Dictionary of Human Evolution and Biology

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Eye Color

Pigmentation on the iris that ranges continuously from light blue to dark brown including mosaics of light and dark areas. Iris pigmentation is produced by melanocytes, but unlike the melanocytes elsewhere in the body, eye melanocytes retain the melanin granules they produce for life. Light eyes lack pigmentation in the superficial layers of the iris; consequently, as light enters these layers blue light is reflected, giving the appearance of a blue iris. The posterior of the iris is also pigmented, but this serves to keep stray light (which would interfere with vision) out of the inner chamber of the eyeball. The inheritance of eye color is not well known except that it is polygenic. The adaptive value of different eye color is unknown, but it has been suggested that light eyes may be a response to low light conditions (due to cloud cover and dark conditions in winter) such as those found in Europe.

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