Why so many eye colors?
What caused the natural selection for eye color?
Sara, Lafayette, Indiana
 "This is a very puzzling question at the moment — one that may take some
time to work out," emails
Richard A. Sturm,
Principal Research Fellow at the Institute for Molecular Bioscience at the
University of Queensland in Brisbane, Australia. Sturm is one of the
leading researchers of the world who delves into eye-color genetics.
A small selection of eye colors. Photo courtesy of Richard A. Sturm and the
Institute for Molecular Bioscience at the University of Queensland.
Most evolutionists think eye-color variation came about as a side
product of skin-color selection. Humans most likely evolved in Africa.
The direct sunlight shining on the equatorial regions of Africa favored humans
with dark skin that gave protection against the sun's harmful radiation.
When Homo sapiens, however, moved out of Africa into northern regions,
darker-skin protection was no longer needed, and natural selection for dark skin relaxed.
Genes controlling eye and hair color may be
closely linked to those controlling skin color. So, when skin-color
selection relaxed, eye and hair color also also were free to change. Genetic freedom to mutate gave
rise to the many eye colors now found
among European-descent peoples.
There is no doubt, Sturm says, that
the genes controlling eye color also influence skin color, and contribute to the
lightning of skin in European peoples. But his personal viewpoint is that
the eye-color genes responsible for switching on blue or brown eye color have a
much greater impact on the color of a person's eyes than on the color of the
skin. Sturm has absolutely no evidence supporting his view yet, but he's
working on it.
"We need a more mechanistic understanding of OCA2 (eye-color) gene regulation
before we can go much further on what may have been the selection pressure."
So, to answer your question: we don't know yet what caused eye-color
selection. But we continue to speculate. Sturm discussed the issue
with his colleagues; some thought people with blue eyes "may have been able to
better stand the dark, depressing days of Northern European ice-age winters than
those with brown eye colour."
Indeed, recent Seasonal Affective Disorder (SAD) studies support this idea.
Goel et al found that among 165 depressed (bi polar disorder or major depressive
disorder) people that "darker-eyed patients were significantly more depressed
and fatigued [in the winter] than blue-eyed patients." He and his team
concluded that lightly pigmented eyes increase the amount of light the eyes
receive during the winter, which relieves depressive symptoms in vulnerable
populations. Terman et al found similar results.
Further Reading:
Eye colour: portals into pigmentation genes and ancestry by Richard A. Sturm
and Tony N. Frudakis, TRENDS in Genetics Vol.20 No.8 August 2004.
Depressive
symptomatology differentiates subgroups of patients with seasonal affective
disorder, by N. Goel, M. Terman and JS Terman, Depress Anxiety,
2002;15(1):34-41.
Photopic
and scotopic light detection in patients with seasonal affective disorder and
control subjects by JS Terman and M Terman, Biol Psychiatry. 1999 Dec
15;46(12):1642-8
(Answered May 5, 2008)
Comments
Very interesting. I
read recently, all blue-eyed people can be traced back to a single ancestor.
Sukumar, Chennai, India
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