I was just wondering what the purpose is of the iris of the eye being coloured, if there is indeed a purpose. And what makes up the colour of the iris?
I don’t have much else to say beside that, it’s just an honest curious question.
The iris is made up of cells. They contain pigment, therefore they’re colored. Except in albinos of course, who lack pigment. Then they’re pink, because the capillaries show thru.
Coloration has benefits, like reducing the amount of light that gets thru to the retina. Otherwise, mix a clear iris with strong sunshine, and the individual would be unable to see much.
The function of the iris is to regulate the amount of light entering the eyeball. It’s like windowshades or drapes - you might have them open at dawn or dusk, but noon might be too much light so you close them to adjust the light level.
The iris is colored because, in order to do this job, it needs to be opaque and opaque objects have color. WHAT color really isn’t that important, which is why eyeballs come in all sorts of colors in natures. In the case of mammals, the pigment used to color eyes is frequently the same as that used to color hair and skin - why not, it’s available, right? - and thus is some form of melanin, which ranges from light tan to dark brown or almost black. The amount of pigment determines the color, and with oddball variations of the basic pigment you wind up with a range of different shades. A lot of critters have brown irises, because brown seems to be a color easily made in nature, but human irises come in shades of gray, blue, and green. Some other critters, like cats, have yellow and orange as options. But from a functional viewpoint the actual color doesn’t seem to matter a whole lot.
The exception to all this is albinos. Not having any pigment, their irises are transparent, more or less. Their eyes are “pink” because you are actually seeing the color of the eye’s blood vessels through the transparent iris. And, needless to say, a transparent iris works about as well as transparent drapes when it comes to blocking out light. As a result, albinos can have great difficulties seeing in bright daylight and can even suffer eye damage over the long term if exposed to too much sunlight or very bright indoor light.
How come only whites have eye-colors (blue, green, etc). Although I’m sure there’s exceptions, blacks, hispanics, pacific islanders, and asians have only different shades of brown.
Nope, Hispanics can have all shades of colours they want, I’ve seen them all. And brown as an eye colour gene is dominant, while green or blue isn’t. Therefore, finding someone with blue or green or gray eyes is already uncommon. In a population that has mostly brown eyes, and that hasn’t mixed with other populations, then it will be very rare to find someone with light eyes.
Cecil did a column on this, but I’ve been unable to dig it up. IIRC, his explaination was no one really knows why there’s all different colored eyes.