Evolutionary purpose of eye color

[ebbixx]

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?

Wouldn't that relate at least as much to the prominent sclera of human eyes as it would to iris color? After all, I'd imagine that prominent sclera also might have been a liability at times, in allowing predators to notice a human child or adult otherwise hidden from view? (this 'graph was written before I'd looked at the paper).

The article cited mentions the light sclera of humans in the first sentence of the abstract, but says nothing there about iris pigmentation per se. Are brown eyes any harder to follow than green ones?

Still it's a fascinating paper. Looking forward to reading it more closely, even if it doesn't wind up bringing up pigmentation in the text.

Here's the only section it seems that mentions the iris by name, largely in relation to comparing its visibility with that of the irises of other animals:

It turns out, as is well-known, that humans indeed do have especially visible eyes (Kobayashi and Kohshima, 1997). Human eyes are colored in a way that helps advertise both their presence and their gaze direction much more saliently than in other primates. Kobayashi and Kohshima (2001) examined 92 primate species (including humans) and found that 85 had exposed sclera that were uniformly brown or dark brown. Microscopic analysis showed that the brown coloration of the exposed sclera was created by pigmentation deposition in the epithelium cornea, conjunctiva, and sclera. In addition, when eye coloration was compared to facial skin coloration in a subset of 81 species (including humans), 80 species were found to have low contrast in eye and facial skin coloration (i.e., the outline of the eyes and the position of the iris were difficult to distinguish due to the similarity in color of the facial skin, sclera, and iris). The only species with a transparent conjunctiva and white sclera without any pigmentation was humans. In addition, humans were the only species in which the eye outline and the position of the iris were clearly visible, since the exposed sclera was paler than the lightest colored iris or surrounding skin. Finally, the human eye and its visible regions were found to be disproportionately large and horizontally elongated for body size (i.e., the visible regions of human eyes were bigger than that of the much larger gorilla). In a quantitative comparison, Kaplan and Rogers (2002) found that the amount of visible sclera was three times greater in humans than in orangutans (when looking straight ahead -- twice as large when looking to the side).

This is much more interesting, though, given that it focuses on a human trait that is quite distinct from all other animals I can think of, and one that might well have serious ramifications for the evolution of consciousness or at least social behaviors that became of much greater value in the context of human consciousness, tool making, cooperative play and the invention of civilization.

 

[ebbixx]

Having read Tomasello et al. more closely now, I just wanted to add something that occurred to me as I read their concluding paragraph. Namely, how much of the not verbal communications that is missing in online comms (that we often try to compensate for with emoticons) is very often centered on gestures that sometimes can be delivered face-to-face almost entirely with a motion or gesture of the eyes. It's a very compelling and suggestive hypothesis.

 

[williamcardno]

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.

Alright, I can see the misunderstanding here. All I am trying to say is that given there are many genes controlling it, the trait is not likely to exist in your direct offspring but could re-emerge in your descendants, as you pointed out. (One parent with brown eyes and one with green eyes gives birth to a child with blue eyes)
Given that it's not directly affecting the immediate offspring, my hypothesis is one of pure aesthetic and social/psychological triggers based on association and colour.
Example:
You notice most people with blue eyes are kind to you
You are more trusting of blue eyes
Or:
You associate a positive emotion with the colour green
You are drawn to people with green eyes
OR:
People with blue eyes are constantly cheating you
You associate distrust with blue eyes
(how you get a "bad feeling" about an individual that you cannot explain)

Those are the implications I was trying to make at the unearthly hour I was trying to make them. lol
Sorry for the confusion.

 

[Aught3]

If I just take your first example:

You notice most people with blue eyes are kind to you
You are more trusting of blue eyes

The reasoning is fine for the individual, but what about the offspring? Unless they have a similar experiences to the parent they may end up with completely different views on which eye colour they find to be more trustworthy. This is why I objected to your earlier explanation as too subjective. Unless the purely 'attractive' trait (judged on a highly subjective basis) is actually indicative of another, say trustworthiness, it can't be selected for over evolutionary time.

 

[PuppetXeno]

(what he said)

I think that is perfectly plausible...


Once more, the mutation required for blue eyes will have been spread through a population before it is ever expressed, it might have already passed it's "minimum viable population" before it was expressed.

Then it was just a novel trait - interesting merely for esthetic value, has nothing to do with extra survivability or anything. Don't over-rationalize it, people back then were conscious enough about their choice of partners to disagree with you today :lol:

Blue eye color first emerged around 10000-6000 BCE according to this so it's very recent, well after environmental pressure could select for/against something like it.

Some arguments I've heard over the years is that it's easier to read emotion and true intention in light eyes than in dark eyes, I kinda agree. Emotions cause more or less moist in the eyes of variable viscosity, causing different reflections of light which is just easier to spot on a light background than on a dark one. Anyway, this may be why light-eyed people have been a choice of preference in some populations... Similar stuff has already been said, I'll say no more.

 

[williamcardno]

The reasoning is fine for the individual, but what about the offspring? Unless they have a similar experiences to the parent they may end up with completely different views on which eye colour they find to be more trustworthy. This is why I objected to your earlier explanation as too subjective. Unless the purely 'attractive' trait (judged on a highly subjective basis) is actually indicative of another, say trustworthiness, it can't be selected for over evolutionary time.

The direct effect on the offspring is subjective association still imo - but I'd explain it further with relation to the parents via the eye colour of the parents as indicators as well.
If your mum has green eyes, you would associate green eyes with a nurturing instinct, etc.

 

[Aught3]

PuppetXeno - it sounds more like you agree with genetic drift than sexual selection.
williamcardno - you explicitly said sexual selection in an earlier post, are you now more in agreement with genetic drift and I'm totally missing your point?

 

[PuppetXeno]

PuppetXeno - it sounds more like you agree with genetic drift than sexual selection.
williamcardno - you explicitly said sexual selection in an earlier post, are you now more in agreement with genetic drift and I'm totally missing your point?

I think it's a combination of both, not one OR the other.

 

[Spase]

I'm having a bit of a hard time with the idea that eye color is related to how well we communicate non-verbally.

I haven't been doing peer reviewed studies on it so this is just conjecture... but I don't see why blue eyes are easier to follow than brown eyes. The white sclera (the whites of your eyes) is what makes it easy to tell which way someone's looking. In fact it would seem to me that the contrast of a dark brown on white is much easier to follow than a light blue on white.

There was a post made at some point that humans are the only critters running around with white sclera and that's related to readability... but the color of our irises seems like a bit of a stretch to me.

My best guess is still that blue eyes were attractive as a mark of clear genetic diversity. In other words they were 'exotic.'

 

[PuppetXeno]

I'm having a bit of a hard time with the idea that eye color is related to how well we communicate non-verbally.

I haven't been doing peer reviewed studies on it so this is just conjecture... but I don't see why blue eyes are easier to follow than brown eyes. The white sclera (the whites of your eyes) is what makes it easy to tell which way someone's looking. In fact it would seem to me that the contrast of a dark brown on white is much easier to follow than a light blue on white.

From a distance, it doesn't matter - the difference in contrast between light eyes/white sclera or dark eyes/white sclera is negligible.

From up close... I've mentioned it in an earlier post how moist eyes/light reflection is much better visible on light eyes than on dark eyes = reading emotion. Anyway, you're free to experiment with this yourself. Just look people straight in the eyes when you communicate with them, and experience the differences.

There was a post made at some point that humans are the only critters running around with white sclera and that's related to readability... but the color of our irises seems like a bit of a stretch to me.

Yes, from a distance. White sclera appeared much earlier than light eye colour. Again, light eye colour is just a novel trait and has no actual purpose, except from up close & intimate. Where white sclera certainly do have a purpose, because the distance it works on is much bigger.

My best guess is still that blue eyes were attractive as a mark of clear genetic diversity. In other words they were 'exotic.'

Call it "exotic" or a "novel trait" - I doubt people were (or are, for that matter) consciously bothered with genetic diversity, they just found it interesting for esthetic reasons. Well, on a deeper level this could be linked to an interest in genetic diversity, but on any conscious level people wouldn't be bothered with that. As I said earlier, the genes for blue eyes could well have been spread beyond the "minimum viable population" to survive before being expressed, and even attempts of genocide would've been futile, because the genes can spread with a decent chance of not being expressed for generations in a row.

I hope this clears things a bit.

 

[Spase]

[...]
From up close... I've mentioned it in an earlier post how moist eyes/light reflection is much better visible on light eyes than on dark eyes = reading emotion. Anyway, you're free to experiment with this yourself. Just look people straight in the eyes when you communicate with them, and experience the differences.
[...]
Call it "exotic" or a "novel trait" - I doubt people were (or are, for that matter) consciously bothered with genetic diversity, they just found it interesting for esthetic reasons. Well, on a deeper level this could be linked to an interest in genetic diversity, but on any conscious level people wouldn't be bothered with that. As I said earlier, the genes for blue eyes could well have been spread beyond the "minimum viable population" to survive before being expressed, and even attempts of genocide would've been futile, because the genes can spread with a decent chance of not being expressed for generations in a row.

I hope this clears things a bit.

I'm not meaning to be difficult but that just isn't what I've observed personally. I do look people in the eyes when I talk to them to the point I've been told I should make less eye contact because it's disconcerting. It's possible that I'm not using eyes as my main source of emotional information when I talk to someone because facial expressions are clearer to me or simply that I do it unconsciously but I haven't noticed the difference you describe.

For the genetic diversity argument,
Not only have I read many anecdotes of people actively believing that a lineage needed infusions of new blood to continue to be strong but it's not actually important if people are conscious of their attraction to genetic diversity. In the somewhat famous set of experiments where it was shown that people generally prefer the smell of sweat from people who are more genetically different from themselves (and do amazingly well at ranking a set by genetic similarity to themselves) the participants were not thinking about genetic diversity, they were thinking about smell preference. Similarly there is no reason to expect we need to be thinking about genetic diversity to be attracted to visual cues for it. Aesthetics are not without biological basis.

Of course I understand that the allele could easily be spread far and wide before there was much evidence of it and I understand the math behind why even deadly recessive traits don't pass out of the gene pool once they're in. I'm trying to think of reasons it's been selected for because it makes for a more interesting hypothesis and more thinking than assuming that it showed up by chance and propagated well. It may very well be that there's absolutly zero selective benefit (thought that would be hard to argue with the concentrations you see in parts of Europe) but it's a less interesting discussion I think.

 

[Aught3]

For the genetic diversity argument...

The genetic diversity argument makes more sense, if someone is visually quite different from you then it is likely they have different genes. In terms of the smell tests, didn't the researchers narrow it down to the MHC molecules which are used in the immune system? It's been demonstrated that differing MHC's are important in mice mating anyway. Having a diverse immune system is probably one of the most important factors for survival, but eye colour? It's a bit harder to built a case maybe a link to lack of melanin. Actually, mate selection based on smell might be one reason that people aren't generally attracted to their siblings - they smell too similar.

The varying alleles could certainly have spread through the population just by random births and deaths but this is genetic drift. In fact, blue eyes could have gone all the way to fixation purely by genetic drift. Where does sexual selection kick in? A highly subjective opinion based solely on the aesthetic value of the eye makes no sense unless there is something underlying it to re-enforce the sexual selection and to maintain a particular direction of selection (a Red Queen race even). A subjective preference that could easily change between each generation or even each individual and has no value would be indistinguishable from genetic drift. If the recessive genes happened to go to fixation, I would submit that it would be chalked up to genetic drift because it is completely random with no direction of selection.

 

[Spase]

The genetic diversity argument makes more sense, if someone is visually quite different from you then it is likely they have different genes. In terms of the smell tests, didn't the researchers narrow it down to the MHC molecules which are used in the immune system? It's been demonstrated that differing MHC's are important in mice mating anyway. Having a diverse immune system is probably one of the most important factors for survival, but eye colour? It's a bit harder to built a case maybe a link to lack of melanin. Actually, mate selection based on smell might be one reason that people aren't generally attracted to their siblings - they smell too similar.

Yes, if my memory serves me it was MHCs that were the primary factor in the smell tests. I was using it more as an example of how aesthetic senses are subject to evolutionary mechanisms we are not conscious of than as an example of a general preference for diversity.

The non-attraction to siblings was definitely one of my first thoughts when I saw those smell tests the first time. I'd be a bit surprised if there are not a number of other mechanisms to select for individuals who prefer genetically different mates. Visually different individuals seem like a natural target but there's a hole in the idea as it relates to eye color. Since in Europe blue eyes actually reached as high as 90% in some areas it ceased being a sign of genetic diversity at some point but continued to be successful.

Hm. There're probably some studies on it.. I have a hard time believing there aren't.

 

[Grimlock]

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.

That,´s not quite true neither my Mother nor my Father have Blue eyes, in fact the only one who had blue eyes where my mother,´s father.

 

[GoodKat]

That,´s not quite true neither my mother nor my Father have Blue eyes, in fact the only one who had blue eyes where my mother,´s father.

Both your mother and father have the gene for blue eyes, but they weren't expressed because they are recessive.

 

[patduckles]

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.

it could be for breeding purposes, because for example i find women with brown eyes generally more attractive than say blue but this could be just a random balance of chemicals in my brain that make me find them more attractive.

 

[Netheralian]

This may (or may not!) add to the discussion.

http://www.news.com.au/story/0,,24174772-1702,00.html

<sic> exposure to sunlight fosters the release of dopamine which is known to slow eye growth.

Myopia is caused by excessive eye growth.

OK, I'm an Engineer and not a biologist so have no idea how this works but does melanin affect the release of dopamine?

 

[Aught3]

does melanin affect the release of dopamine?

Not that I've heard and it seems very unlikely. Albinos have a severe lack of melanin and it doesn't affect their mental capacity.

Cool news story btw.

 

[ebbixx]

This may (or may not!) add to the discussion.

http://www.news.com.au/story/0,,24174772-1702,00.html

" <sic> exposure to sunlight fosters the release of dopamine which is known to slow eye growth.

Myopia is caused by excessive eye growth. "

OK, I'm an Engineer and not a biologist so have no idea how this works but does melanin affect the release of dopamine?

Here's the likely citation:

Rose KA, Morgan I, Smith W, Burlutsky G, Mitchell P, Saw S-M. Myopia, lifestyle and schooling in students of Chinese ethnicity in Singapore and Sydney. Archives of Ophthalmology, accepted June 2007

Or it might be contained in some other part of Dr. Rose's publications list.

 

[ebbixx]

"Netheralian" said: "does melanin affect the release of dopamine?"

Not that I've heard and it seems very unlikely. Albinos have a severe lack of melanin and it doesn't affect their mental capacity.

You might be looking at an interaction with melatonin (implicated in SADD, for instance) as a neurotransmitter that may interact in some way (though there's not a lot of research suggesting this) with dopamine, rather than a direct effect of melanin levels? However, there do seem to be some linkages between neuromelanin (different from the 2 forms involved in skin pigmentation) and dopamine, most notably implicated in Parkinson's disease.

It should be noted that production of a substance is not the only issue that can have an impact or interaction with other bioactive compounds, such as dopamine, seratonin and so on. For instance, darker skin results not from significantly greater production of melatonin, but from a slower rate of breakdown.

Also, the fact that natural dopamine was only discovered in 1958 (despite having been synthesized first in 1910, unless there's an error on the page) could indicate that the medicinal understanding of its interactions with hormones and other compounds may not be entirely understood, and that a lot of the most current research may be of uncertain validity or may reflect an incomplete understanding of its actions and interactions within humans.

Add to that the knee-jerk prescriptive language of a popular press article, suggesting that the "solution" to myopia is just to kick the kids out onto the playground ignores multiple other issues, for instance the greater light sensitivity of many people predisposed to myopia or other issues where melanin (and perhaps melatonin?) may be factors, and the higher risks of those with greater light sensitivity may have for various cancers correlated with sun exposure.