Blue eyes may be an example of culture-gene co-evolution:
Thanks to the work of the appropriately named (and blue-eyed) Danish geneticist Hans Eiberg and his colleagues, we now know that the chief mutation that causes blue eyes is a single letter change, from A to G, at the 26,039,213rd position on chromosome 15, within a gene called HERC2.
HERC2 has no effect on eye color, but it contains an unexpressed segment of DNA that is needed for the switching on of a nearby gene called OCA2, as demonstrated by newly published work by Robert-Jan Palstra and others at Erasmus University in the Netherlands. The mutation that causes blue eyes reduces the expression of OCA2 and hence reduces pigment concentration. Paler eyes look bluer.
Why did this mutation become so common somewhere around the shores of the Baltic sea around 6,000 years ago? The answer may lie in the fact that the date coincides with the arrival of agriculture in the area. When people began relying heavily on a diet of bread at such a northern latitude, they probably became chronically deficient in vitamin D, for bread is generally low in vitamin D.
This wouldn’t matter in a lower latitude, because the body can synthesize vitamin D if exposed to ultraviolet sun rays. But in northern Europe, diseases related to vitamin D deficiency, such as rickets, would have become common. Any individual who had a genetic mutation that lightened his or her skin (and eyes) would absorb more sunlight, boosting health and the ability to survive and breed. Paleness was selected.
When Nordic people started depending more on bread than on fish, they got less vitamin D from their diet. As a result, they got paler, improving the capacity of their skin to generate this crucial nutrient just from scarce sunlight. How they lived changed, in effect, how they looked.