A rapid evolutionary shift has caused this deer mouse to have lighter fur (Image: Emily Kay)

Deer mice have pulled off the opposite trick to the famous peppered moth, evolving a light coat to disguise themselves from predators. What’s more, they did it even though their ancestors had no genes for light coats.

Most deer mice have dark fur, which would stand out vividly against the pale-coloured Sand Hills of Nebraska. The hills formed around 10,000 years ago, but genetic analysis performed by Catherine Linnen of Harvard University and colleagues found that back then the deer mouse genome didn’t contain the genes for light fur. That means the trait arose from a new mutation which rapidly spread through the local deer mouse population.

The mutation is in a gene called Agouti, which helps control coat colour in many mammals.


“We found a very strong association between a single amino acid deletion, in a functionally important part of the gene, and the colour phenotype,” Linnen says.

Ron Woodruff, an evolutionary biologist at Bowling Green State University, Ohio, says the results are striking. “Any cases where you can find positive selection for a mutation are interesting,” he says.

Standing start

The amount of genetic variation in the new version of Agouti suggests the mutation occurred less than 10,000 years ago, Linnen says – after the formation of the Sand Hills.

“An older mutation would show lots of variation,” she says.

Normally, to achieve such a rapid evolutionary shift a species needs to have an alternative version of a gene already in circulation. A change in conditions – such as trees becoming darker as they get covered in soot, in the case of the peppered moth – can then provide the selection pressure that causes the alternate gene to spread. But in deer mice the new version of Agouti spread rapidly from a standing start.

Woodruff says that these “de novo” mutations – which occur after a species has encountered the situation where they will be useful, rather than before – may be more important for evolution than biologists think.

If the Agouti mutation occurred in just one mouse, the chances of it spreading rapidly through the whole population would be slim, which is why newly arising mutations like this have been thought to be unimportant.

But Woodruff points out that mutations can occur while an animal is in the womb. If this occurs early enough, the mutation may be present in many of its sperm or egg cells. Then the animal could have several offspring with the mutation, making it easier for the mutation to spread.

He says, “Beneficial mutations occur by both mechanisms: pre-existing variations and de novo mutations. The debate is, what is the proportion of the two?”

Journal reference: Science, DOI: 10.1126/science.1175826