A Whiter Shade of Pale

As it was with the peppered moth, so it is with the deer mice of Nebraska’s Sand Hills. Covering roughly a quarter of Nebraska, the Sand Hills are grassy dunes of light colored sands deposited as ice sheets receded some 10,000 years ago. Deer mice (Peromyscus maniculatus) colonized suitable habitat, including the Sand Hills, shortly after the ice sheets retreated northward.

Most deer mice, including those from regions neighboring the Sand Hills, have dark brown pelage, but those inhabiting the Sand Hills have evolved a much lighter tan pelage (Figure 1). Their lighter fur makes them less conspicuous to aerial predators (such as hawks and owls). Therefore selection favors lighter coloration against the lighter background of sandy dunes.

deer_mice_pelage
Figure 1. Deer mice from the Sand Hills region of Nebraska on a contrasting soil type (above) and their normal soil type (below).


If that were the whole story, deer mice populations from the Sand Hills would represent one more case of natural selection for camouflaged pelage. However, what Linnen and colleagues (2009) demonstrate is that this pelage color change occurred within roughly 8,000 years and that it was triggered by changes in a single gene.

Like most mammalian fur, deer mice fur has distinct color bands. These bands reflect different quantities of two pigments; the blackish-brown eumelanin and the yellowish-red pheomelanin. When the researchers examined the hairs of the Sand Hills deer mice, they noticed that the pheomelanin band near the tips of the hairs was much wider, giving these mice a lighter coat than deer mice living on darker soils nearby.

The researchers then turned their attention to a gene, called
Agouti, which is responsible for producing pheomelanin. Using a series of crosses between light-colored mice and wildtype brown mice, they showed that a single amino acid deletion led to an over-expression of pheomelanin in the hairs, which makes the pelage appear lighter. Interestingly, this Agouti mutation is genetically dominant to the darker wildtype pelage.

Based on predation experiments and data from other studies, Linnen and colleagues (2009) suggest that this mutation occurred de novo in deer mice already living in the Sand Hills some 8,000 years ago. Since that time selection has strongly favored the
Agouti allele in Sand Hill deer mice populations.

References

Linnen, C.R., Kingsley, E.P., Jensen, J.D., and H.E. Hoekstra. 2009. On the origin and spread of an adaptive allele in deer mice. Science, 325:1095-1098.