Why are irises (not the flowers!) coloured?

Question

A while back I badly phrased a question about the iris that surrounds the pupil in vertebrate eyes. I know about its function in reducing/increasing the amount of light entering the eye, but why is there a variety of possible colours? (Not how is the variety produced, but why does variety exist?)

Answer 1

Pigmentation; in the same way that skin is differently pigmented.  To protect the eye from excess UV radiation.  Thus Africans have very dark, almost black irises, whereas northern europeans have blue eyes, a consequence of there being less solar radiation to contend with.

The variation in colour is caused by differential pigmentation within the iris.

Answer 2

Excellent question. Nathan has told you why dark irises are favoured under conditions of strong sun, but not why there should be a mixture of types among, say, northern Europeans.

When variety exists, as in eye colour or even more strikingly in blood groups, it requires explanation of some kind.  One possibility is that it goes with slight differences in biochemistry, which affect susceptibility to infection by viruses.  Then there is an advantage in belonging to the minority variety, because there are fewer people around most able to infect you. The result is that as soon as a variant becomes rarer, selection works in its favour.

This kind of thing can go on from millions of years. We actually share variety of blood type with chimpanzees, although we parted company from them some 4 million years ago.

Answer 3

The iris is made up of four layers:

The anterior border layer (the front layer facing out)

The stroma

Two layers of endothelium (at the back of the iris)

The double layer is responsible for dilating the pupil and absorbing any stray light that reaches the back of the iris. It is only the first two layers that determine iris colour.