Matt Walker

Editor, Earth News

Pale-coated deer mice usually live on sandy soils

A tiny pale deer mouse living on a sand dune in Nebraska looks set to become an icon of biology. Within just a few thousand years, generations of the mice have evolved a sandy-coloured coat camouflaging themselves from predators. Most striking is that these mice acquired the mutation for pale fur naturally, then rapidly passed it on. That makes the fast-evolving deer mouse one of the best examples yet studied of "true" natural selection in action. Deer mice are one of the most abundant and widespread mammals in North America. Ours is a very complete story. We've been able to connect changes at DNA level to the ability of deer mice to survive in nature

Dr Catherine Linnen Usually the mice have a dark coat, which enables them to blend in with dark soils and avoid being seen by predators such as owls and hawks. But at Sand Hills in Nebraska, pale-coated mice abound. "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 colour of the Sand Hills mice became advantageous only recently." Fair gene Linnen and colleagues at Harvard and the University of California at Berkeley have now worked out exactly how the mice evolved so quickly. They have published the details in the journal Science. They discovered that the light coat colour is coded by a single gene, dubbed Agouti. This is expressed at a higher amount, and for longer, than the genes that code for dark hair. Most animals known to quickly evolve new features do so by expressing a variation of a gene that already exists, rather than evolving a new type of gene altogether. But the researchers found that the Agouti gene only appeared among wild deer mice in Sand Hills around 4,000 years ago, just a few thousand years after dark mice colonised their new home. That means it first evolved 8000 generations of mice ago. The darker, more usual coat colour

They also ascertained 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." The researchers say it is the first time that it has been possible to document the appearance of a gene, its selection and subsequent spread through a population of wild animals. And that has allowed them to estimate the "strength" of the natural selection pressure. Having light coloured fur gives the paler Sand Hills mice a 0.5% survival advantage. "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." Rival icon In some respects, the dune-living deer mice are similar to the famous peppered moths of northern England. For decades, the peppered moths (Biston betularia) have been heralded as one of the best-examples known of a wild animal adapting to its environment due to natural selection. Originally, most peppered moths were lightly coloured, to blend with the lightly coloured bark of trees. Due to widespread pollution caused by the Industrial Revolution, soot blackened the trees and newly conspicuous lightly coloured moths were picked off by predators, a selection pressure that triggered the rise of more dark coloured moths. "In both species, changes in colour evolve rapidly due to selection by visually-hunting predators," says Prof Hoekstra. But the study by Dr Linnen and Prof Hoekstra's team takes our undertaking of natural selection to a much deeper level. The selection pressure on the moths was technically artificial, caused by pollution produced by people. Whereas the selection causing the pale mice is truly natural. What is more, the scientists have found the gene responsible, and worked out exactly how long it took to evolve and take hold in the population. "Despite the fact that the peppered has been an icon of 'evolution in action', we do not yet know the genetic changes involved," says Prof Hoekstra. "Once researchers find the pigmentation gene responsible for moth colour change, they can do the same types of analyses we have done. It will be really interesting to compare these estimates between mice and men."



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