The 10,000 Year Explosion by Gregory Cochran

Author:Gregory Cochran [Cochran, Gregory]
Language: eng
Format: epub
Published: 2011-02-10T07:15:07.365000+00:00

BLUE EYES

A strong state fosters gene flow within itself through trade, free movement, and sometimes forced movement. At the same time, it tends to limit gene flow from outside, particularly when it upholds its borders with military force. When such a state disintegrates, there can be a massive movement of peoples—in part because the borders are no longer defended, but even more in those cases in which the inhabitants have lost their military habits over the course of a long imperial peace. In classical times, one obvious sign of that kind of relative domestic tran- quility was unwalled cities.

The fall of the Roman Empire followed this pattern. As the state weakened, many ethnic groups entered. In the early days they were often mercenary soldiers, but eventually some came as plundering bands. Those invaders also carried new al- leles. We will discuss the Vandals, one of the most spectacular of these groups. We think they may have played a part in spreading a particular far-flung new allele, the one responsible for blue and green eyes.

Blue eyes are common in Europeans and their descendants and are found to some extent in adjacent populations, but they are essentially nonexistent in most of the world. Some 10,000 years ago there seems to have been no such thing. There are other shades of eye color, and other genes have some influence, but most of the story boils down to a single new allele of the gene named OCA2 (for oculocutaneous albinism II, also discussed in Chapter 4). To be exact, blue eyes are caused by a change in a DNA sequence that regulates the expression of OCA2, a sequence that is embedded in HERC2, the gene next to OCA2.16That allele accounts for 75 percent of the variation in eye color in Europe. It's the third longest haplotype in Europeans and therefore can't be very old: Analysis of the unshuffled region associated with OCA2 suggests that it originated about 6,000 to 10,000 years ago. Blue eyes are most common in northern Europe, centered around the Baltic. The simplest assumption

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