Much of evolutionary biology focuses on ultimate causation--identifying the evolutionary forces that have led the world to be as it is today, without much regard for the nuts and bolts of how organisms work. Much of molecular biology, on the other hand, focuses on proximate causation--understanding how the world works without much regard to how it came to be that way.

It's nice, then, to see the two joined in an elegant fashion; a recent paper on the evolution of pigmentation in deer mice does just that. About 8-10 thousand years ago, a novel habitat arose in the middle of Nebraska in the form of a large field of sand dunes. Since then, the a population of deer mice adapted to the habitat by evolving a lighter coat color (though I'm having issue with images on blogger, an example is in part A of this figure).

The authors show that this decrease in pigmentation is due to changes in the temporal expression of the agouti protein (this protein is part of the "canonical" pigment type-switching pathway)--light mice express agouti for longer than dark mice during hair development. They're able to show that this change in expression pattern correlates perfectly with a deletion of a single amino acid in agouti, though they hesitate to claim that this is the true causal mutation (this polymorphism is perfectly correlated with others in the region).

The allele shows classical signatures of having experienced a selective sweep, and the authors estimate that the allele arose sometime after 10kya, after the formation of the dunes that produced the selective advantage. It's difficult, of course, to prove that this polymorphism wasn't segregating prior to the formation of the dunes, but if it was, it was certainly at very low frequency. In all, this is a very nice story, and a great example of the power of molecular approaches to the study of evolution, even in non-model organisms.

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Citation: On the Origin and Spread of an Adaptive Allele in Deer Mice. Catherine R. Linnen, Evan P. Kingsley, Jeffrey D. Jensen, and Hopi E. Hoekstra (28 August 2009). Science 325 (5944), 1095. [DOI: 10.1126/science.1175826]