When researchers study evolution through natural selection, they typically focus on just one part of it. The essence of the process is this: Some genes confer beneficial traits. Those traits make their owners more likely to survive and reproduce in a given environment. Over time, those genes and traits become more common. So researchers might, for example, find genes behind certain traits (such as striped coats). Or they might link certain traits to success in a given environment (such as longer-legged lizards in hurricane-hit islands). Beyond some experiments with lab-grown microbes, they have rarely connected all the dots together.

That’s what Barrett accomplished. With hundreds of mice and years of research, he and his colleagues were able to show and measure, in the real world, “the full process of evolution by natural selection,” says Hopi Hoekstra of Harvard University, who led the study. “It’s all in one.”

It was also a pain in the ass. “Utter ignorance was a good thing,” said Barrett, who had, until this point, only ever worked with small fish. “Anyone who had worked with mice would have never attempted this.”

credit: Rowan Barrett

Once the team had Bill Ward on board, they ended up buying 30,000 pounds of stainless steel plates from a local hardware store, and carting them over to the farm using flatbeds and forklifts. There, they erected the plates in trenches two feet deep, creating square enclosures that were 164 feet across on each side. They built three such pens on light sand, and three on dark soil.

At first, the steel pens seemed to work. Mice could neither dig beneath the plates nor climb over them. They were, however, exceptionally good at sneaking through gaps where adjacent plates didn’t quite meet, so the team had to dig everything back up and pour concrete around the joints.

Nature itself seemed eager to select against the team. On one trip, high winds almost flipped the truck carrying the steel plates. Once, a team member fainted and cut himself on a piece of steel. During winter, ramps of snow would accumulate along the walls, so the team had to add an extra layer of mesh along the plates. They also had to catch all the rattlesnakes in the enclosures and throw them over the walls; Bill helped. “Everything goes wrong in the field,” Hoekstra says. “And we’re used to dealing with pipettes, not backhoes.”

When everything was finally set, the team evicted every mouse already inside the enclosures, and caught around 500 more from the surrounding hills. They photographed each rodent, took a DNA sample, implanted a tiny radio chip between its shoulders, and released it into one of the enclosures.

As time passed, many of the mice fell prey to owls, but after three months, the team returned and recaptured the ones that were left. Sure enough, they found that, compared with the average founding rodents, the average survivors were noticeably lighter in the light-sand enclosures, and darker in the dark-soil ones. Through the deaths of the most conspicuous individuals, the survivors from two initially identical populations had shifted in different directions thanks to their different environments. “It’s intuitive that if you match your background, you’re more likely to survive,” Hoekstra says. “But that’s been a just-so story for years.” This experiment showed that it matters—a lot.