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On the Origin and Spread of an Adaptive Allele in Deer Mice

Adapting Coat Color Simple phenotypic changes can often be the target of selection—for example, variations in coat color that provide protection against detection by predators. Linnen et al. (p. 1095) explore the underlying molecular mechanisms behind the production of pale deer mice living on the light-colored Nebraska Sand Hills. The mice that live on the sand are significantly lighter in color than conspecifics living nearby on darker soils. This lighter color was found to be due to de novo changes at the Agouti coat color locus. Thus, rapid adaptive change does not always rely on preexisting genetic variation. The light coat-color variant in deer mice is a mutation selected for its adaptive value for living in sand hills. Adaptation is a central focus of biology, although it can be difficult to identify both the strength and agent of selection and the underlying molecular mechanisms causing change. We studied cryptically colored deer mice living on the Nebraska Sand Hills and show that their light coloration stems from a novel banding pattern on individual hairs produced by an increase in Agouti expression caused by a cis-acting mutation (or mutations), which either is or is closely linked to a single amino acid deletion in Agouti that appears to be under selection. Furthermore, our data suggest that this derived Agouti allele arose de novo after the formation of the Sand Hills. These findings reveal one means by which genetic, developmental, and evolutionary mechanisms can drive rapid adaptation under ecological pressure.