Modified Mouse
Ever since a 2000 study on fruit flies showed that genes can evolve over decades rather than millennia, scientists have been searching for evidence of rapid evolution elsewhere. It seems to have scurried onto the world stage as the white-footed mouse of Chicago Wilderness.

Appearing in the journal Nature this past May, a study by Oliver Pergams of the Brookfield Zoo, Wayne Barnes of Washington University in St. Louis, and Dennis Nyberg of the University of Illinois at Chicago has uncovered a dramatic shift over the past 150 years in the genes of the white-footed mouse, Peromyscus leucopus, in northeastern Illinois.

The scientists searched museums as far away as Alaska and Switzerland for specimens that originated in Chicago Wilderness. They located 56 such museum mice, collected as early as 1855, and took small strips of tissue from them. Then they trapped 52 live white-footed mice in the same five Chicago Wilderness locations that the museum specimens came from.

The researchers determined the sequence of the mitochondrial DNA in all the samples. Working with a specific section of DNA, the researchers found two unique DNA sequences (called haplotypes), labeling one "A" (for ancient) and one "M" (for modern). Comparing the sequences in mice from the 1800s to those of modern mice, they saw something remarkable: the proportion of A haplotypes to M haplotypes gradually switched from all A's to almost all M's. That is, the genetic sequence most common 150 years ago has now become very rare. The mice with the different haplotypes don't look any different from each other, Pergams points out, but the differing haplotypes "are likely to be markers of changes elsewhere in the P. leucopus genes."

"Where did this new haplotype come from?" Pergams asks. Given the undisputed backdrop of European settlement and expansion, he says, this major historical shift may have played a role. If it did, then one possible explanation is that settlers brought the M haplotype with them, in mice from outside the region — an invasive genotype. The second possibility is that the M haplotype already existed in local mice in low frequency, and that European expansion gave M some advantage over A.

Whatever the case, such rapid evolutionary shifts could have implications for other native species. The change in genotype in the white-footed mouse, for instance, may have negatively affected populations of the once-plentiful prairie deer mouse, P. maniculatus bairdii (see "White-Footed Mice," CW, Winter 2000). Pergams and Nyberg have done two other studies documenting the decline of the prairie deer mouse in Chicago Wilderness. One study used museum data to predict the local disappearance of the prairie deer mouse in the year 2009. The second study began to test this prediction by setting out live-traps for prairie deer mice in locations where they had previously been found, as well as in their preferred habitat, high-quality prairies. Though Pergams and Nyberg caught 477 small mammals, including 252 white-footed mice, they caught only a single prairie deer mouse. Pergams theorizes that M white-footed mice may have been better able to compete with prairie deer mice for resources.

On the other hand, Pergams speculates, hard times for the prairie mouse could be due more to loss of prairies and the suppression of natural prairie fires since European settlement. The prairie deer mouse lives in burrows among the deep roots of prairie plants, thus surviving fires well. The white-footed mouse, on the other hand, does well in the woody plant growth that has advanced on many remaining prairies in the absence of fires.

— Mary Carvlin