The deer mouse, Peromyscus maniculatus, is the most populous mammal on the continent, a versatile little rodent that can thrive in virtually any environment. Before the sandhills formed, Nebraska’s soil was dark — and so were its deer mice. Even today, most of the region’s mice have medium- to dark-brown coats, except for the ones that scurry around on the sand. Their fur tends toward tan or orangey-blond.

The reason seems obvious enough: Mice that have dark fur and live on light sand are easy pickings for hungry hawks and owls. A genetic mutation led to lighter fur, and the sandhills mice lucky enough to be born mutants were more likely to survive and breed. With each generation, the population of light mice would grow, while that of the dark mice would shrink, until eventually, the bulk of the sandhills mice sported lovely beige fur. It’s a textbook tale that any high school student could explain.

The only problem is, it might not be true. To Dr. Hoekstra and Rowan Barrett, a postdoctoral researcher in her lab, the fact that it makes sense says nothing about its scientific credibility. “People love to tell stories about how species adapt and evolve,” said Dr. Barrett, “but no one ever sees it happen.”

Thanks to an accumulation of knowledge about both the ecology and the genetics of the Nebraska deer mice, the animals offer a rare opportunity to directly test the story, and to measure the effect of environmental conditions on evolution. Several years ago, Dr. Hoekstra and another postdoctoral researcher, Catherine Linnen — now at the University of Kentucky — discovered that a specific gene controls much of the fur coloring in P. maniculatus by telling pigment-producing cells how long to spend making certain shades. More recently, they identified four different parts of the gene where mutations affect coat color, and showed that natural selection is involved in the color changes.

“It’s kind of fun to find the mutations,” said Dr. Hoekstra, “because that’s the basic material of change. But at a bigger level, there are two general things we can learn. One is about how changes in gene expression evolve. And the other is, does evolution occur through big leaps or the accumulation of small, gradual changes?”