August 29, 2009

Scientists Track Mouse Evolution Icon

Generations of tiny pale deer mice have evolved a sandy-colored coat, which camouflages themselves from predators, just within a few thousand years.

These mice acquired the mutation for pale fur naturally and then rapidly passed it on to the next generation.

This makes scientists consider the deer mouse one of the best examples yet studied of "true" natural selection in action because of the fast evolving of the creature.

The mice are most abundantly found in North America.

They usually have a dark coat that enables them to blend in with dark soils and avoid being seen by predators like owls and hawks.

However, at Sand Hills in Nebraska, pale-coated mice flourish.

"We decided to investigate the striking contrast between mice living on the pale Sand Hills and mice living on darker soils just a few miles outside," says Dr Catherine Linnen of Harvard University in Cambridge, Massachusetts, US.

"We were also intrigued by the fact that Sand Hills had formed within the last 8,000 to 15,000 years, which implies the light color of the Sand Hills mice became advantageous only recently."

Linnen and colleagues at Harvard and the University of California, Berkeley have figured out exactly how the mice evolved so quickly.

Their findings were published in the journal Science.

They discovered that a single gene, known as Agouti, codes the light coat color.  This is considered to be a higher amount than the genes that code for dark hair.

Most animals evolve new features quickly by expressing a variation of a gene that already exists, rather than evolving a new type of gene altogether.

However, researchers discovered that the Agouti gene only appeared in wild deer mice in Sand Hills around 4,000 years ago, just a few thousand years after dark mice colonized their new home.  This means that the mice first evolved 8,000 generations of mice ago.

Also, the researchers found that this new gene has since become very common among the Sand Hills mice.

"The light gene wasn't in existence, so the mice had to "wait" until a particular mutation occurred and then selection had to act on that new mutation," says team member Professor Hopi Hoekstra, also of Harvard University.

"It's a two part process. First the mutation has to occur and second, selection has to increase its frequency."

Researchers say that this is the first time it has documented the appearance of a gene, its selection and subsequent spread through a population of wild animals.

That has allowed them to estimate the "strength" of the natural selection pressure.

The paler Sand Hills mice have a 0.5% survival advantage with the light colored fur.

"It doesn't seem that much, but multiplied over thousands of individuals over hundreds of years, it makes a huge difference," says Prof Hoekstra.

"Ours is a very complete story," adds Dr Linnen.

"We've been able to connect changes at DNA level to the ability of deer mice to survive in nature."

Image Courtesy National Park Service - John Good


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