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Seeing “Evolution in Real Time”: Mice Blend in to Survive

By watching a population of deer mice shift in the span of a single generation, scientists have captured evolution in action, connecting genes to survival in the wild.

ByKatherine J. WuNOVA WondersNOVA Wonders
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A light-colored deer mouse (Peromyscus maniculatus) on dark soil in the Sand Hills of Nebraska. This poorly camouflaged rodent might not survive much longer, as it's easily spotted by hungry owls scouring the landscape for prey. Image Credit: Rowan Barrett, McGill University

On this particular morning, as he checked the contents of his live traps in the Sand Hills of Nebraska, Rowan Barrett found himself three for 800. It wasn’t a great start to the day.

Not all the traps he’d set out were empty; far from it. But the cache of sunflower seeds within had mostly drawn unwelcome guests: pocket mice, grasshopper mice, and, bizarrely, rattlesnakes. Only a handful contained the deer mice Barrett, now an evolutionary ecologist at McGill University, was after.

For weeks on end, Barrett and his team had been rising before the sun, scouring the dusty landscape for traps jiggling with a catch. Their mission was simple: collect about 500 wild deer mice in a variety of colors, and monitor their survival in the Sand Hills—a cascading series of grass-speckled dunes where the shades of soil range from brown to beige.

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The Sand Hills of Nebraska, where a team of researchers led by biologists Rowan Barrett and Hopi Hoekstra conducted a large-scale experiment to monitor the survival of a population of deer mice. Image Credit: Rowan Barrett, McGill University

Common sense would predict that rodents with coats that blend into their surroundings should be better camouflaged from hungry birds of prey, while mismatched mice, like brunettes on pale sands, would stick out and be quickly gobbled up (spoiler alert: that is, in fact, exactly what they found).

It sounds simple; even obvious. “You might think, ‘Well, duh, the camouflaged mice survive better,’” Barrett says. “But actually demonstrating that? It’s different than having an intuition about something.”

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In fact, the discovery Barrett truly sought was much bigger than mice, an elusive goal biologists have chased for centuries: Capturing a snapshot of evolution happening in real time.

It was a daunting task, requiring a tricky trifecta of variables: genes, traits, and survival. Plenty of studies have shown how genetics affect outward characteristics, like BRCA mutations increasing the risk of breast cancer; others clearly demonstrate that observable, physical features can make the difference between life and death, as with black moths that blend into a backdrop of soot-covered trees. But few have brought all three elements together in a single experiment, pinpointing how genes can dictate survival—especially in mammals.

The work of Barrett and his team, published today in the journal Science, highlights how the stresses of the natural environment can directly alter the fate of one gene—all within the span of a single generation. In light of our rapidly-changing world, these findings underscore the idea that, even against the complex backdrop of nature, one small shift can permanently tip the scales for an entire population.

“This is a beautiful study,” says Obed Hernández-Gómez, an evolutionary biologist and geneticist at the University of California, Berkeley who was not involved in the study. “Recently we’ve looked at genes and understood that evolutionary processes act very quickly. To see that confirmed in this study, and not in a human-mediated environment... This could end up being one of those prominent examples we use to teach biology students in the future.”

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A young, dark-colored deer mouse on dark soil studded with bright green grass. This rodent might be better camouflaged from birds of prey because it carries a functioning copy of the Agouti gene, which contributes to fur pigmentation. Image Credit: Rowan Barrett, McGill University

It’s been over a century and a half since Darwin first put forth his theories on evolution. Scientists have understood the basics of the process for decades. But putting this knowledge to work to forecast the fates of the species that populate the planet, including our own, is still mostly an exercise in futility.

“Even after decades of evolutionary research, we’re only beginning to integrate the different types of approaches we need to truly understand the action of evolution,” says Lucy Tran, an evolutionary biologist at Arizona State University. Tran is now a postdoctoral researcher in the lab of study author Susanne Pfeifer, but was not directly involved in the study.

One thing, though, is certain: Evolution isn’t exclusively the slow, plodding process that Darwin originally envisioned. Time and time again, researchers have shown that it can be witnessed in just a matter of years—or even within the span of a single experiment.

“We took our motivation from microbes in lab environments,” Barrett says. One particularly well-known experiment has been monitoring bacteria for decades. “The crazy idea was, can we do this with a mammal in nature?”

For their test subjects, the team chose deer mice (Peromyscus maniculatus). In these rodents, the two halves of the equation were already there: The general importance of stealth for survival, and a gene, Agouti, known to control the color of fur. “I wanted to put the two bits of this puzzle together,” Barrett says. “No one had done that before.”

In retrospect, Barrett says, there was probably a reason for that. The setup was far from trivial.

To test mouse survival, the team needed a way to simulate the natural environment. That meant building eight 80-foot-by-80-foot enclosures, four each on dark and light soil. Six enclosures would house between 75 and 100 microchipped mice each; two others would remain empty. This required finding a lot of flat, rodent-friendly land—in a place named for its hills—and corralling herds of mice onto distinctly colored patches of dirt.

Easier said than done. “Deer mice are like little ninjas,” Barrett says. “They burrow, they climb. People told me, ‘You’ll never hold them.’”

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Evolutionary biologist Rowan Barrett basks in the triumph of the research team's manmade mouse enclosures in Nebraska's Sand Hills. Image Credit: Naomi Man in’t Veld

But with steel plate walls buried two-and-a-half feet into the ground, the barriers held. And while each enclosure started off with a mix of light- and dark-colored mice, by the end of three months, their compositions had changed drastically. First off, the populations had been decimated: Hundreds of mice had died, likely picked off by keen-eyed owls. And the surviving mice were only a very particular subset of the original population. Mice with fur to match the soil—like light mice on cream-colored soil—had been the ones to emerge victorious. Their standout brethren, on the other hand, had not.

“I didn’t really think that we would see much change over such a short-term experiment,” Barrett says. “This was kind of a Hail Mary.”

And when the researchers sequenced the mice’s DNA, they found they had the genes to match. Surviving mice on the dark side tended to carry a normal version of the Agouti gene, which aided the production of pigment in their coats, while the remaining mice in the light enclosures were more likely to carry a broken copy of Agouti that kept them blonde.

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Too many mice died in those early months for the experiment to look past the initial cohort of 500 mice, Barrett says. But one look at the Agouti genes of the remaining mice in each enclosure told him that it would be essentially impossible for their offspring to be very diverse, in terms of coat color. For the most part, lighter mice carrying broken Agouti had been left on light soil, and vice versa; future generations would likely reflect this as well. And for Barrett, this probably represented a sped-up version of what had happened naturally on the Sand Hills over the course of many years, ultimately partitioning the rodents by color.

“Typically, we would say, for this to be evolution, you need to see if it’s reflected in the next generation,” he says. “But we know [the traits are] going to be transmitted, because we showed changes at the genetic level.”

Others, like Tran, are slightly more hesitant to make this conclusion, however. “Multiple generations would provide stronger evidence [of evolution],” she says. “What’s missing in the study is whether these changes are definitely heritable.”

Regardless, the study’s ability to link natural selection in the field to a specific gene makes it a “rare gem,” says Melissa Pespeni, an evolutionary biologist at the University of Vermont who was not involved in the research.

In the future, Barrett and his colleagues hope to extend the study into a multi-generational approach. “This is just the first step in a long-term project,” says study author Hopi Hoekstra, an evolutionary geneticist at Harvard University. But the staggering rates of change the researchers found might still have a lot to say in other contexts—including species conservation.

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“This study is sure to be a classic example of how natural selection works on both an animal’s [physical characteristics] and the genetics associated with them out in the wild,” says Rebecca Safran, an evolutionary ecologist at the University of Colorado Boulder who was not involved in the study. Image Credit: Rowan Barrett, McGill University

“The more we can learn about how organisms adapt, the more we’ll know about how organisms might adapt to, for instance, a changing climate,” Hoekstra says.

Already, it’s clear that shifts in global temperatures and other human-motivated changes have done some serious damage to Earth’s inhabitants. Owls are molting out of paler shades in response to warming winters; sea turtle populations with temperature-dependent sex determination systems are skewing heavily female.

“A major lesson here is to preserve genetic diversity,” Hernández-Gómez says. “We might lose genes that might be beneficial in the future if we don’t.”

Some species may be flexible in the face of harsh environmental change. With others, though, one variable could be all it takes to transform an entire population in just a handful of weeks.

As for passing these changes down through the generations? Well—that’s a mouse of a different color.