What happens when you put together several tons of steel plates, hundreds of mice, and a few evolutionary and molecular biologists in a rural location? You get one of the most complete pictures of vertebrate evolution. 

Led by Hopi Hoekstra, professor of organismic and evolutionary biology and molecular and cellular biology, an international team of researchers conducted a years-long study in which hundreds of mice were released into massive, custom-built outdoor enclosures to track how those that were light- or dark-colored survived in light- and dark-colored habitats. 

The results confirmed the intuition that light-colored mice survive better in light-colored habitats, and dark-colored mice in dark habitats. But the experiment also allowed researchers to pinpoint a mutation related to survival, one that affects pigmentation, and to understand how it produced a novel coat color. The study is described in a Feb. 1 paper published in the journal Science. 

“Part of the inspiration for [this study] came from the experimental evolution studies people have been doing for many years now using microbes in the lab,” Hoekstra said. “The idea has been that you start with a particular population, genotype it, and then give it environmental challenges and watch how the population evolves over generations. Then you genotype it at the end, and you can see, at the genetic level, what changes.

“We were interested in replicating that approach but doing it in vertebrates, and doing it in a natural environment. And letting them evolve in habitats that — importantly — are open to predators, or at least visually hunting, avian predators.” 

To do that, then-postdoctoral fellow Rowan Barrett (now on the faculty at McGill University) and colleagues traveled to the tiny town of Valentine, Neb., to take advantage of an important natural habitat, the sand hills. 

As early as the 1930s, Hoekstra said, it had been observed that mice living in the sand hills — a large area of contiguous dunes with sandy, light-colored soil — were lighter-colored than those living in surrounding areas with dark, loamy soil. To understand what was behind those differences, Hoekstra, Barrett, and colleagues came up with an ambitious plan to build eight enclosures, each 2,500 square meters, or just over a half-acre, four on the sand hills and four on the darker soil. 

They then “seeded” each enclosure with 100 mice, half trapped from the sand hills and half trapped from the surrounding dark soil. They embedded each with a tiny radio-frequency identification tag and took the very tips of their tails for genetic sequencing. Three months later, the researchers returned to identify which of the mice had survived.