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Island mice may evolve faster: From one species to six
By Bijal P. Trivedi

Janice Britton-Davidian spent several weeks in 1999 placing hundreds of mousetraps all over the semi-tropical island of Madeira and discovered what may be an example of "rapid evolution." She caught hundreds of small brown mice that look pretty much alike but that are genetically distinct—a very unusual thing for such a small, geographically contained place. It normally takes thousands to millions of years for one species of animal to diverge to become two. On Madeira, one species may have evolved into six in the space of just 500 years.

Britton-Davidian, an evolutionary biologist at Université Montpellier II in Montpellier, France, showed that populations of Maderian mice have between 22 and 30 chromosomes, even though their ancestors, who first arrived with the Portugese in the 15th century, had 40.

Madeira is a rugged volcanic island with sharp black cliffs that block all but a few isolated rocky shores. Only a few small villages decorate the strip of coast. The Portuguese were first to inhabit the island, bringing with them the mice that Britton-Davidian so avidly seeks. As the Portuguese founded small settlements around the island, they inadvertently deposited small groups of mice at each stop. And, for the last five centuries, mountainous barriers have prevented these coastal colonies of rodents from commingling.

Britton-Davidian collected hundreds of mice from about 40 locations around the island and found six distinct populations. The common brown house mouse of Europe, presumably the ancestor of the Madeira mice, has 40 chromosomes, but the six families of Madeiran mice have between 22 and 30.

The current families of Madeiran mice are not short of genetic material. They have not lost any DNA. What happened is this: over time, some of the chromosomes fused together, packing more DNA into some chromosomes. Each of the six unique populations of mice on Madeira has its own special assembly of fused chromosomes. Each group of mice may now be its own species.

The diversity of fused chromosomes seems to have occurred in just 500 years, or between 1,500-2,000 generations of mice, says Britton-Davidian. Furthermore, the huge diversity in chromosomes has evolved solely from geographic isolation rather than adaptations to different environments.

"What is surprising is how fast this has taken place," says Scott Edwards, an evolutionary biologist from the University of Washington, in Seattle. Based on fossil records of sea urchins and invertebrates, evolution of different species is thought to take thousands to millions of years. "But this is an interesting case because it may prove to be an extreme case of rapid speciation," says Edwards.

Britton-Davidian wants to know whether these populations of mice have evolved into different species or whether they are on the cusp of speciation. A species is defined as a group of organisms that can mate and produce fertile offspring.

One of Britton-Davidian's most surprising findings is that she and her colleagues found no mice that are hybrids among any of the six groups. "This might be because the hybrids are infertile or they may be less fit than the parents and unable to survive," says Britton-Davidian. Other explanations could be that the groups have been geographically isolated and have not had the chance to mate, or that the mice "recognize each other as different and choose not to mate."

Britton-Davidian has taken some mice from Madeira back to her lab in France and will try interbreeding the six populations to confirm whether the hybrid mice are infertile, which, if they are, would imply that the different groups were in the process of speciation. Her team will also observe the mice to see whether they show behavioral or physical differences.

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Britton-Davidian, J. et al. Rapid chromosomal evolution in island mice. Nature 403, 158 (January 13, 2000).

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