Evolutionary purpose of eye color

[Grimlock]

One thing that has puzzled me in some time ever since i got a greater understanding of 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?

 

[ImprobableJoe]

It might not have any particular reason at all.

 

[GoodKat]

Perhaps the same as peacock feathers.

 

[Ozymandyus]

Perhaps the same as peacock feathers.

Naw, peacock feathers are a result of sexual selection. Eye color variation is just a few gene variants that have no discernable effect on survival. Non-harmful mutations and genetic drift. Not everything has a purpose.

 

[Spase]

Wikipedia has a pretty good article on eye color.

I'm not sure what the advantage of having light eyes is. It's in large part the result of less melanin (same stuff as give you darker skin). Light eyes do have higher rates of eye problems as you age including eye cancer (melanoma).

Based on some of the stuff in the article my best guess would be that light eyes do okay places where there's less strong light or where individuals spent less time outside in the light which is why the phenotype is relatively common in Europe and not at all in Africa... there doesn't seem like there's any reason for strong selection either way though.

It *may* be that light eyes allow a very small amount more light in than darker eyes but there's not direct evidence for this that I know of.

 

[Aught3]

It's in large part the result of less melanin (same stuff as give you darker skin).

Yeah, this was the only thing I could think of, producing less melanin makes sense in an environment with less sunlight. So blue eyes might just be 'linked' to lighter-coloured skin.

 

[GoodKat]

So blue eyes might just be 'linked' to lighter-coloured skin.

Just add in blonde hair and you have the perfect human... until solar intensity gets too high :lol:

 

[Aught3]

Well lighter shades of hair also go along with the less melanin idea.
Maybe albinos are the perfect humans after all.

Albi-YES, I mean :lol: .

 

[ebbixx]

It *may* be that light eyes allow a very small amount more light in than darker eyes but there's not direct evidence for this that I know of.

Since as you point out it's most likely related chiefly to melanin production, and that is related to things like Vitamin D uptake, perhaps any marginal ill effects of the underlying genes (succeptibility to cancer, for instance, which has almost no impact on reproduction, given that few skin/eye cancers emerge until an individual is past their reproductive prime, and in fact past the age at which ancient humans would have in all probability been dead) -- perhaps such effects are entirely irrelevant to natural selection and therefore about as important as the specific distribution patterns of freckles or minor individual variations in the coats and feathers of various other species.

There is likely an underlying selection issue (probably related to sun exposure needed to process/synthesize adequate amounts of Vitamin D and possibly other benefits) that may be at work in the divergence in geographically different populations, but is incidental (but causal) to the nature of variations in eye color.

Still, the map at Wikipedia is interesting in this regard, especially since the distribution is NOT entirely based on latitude -- though perhaps it does reflect areas of greater and lesser genetic mixing with migrant populations?

Another factor though, might just be that very few animals seem to have the prominent irises that humans do. Most mammals whose eyes I think of have eyes that bear little resemblance to those of humans. There could be a structural difference that also accidentally leads to such a noticeable trait. I'm also unpersuaded that there is no element of sexual selection involved, since there are certainly people (and categories of porn) devoted to just about any trait one can imagine, albeit, not all of them are genetic traits.

 

[BrentonEccles]

So your location (environment), through evolution over time, affects things like eye color variant?

 

[Aught3]

So your location (environment), through evolution over time, affects things

Yes.

like eye color variant

Maybe, that's what we're talking about. It certainly has for skin colour, there's just not an obvious causal link for eye colour (that we can come up with). Though melanin seems to be a common factor.

 

[Canto]

Argh, I had this long question written up, then realized I was making what is probably a fatal error in my question. Is eye color strictly human based? I was trying to figure out why humans would need different colored eyes, and realized that I was forgetting to think about all the creatures that led up to humans and why they might need different colors.

I can see a logical reason for eyecolor to matter to humans(group dynamics, family structure, recognition of outsiders in tribal/small group lifestyle), but I cant see a logical reason for the color differences to be there in the first place.

 

[PuppetXeno]

Naw, peacock feathers are a result of sexual selection. Eye color variation is just a few gene variants that have no discernable effect on survival. Non-harmful mutations and genetic drift. Not everything has a purpose.

Sexual selection may very well be involved in early european tribes and their eye color and getting a lighter skin. I don't see how that should not be the case.

I found this article which is funny.

 

[williamcardno]

PREFACE:
I am not a biologist, I am an amateur philosopher. This is nothing more than my speculation on the issue, but I feel I made a good case. If there is anything factually incorrect about this, I encourage you to please point it out and demonstrate it to me so I can be all the wiser. I also encourage you to not just take my word as fact but as a subjective understanding which should be scrutinized and studied for yourself. I'm trying to point you in a direction and give you my conclusion, not postulate a definitive argument.

The first thing that needs to be established about eye colour is that it is a polygenic trait within a species. Since more than one gene controls it, this means that it doesn't directly affect the next generation or get passed on in the same way other characteristics might.
Now we step a little bit into the realm of social philosophy and observe how we interact and why, as well as subjective understanding of colours and our interactions which lead us to that conclusion.
At first we can look at this from a categorical standpoint. That is to say that if everyone or most people i know with blue eyes screw me over, unconsciously I am going to perceive blue eyes as a trait that denotes an untrustworthy individual. Same can be said of any eye colour or hair colour, really. This is why I think certain people actually believe they have ESP, because they are operating on this visceral and emotional reaction without thinking about why they could have these reactions. Things like "I have a bad feeling about this person that I cannot explain" get explained when we think about these kinds of interactions.
Also, based on my understanding of Evolution - it's about who can pass on their genetic material, so attractiveness does play a part here. On a broad scale, different eye colours play to different characteristics of people's taste. I don't mean they give us a satisfactory picture of the individual's actual character - but more that they emphasize certain broad things we all share.
For example - blue eyes are cold and striking, usually grabbing the attention of a potential mate quickly. If you play any tabletop RPGs, you can think of this as a CHARISMA enhancement.
Brown eyes tend to accentuate aspects of the person which beg for sympathy. Someone who is sad and has brown eyes appears just that much sadder. The "puppy dog" look is easier and works to optimal effect here where as someone with green or blue eyes would have to work harder to achieve the same effect.
Darker correlates with deeper and usually causes the other person to want to examine the individual more because there's such a perceived depth.

This is all a long way of saying "different strokes for different folks" but gives eye colour a purpose in evolutionary terms and in social terms, as the two often go hand in hand. Just like people look for different and distinct traits in what makes someone attractive, the various eye colours seem to provide instinctual visual indicators that could affect your first impression on a mate.

 

[Aught3]

I find the sexual selection role quite hard to credit for the evolution of eye colour. Sexual selection appears to have had a role in some human traits, fat distribution for example. What gets selected is usually some signal about the health or ability of animal. Slightly more expressive or attractive eyes seems very subjective and doesn't confer much in the way of information.

 

[williamcardno]

I find the sexual selection role quite hard to credit for the evolution of eye colour. Sexual selection appears to have had a role in some human traits, fat distribution for example. What gets selected is usually some signal about the health or ability of animal. Slightly more expressive or attractive eyes seems very subjective and doesn't confer much in the way of information.

This is a good point, but an incomplete one and from a perspective outside the context of the reply I was giving. The topic isn't how eye colour evolves, but the role it plays in our evolution. I addressed that it's polygenic earlier which means that it won't have a direct impact on the next generation. This is why two parents can have a child with different eye colour than the parents. So it's out of the realm of achieving eye colour by natural selection - so what role does eye colour play in evolution if not that one? Sexual selection is the only answer I have been able to come up with given that it is a quality of attraction and little more. Like the feathers a peacock has.

Also keep in mind that heathy features only get selected like that when they prolong the ability to pass on genetic information. If something is appealing, yet detrimental, the creature will die long before the healthy one and this will stunt the amount of genetic information being passed on.
Something benign like eye colour, which doesn't correlate with health as far as I'm aware, can still be selected in this manner but its only pressure is that of aesthetic appeal and psychological associations. Just because it doesn't affect health doesn't mean that it can't be explained by sexual selection... it merely means that it has no affect on the health of the creature.

Essentially - I have brown eyes so that I can attract those drawn to brown eyes. Much in the same way your hair colour serves to attract people without having any true affect on your physical health or reproductive capabilities.

 

[PuppetXeno]

I find the sexual selection role quite hard to credit for the evolution of eye colour. Sexual selection appears to have had a role in some human traits, fat distribution for example. What gets selected is usually some signal about the health or ability of animal. Slightly more expressive or attractive eyes seems very subjective and doesn't confer much in the way of information.

Not so much the evolution of the colour, but as I recall, blue eyes is a recessive trait. You need to inherit the blue-eye gene anomaly from both parents to have blue eyes. This blue-eye gene anomaly would be a mutation, which goes unnoticed at first (not expressed with only one parent providing the anomaly), so does get spread throughout a population until children are born who inherit the blue-eye genes from both parents, and end up with blue eyes.

This could be seen as a novel trait, and make the individual more interesting to the other sex. So it is selected for (however brown eyes are not selected against), eventually becoming more prevalent in some tribes because it could be classified as a standard of exceptional beauty, or even attributed to some supernatural power (green eyes).

Further, you know blonde jokes - just a fraction of the biases around blondes. Somewhere there's an association between "blonde" and "easy" aswell. There is certainly an active mechanism "promoting" blondes as sexual partners, low melanin (pigment) levels in hair, skin, eyes are quite the european phenotype.

In all, not all selection has to do with actual usefulness, there is esthetic and novelty value aswell. Atleast in humans...

 

[Spase]

[...] The topic isn't how eye colour evolves, but the role it plays in our evolution. I addressed that it's polygenic earlier which means that it won't have a direct impact on the next generation. This is why two parents can have a child with different eye colour than the parents. So it's out of the realm of achieving eye colour by natural selection - so what role does eye colour play in evolution if not that one? [...]

Just because a trait doesn't pass directly to your child as a dominant trait doesn't make it fall outside of the range of natural selection. In the case of individuals that are homozygous mutants for the trait (have blue eyes), even if none of their kids have blue eyes, that individual reproducing more because of their eye color has a direct impact on the allele frequency in the population. The trait being polygenic should be no different in that respect.

I may be misunderstanding you... and it's possible I'm wrong in the case of polygenic traits but I don't think so.

It's possible there could be sexual selection even if there isn't an apparent advantage to having blue eyes. One thing humans seem to be programmed to breed for is genetic diversity in a population. Don't marry your sister, and better yet, go find a mate that's genetically different from the whole tribe. Blue eyes may have served as a strong visual indicator of genetic difference. Things that are exotic are often considered attractive and that may have been the allure of blue eyes at least at first.

It still doesn't explain the very high prevalence in certain parts of Europe since at some point these stopped being exotic and became the norm but it's an idea of why they would be attractive, especially initially. It also explains why they would be spread once they appeared to neighboring tribes or groups which would be attracted to this genetic marker for difference.

 

[ebbixx]

I can see a logical reason for eyecolor to matter to humans(group dynamics, family structure, recognition of outsiders in tribal/small group lifestyle), but I cant see a logical reason for the color differences to be there in the first place.

The primates we're related to were largely limited in geographic scope to Equatorial Africa, where melanin production (or more accurately its rate of breakdown) needed to be optimized locally to balance between the need for Vitamin D, vs. the risks of melanoma, sunburn etc. Particularly since none of those species appear to have developed tools necessary to become fashion designers, and do not build huts or underground bunkers (to the best of my knowledge).

I expect we inherited a very similar complex of genes regulating melanin production and breakdown and that the effect on eye color was only "uncovered" once humans began to migrate elsewhere and there were enough generations of humans at various latitudes, staying fixed in one general location (and away from a fish-rich diet) for long enough for the traits to express themselves as they have.

See the Wiki articles on eye color and skin color for more details. And if you find inaccuracies be sure to fix them.

I'm sure it's more complicated than that, but this seems to cover most of the generally accepted data and deduction, at least what's easily available.

In other words, if bonobos (or another closely related species of ape) could have managed to establish niches all over the world (or at least at more extreme latitudes), they too would probably have developed a wider range of iris colors. Though I'm not sure about the difference in proportions between simian and human eyes. That could have been a development strictly on our own branch.

 

[Aught3]

Wow, this has turned into a great topic Grimlock, eKudos

I addressed that it's polygenic earlier which means that it won't have a direct impact on the next generation. This is why two parents can have a child with different eye colour than the parents. So it's out of the realm of achieving eye colour by natural selection

Like Spase, I'm a little worried about the direction you're going here. I gave you the benefit of doubt on your last post when you addressed polygenic traits but this one seems to go too far. Most traits are polygenic and can still be influenced by natural selection, skin colour is a good example. Please explain further if I am misunderstanding you.

The topic isn't how eye colour evolves, but the role it plays in our evolution.

I don't really see the difference here unless you thought I was saying eye colour has no role in our evolution. Ozymandyus gave a good answer to this effect; non-harmful mutations plus genetic drift. If pressed I would say my answer addresses the second category, I would like to understand the role eye colour has in our evolution.

Also keep in mind that heathy features only get selected like that when they prolong the ability to pass on genetic information. If something is appealing, yet detrimental, the creature will die long before the healthy one and this will stunt the amount of genetic information being passed on.

This is what makes sexual selection so tricky to think about. You're right in thinking that the trait cannot be so detrimental that the organism will die before it has the opportunity to reproduce. However, if both organisms make it to adulthood then the one with the 'healthy signal' will be more successful, leaving more offspring and genes, than the one without it. The example of peacocks is a good one, tails feathers aren't fully grown until late in life and by then the peacock is able to escape from predators. When it comes to mating time the peahens choose the males with the 'sexiest' tail feathers. Long tail feathers are detrimental to the peacock's survival but pay off during reproduction and so the genes are passed on.

I found this interesting article talking about the cooperative eye hypothesis. Basically, it says early human communication was facilitated by brightly coloured eyes, because they are easier to follow and maintain eye contact with. Compare with dull ape-like eyes. Intriguing, any comments?