eliminated from the collective gene pool. It could be a combination of founder effect and/or genetic drift in populations that later greatly expanded. Some traits are adaptively neutral.
It’s also worth noting that blue eyes aren’t actually blue. There’s no blue pigment in blue eyes.
Your eye’s iris has two layers. When light hits these two layers, it gets scattered in a process that is similar to how sunlight gets scattered by our atmosphere. And, similar to our atmosphere, the scattered light from our eyes looks more blue, just like the sky looks blue. If you want to get nitpicky, the type of scattering in the sky is called Raleigh Scattering and the type of scattering in our eyes is called Tyndall Scattering (hope I got those names right - this is from memory). They are similar but not quite the same.
People with brown, green, and blue eyes all scatter the light in the same way.
People with brown eyes also reflect back brown and black from the melanin in the outer layer of their iris, so their eyes look brown even though there is technically some blue scattering mixed in with the brown. You just see the brown a lot more than the blue, so the eyes look brown.
People with green eyes also have melanin in their eyes, and that melanin is also brown and black, but their eyes look green because they have less melanin and that smaller amount of brown reflection mixes in with the blue scattered light and makes their eyes look green.
People with blue eyes have no melanin in their eyes, so all you get is the blue scattering.
So as far as color schemes go, there is no blue or green pigment in eyes. It’s all just varying levels of brown plus some light tricks caused by how the eye is structured.
Fair point.
There’s an interesting article on the subject:
Primarily …
It basically debunks the vitamin D hypothesis, but raises the “Paul Newman effect:”
the idea that they were more sexually attractive than brown eyes
Interestingly, they also offer:
There’s also the idea that blue eyes were advantageous because they perceive stationary objects better than moving things. This could have been an advantage to hunter gatherer women who needed to identify and collect plant foods — indeed blue eyes may even have evolved in women first.
and …
But Sturm has another idea. He says blue eyes have been linked to people coping better with seasonal affective disorder, a major depressive illness that occurs when there are long periods of low light.
Notably, he says, the eye has special neurones in the retina that can detect blue light and use this to help regulate circadian rhythms.
“Perhaps those with blue eyes may have been able to withstand the dark, depressing days of the Neolithic European winters better than those with brown eye colour?”
“They may have been actually active enough to go out hunting while all the rest were sitting in the cave depressed.”
[My uncle always referred to blue-eyed people as being ‘a quart low’]
Makes sense, thank you.
Not necessarily. Cheddar Man, who lived in Britain 10,000 years ago, had blue eyes, dark hair snd dark skin. The eye colour and skin colour are not necessarily linked.
Previous thread on topic:
10000 years! Must be an Xtra-Old Cheddar.
Extra mature, my dear chap.
Since when is a structural color not a color, i.e. a perceptual visual property?
The cite basically debunks the vitamin D hypothesis with respect to eye color, but doesn’t say a word about skin color. It argues, on the basis of DNA analysis that there were blue eyes before light skin.
I, for example, have blue eyes, (used to have) nearly black hair, and relatively dark skin. My maternal grandfather was a blue-eyed blond. Paternal grandmother and father had blue eyes. All of us Ashkenazi Jews, FWIW.
That probably explains why my eyes change colors.
I have what are usually considered “blue” eyes (that’s what is on my driver’s license). I’ve had multiple times when a person has met me, and then looked with confusion at my eyes because they’re trying to tell what color they are. Most people conclude that they’re blue, and I self-identify as having blue eyes. But sometimes they look green or grey.
It can depend on the light, on what colors I am wearing, and (weirdly) what mood I’m in (my guess is that it has something to do with dilation of the pupil).
If indeed my eyes aren’t actually blue, green, or grey, but the color is a trick of the light, then that explains why the color is different under different circumstances. Because really, it’s none of them.
I have blue eyes. Mrs Piper says she can tell when I’m tired because my eyes look grey.
Not specific to eyes but: evolution doesn’t need traits to be adaptive so much as not maladaptive. Some traits just are, they don’t affect your ability to breed much. But of course that doesn’t discount sexual selection etc. providing an advantage.
That explains my eyes as well. I didn’t realize this is something that happens to others and I thought that it couldn’t possibly be right, but it looks like this is actually common.
I have greenish / blueish / light grayish eyes that aren’t really clearly any of these colors.
Living in Asia, they don’t see the fine distinction so often they say my eyes are blue, but they aren’t bright blue.