February 4th, 2008

thoughtful

Blue Eyes

I've been fascinated to read of the latest research on the origins of blue eyes, carried out by Dr Hans Eiberg of Copenhagen. It seems that all people with blue eyes (including me, my parents, and my children; but not my wife, whose eyes are green) share a single identifiable mutation, descended from a single ancestor (or, as one Australian headline sensitively puts it, "Blue-eyed people 'inbred mutants'"; compare Der Spiegel's "All blue-eyed people related to Brad Pitt"). The original mutation likely occurred in near the Black Sea 8-10,000 years ago. Since it's a recessive gene, the first blue-eyed people would not have been born until several generations after the original mutation, as his or her descendants began to intermarry.
 
The intriguing thing is why we blue-eyed folks became so prevalent in the northern European population - see this map of distribution of "light eyes", taken from a 1960s textbook.
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There's an assumption underlying a lot of the press coverage (also mentioned by Razib Khan in his blog here) that there was strong positive selection for blue eyes in the northern European population at some point shortly after the mutation happened. There is a linkage between blue eyes, blond hair and fair skin, which is supposedly of adaptive advantage in chilly northern latitudes; there also seems to be some kind of link with adult lactose tolerance.  I am struck by the way in which several media sources mention the role of sexual selection, mainly relying on quotes from other geneticists not themselves involved with the Danish research: see here or here. Surely both men and women play a role in this? It seems rather odd to say that blue eyes have persisted and spread because only one gender finds them attractive! (The media reports seem unable to agree as to whether the blue-eye-fancying gender are babes or dudes.)
 
Razib also rightly excoriates the "discovery" that all blue-eyed people are descended from the person with the original mutation. In fact, if that person lived as long ago as 10,000 years before the present, it's very likely that all human beings alive today are descended from him or her one way or another; as I've written before, I'm convinced by the calculations that the most recent common ancestor of all humanity lived in historical times, and the original blue-eyed recessive lived much longer ago.

It's easy (and it's also fun) to mock the press coverage for attempting to sex up the Copenhagen research. All rather fascinating, though, and yet another reminder of how closely inter-related we all are.