The world is filled with examples of introduced exotic species such as African walking fish in Florida, rabbits in Australia and kudzu throughout the southeastern United States decimating native species and upsetting ecosystems. Until now, biologists studying these events have not seriously considered evolution as part of the strategy used by invading species.
But new evidence stemming from the accidental introduction into North and South America of an Old World fruit fly, which has exhibited one of fastest evolutionary changes ever recorded, may alter that perception.
Writing in this week's issue of the journal Science, a team of researchers from the University of Washington, Clarkson University in New York and the University of Barcelona report that the fly, Drosophila subobscura, which was introduced about two decades ago, has already evolved a wing size pattern that mimics that of established populations in Europe In the process, D. subobscura, a black fly less than one-eighth of an inch long, seems to be replacing native fruit flies in the Pacific Northwest.
"Humans are introducing plants and animals all around the globe and, in many cases, those introductions are wreaking havoc on native populations," said Raymond Huey, a UW evolutionary biologist and lead author of the study. "The dynamics of invasions become much more complicated if the invaders evolve rapidly. It is very likely that introduced invading species are evolving, and scientists typically have not appreciated how fast evolution can occur in an introduced species. This probably means native populations will be changing as well in response to introduced species."
D. subobscura is a temperate-zone fly native to a region stretching from Spain northward into southern Scandinavia and from North Africa eastward to the Middle East. In South America, it was first found near the Chilean port city of Puerto Montt in 1978. The flies quickly colonized much of coastal Chile, although scientists detected no change in wing length a decade later. They were initially discovered in North America in the early 1990s in Port Townsend, Wash.
Both introduced populations are nearly identical genetically, leading geneticists to believe they stem from a common stock that hitchhiked on a ship that probably stopped in Chile and in North America sometime around 1978. Since then, the flies have spread over an area spanning more than 16 degrees of latitude in Chile and across the Andes Mountains into coastal Argentina. In North America, D. subobscura has spread from Santa Barbara, Calif., north to the tip of Vancouver Island in Canada. They also have been trapped as far east as near Salt Lake City.
To check for evolutionary change, Huey and his colleagues trapped flies at 11 North American sites in 1997 and 10 in Spain, France, the Netherlands and Denmark in 1998. Then they raised five or six generations of flies in similar environments, measuring the wing lengths of the last generation. Biologists use wing length because it is an easily measured and highly repeatable index of body size.
D. subobscura has occupied temperate Europe for about 10,000 years, since the last ice age, and exhibits a geographic pattern of bigger individuals in areas of higher latitude. Thus wings of flies in Denmark are 4 percent longer than those from Spain. Huey said this probably has some connection with temperature, but the precise reason has yet to be determined.
Measurements of the North American flies revealed that these introduced flies had evolved a similar pattern in less than two decades. Like the Europeans, higher latitude female flies were four percent larger than those from central California. For males, however, the size difference was only 1 to 2 percent. Even so, the two continental populations show subtle differences in the part of the wing that lengthens with increased latitude. European flies have what could be called longer "biceps" while North American flies have longer "forearms."
Stocks of South American D. subobscura recently were obtained but have not yet been studied to see if they too have evolved geographic patterns in size.
Huey said the only other recorded case of a more rapid evolutionary change in size and shape was weather induced and did not involve an introduced species. In that case, Galapagos Island finches were subjected to a severe drought in 1978. Only finches with big, massive bills survived and passed that trait to the next generation.
He said that D. subobscura has more than decimated the native species, D. psuedo-obscura, in Northwest cities.
"If I collected fruit flies in my Seattle backyard, at least 90 percent would be D. subobscura. This is an extremely successful and still spreading introduction and I would bet they may show up in New York City in a couple of years," Huey said.
"These flies evolved quickly and their evolution closely matched to the European pattern. This observation of very rapid evolution may prod ecologists to realize that invading species are not set in stone but can evolve quickly in response to their new environment, thereby changing the dynamics of their interactions with native species. Previously scientists studying invasions have largely assumed that evolution can be ignored because it was thought to occur so slowly relative to the dynamics of invasion. This study shows that an invader can in fact evolve very quickly, in just a few years, and potentially have a big impact," he said.
Other members of the research team are George Gilchrist, a biologist who earned his doctorate at the UW and is now an assistant professor of biology at Clarkson University; Margen Carlson, a UW undergraduate student majoring in zoology; David Berrigan, a scientist at the National Cancer Institute who did post doctoral research at the UW; and Luis Serra, an evolutionary geneticist at the University of Barcelona.
An image showing the difference in wing size between two European D. subobscura can be downloaded at http://www.washington.edu/newsroom/news/images/flies.jpg The image shows females from Valencia, Spain, on the left and Aarhus, Denmark, on the right. Please credit the photo to George Gilchrist, Clarkson University.
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