POSTED 29 MAY 2003

Evolution among fish.

The Galapagos Islands.

City critters.

Oliver Pergams

The white-footed Mouse (Peromyscus leucopus) is the most common rodent in the deciduous forests of North America. It prefers wooded and brushy areas, although sometimes -- as the new study shows -- it sometimes lives in more open ground. Photo by Jim Schulz, courtesy of the Brookfield Zoo.

But sometimes, as any pigeon watcher knows, a species can thrive amid the flurry of human activity. When cities grow, some animals take over where others, who can't adapt in time, move out. Lucky for one Midwestern mouse, it doesn't always take eons to evolve.

Just ask Oliver Pergams. As the University of Illinois biologist (with co-authors Dennis Nyberg and Wayne Barnes) reported in a recent issue of the journal Nature, a remarkable show of natural selection has played out among the white-footed mice of Chicago - in just 150 years. It's very rare to see evolutionary change among mammals in such a short timespan. And, no surprise here, the impetus was probably people.

You can take the mouse out of the country...
But can you take the country out of the mouse? In Aesop's classic fable, a country mouse visits his friend the sophisticated urbanite, and decides he prefers the comfortable security of rural life to the unpredictable dangers of town. But in the windy city, just the opposite may have happened.

In the early 1800s, before the land around Chicago was put to the plow, the land consisted of prairies scattered with woodlands and wetlands. The prairie deer mouse ruled supreme in the prairies, and the white-footed mouse dominated the woodlands.

But the human population grew quickly, and eventually the prairies were fewer and farms and condos were more, so the prairie deer mouse lost its habitat. Today there is less than one percent of the prairie that existed before European settlement. What's more, fire suppression allowed woody vegetation to move into the prairies.

Downtown Chicago: NASA.

Along with the woody vegetation came white-footed mice, further driving the prairie deer mouse out of its remaining habitat. By 1970, fewer than five percent of the wild mouse population (excluding the common house mouse, which is another story entirely) were prairie deer mice. The change happened in mere decades.

The white-footed mouse seems right at home in Chicago. But how has the forest-loving species survived as the city has grown? One way to get at the question is to measure how the animals' genes have changed over time to match the changing environment. If there are big genetic differences between modern white-footed mice and their older relatives, the reason is probably natural selection.

Using museum collections -- some as near as Chicago's Field Museum of Natural History and others from as far away as Switzerland -- Pergams compared museum specimens of white-footed mice captured in Chicago from 1855-1974 to those he trapped in prairie remnants in 1999-2000.

In all, Pergams found 61 museum skins and ear samples from 52 live-trapped mice. Because some of the museum skins were withered and old, extracting the DNA from them proved difficult. So Pergams used DNA from the animals' mitochondria, the cellular powerhouses that mine energy from sugar.

The sequence of a gene is a series of letters that represent the four components of a DNA molecule. The letters A, C, G and T make up this shorthand, so that a sequence looks something like this: ...ACCGTACGATATGTC... Each gene - which can be hundreds or thousands of base pairs long - codes for one protein. Since similar sequences can sometimes code for the same protein, scientists say the genetic code is "redundant." Because of this quality, one gene can have several haplotypes (exact sequences) that produce the same protein product. Double helix graphic: NASA.

Every cell holds thousands of copies of each mitochondrial gene, whereas there are only one or two copies of DNA in the nucleus, where chromosomes are located. Pergams sequenced a stretch of mitochodrial DNA (part of a gene called the cytochrome oxidase c subunit II gene) in all the mice.

Of mice and genes
Pergams noticed some striking changes in the new mice. He observed two common sequences, or haplotypes, of the DNA fragment in the mice. By definition, each haplotype has a unique series of base pairs (the "letters" that together make up a DNA message). And when Pergams looked at how the haplotypes were distributed over time, he found that one form -- which he dubbed haplotype M -- has become increasingly common.

Pergams trapped mice from the same five spots (indicated by arrows) from which researchers collected museum specimens a hundred years ago. The graphs show how the new haplotype (dark blue) has replaced older haplotypes (light blue and red) over time. Courtesy Oliver Pergams.

The first mouse with haplotype M was captured in 1906 at Volo Bog, about 55 miles northwest of the city. Yet among the skins collected before 1950, haplotype M was very rare. Most of the white-footed mice in those years had the now-rare haplotype M. Now, nearly all white-footed mice in the Chicago region appear to have the new version.

But why? You might be tempted to think the newer sequence codes for some profitable new protein. But in fact, haplotype M churns out proteins identical to those produced by the other haplotypes. The new sequence, in this case, doesn't provide a competitive advantage over the old.

It might, however, reflect changes elsewhere in the genome, some of which may give the modern mice an advantage in city life. In this case, the changes are likely not the result of mutations, Pergams says. A mere 150 years is simply not enough time for a species to accumulate meaningful adaptive changes in this way.

No matter. "Evolution occurs in populations, not in individuals," Pergams notes. Just as the transition from Homo sapiens neanderthalensis to Homo sapiens sapiens involved migration and competition, mice populations too wrangle with each other for attractive territory. Evolution can occur when gainful mutations accumulate slowly in a stable population. But it also happens when sudden environmental changes favor some members of a population but not others.

"This replacement in a population of one genome by another is the modern definition of evolution," Pergams says.

Mice with the newer haplotype (and other advantageous differences) may have wandered into the area and out-competed mice with the old haplotype. Or they may have been introduced by people. Either way, says Pergams, "evolution is expected to occur over thousands of years. We've seen it in 150."

The moral? In evolution only one thing is certain: The story never ends.

-- Sarah Goforth

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