What biological reasoning explains the panoply of eye color in humans?
If I could add a second question: do different pigments have any sort of effect on the eye?
Great questions and a favorite topic of mine. Let me try to answer them.
There was a great article in SciAm a year or 2 ago that described the structure of the eye. Basically it mostly consists of "nearly comatose" cells. The cells take this state because it makes them nearly transparent enabling light to transmit through them.
This is the case until the iris. There are working muscles located there that act to change the shape of the lens as needed for focusing. That means the cells located there are very much alive and functioning.
However, they have no "UV protection" from melanin as all the layers of cells in front of them are transparent.
Thus, there are "uveal melanocytes" that inject pigment into that layer to afford some UV photoprotection for the living cells.
Albino's, who have a genetic defect preventing melanin production, have pink iris's.
They are pink because of the blood vessels located there that supply the living cells nutrients.
Next on the color spectrum are "blue eyes", which are just pink eyes with very low quantities of melanin.
Next on the color spectrum are "hazel eyes", which have even more melanin than blue eyes, then "brown eyes", then of course lastly "black eyes" have the most.
So, if photoprotection is the only thing that matters why didn't evolution "naturally select" black eyes for all of us?
The answer is vitamin D needs to be locally synthesized in the iris, which forces a balance between UV absorption and transmission, effectively setting an upper limit on the allowable melanin concentration.
In summary, eye color is essentially "skin or hair color" over a pink substrate instead of the normal "nail color" substrate of skin and hair.
Third question could be that, why does the eye color change with time?
Is the pigment exported from the melanocytes? I'd think the pigment would be held within the melanocyte.
Do you have a reference that states that vitamin D needs to be locally synthesized in the iris?
It has to do with the level of gene expression of the genes encoding the pigments, I don't think it is really understood how or why the gene expressions would change for these traits.
Terribly complicated question. But I will share an interesting anecdote.
The father of an old friend, around 70 now, has pure white hair and ice blue eyes.
But when my friend and I were younger he had brown hair and eyes, he was very proud of his Italian heritage. He wonders now why his eyes have turned blue, but not why his hair is white or why his skin is very pale. Of course he thinks his hair and skin have turned white due to getting older.
But even young people have "blue eyes", right? So his eye color should have stayed brown forever, right?
I should also point out that he has religiously avoided the sun for around 25 years now. Someone convinced him a long time ago about the dangers of skin cancer I guess.
And he did succeed in avoiding skin cancer.
It's curious to me that everyone accepts that "not using your muscles" leads to their atrophy over time, yet hardly anyone makes a connection to a chronic lack of UV exposure and loss of pigmentation in the hair, skin and eyes.
It's like the general public thinks: "there's absolutely no correlation even though most of us tan when we go to the beach."
Hi Monique, I did reply to your questions but looks like it got lost.
I'm out of time at the moment. I'll try to get back to you again later on these.
What about green eyes? Just somewhere in the spectrum as well?
According to one worldwide survey (don't ask me which), green eyes were found most attractive.
Maybe because they're so rare.
Quick reply, yes - it's located between blue and brown.
I don't remember the specifics, it could also be due to a unique mixture of pigments.
Humans produce more than one kind.
I also have green eyes and see them every morning but I still find green eyed women the most captivating.
Monique, obviously you have an interest in green eyes. I'm curious, do you find the explanation given in this thread lacking?
If so, do you have your own explanation?
I ask because I think it may have something to do with lightning flash photoprotection, in addition to to sunlight photoprotection.
Remember, back in the evolutionary day it wasn't easy to find shelter from sudden storms.
You haven't yet provided a reference that vitamin D needs to be locally synthesized in the iris. If you do I can evaluate the information, otherwise it is pure speculation.
Oops, I forgot about that. I may not be able to find it, it's something I read about 2 years ago, it was a paper in a British physiology journal. That's all I remember, not much to go on.
But either way I'm interested in your opinion about green eyes. This thread showed me that there is something of a green eye lovers club in the public, and you're obviously in it. So, I thought maybe you had researched it some given your expertise.
For the record, if it wasn't clear enough, I'm definitely speculating on the possibility of lightning flashes being an evolutionary factor driving human eye pigmentation selection.
Maybe you're concerned this type of conversation is out of bounds here on the forum? If so, please consider this a full diclaimer.
Or maybe you're afraid to put your opinions out there because of possible ridicule? If so, I promise I won't do that and - not that I need to say this - but, please keep in mind it's all anonymous - you're professional standing is not at risk here.
Green eyes reflect in the spectral region where the Sun's optical intensity is greatest. I believe that in daylight they would seem to glow more than eyes with other pigments.
I don't think there is any evidence that vitamin D is needed in the eye, so clearly this can not be the explanation (unless you can provide some evidence otherwise).
I have tried to find publications that have information on the importance of eye color, but I can't find one. A 2007 http://www.nature.com/ng/journal/v39/n12/pdf/ng.2007.13.pdf" had this to say:
It could very well be genetic drift, uveal melanomas due to a light iris is very rare so there is no strong selection against light eye color (maybe only in regions around the equator).
Thanks for the reply. Since I asked for some of your time, I spent a little time looking online for the article I mentioned. No luck yet, but I did find some that I give below relevant to your points =>
Here's a link to a paper on Vit-D and retinal tumor suppresion:
Here's a link to a paper on the correlation of latitude (UV-R exposure levels) with ocular melanoma:
Here's a link to a Wiki article that describes very clearly that there is a strong selection against eye color thats too light (see the "Symptoms and conditions associated with albinism" section):
I also found the statement: "Lights should be yellowish rather than blue and not point towards the usual position of a person with albinism (like their seat at a table)" in the "Sun protection" section very intriguing. No citation was given but sometimes they backfill that type of data. I'll check back in a few months.
Separate names with a comma.