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Clarkson Biologist's Research Into Speed Of Evolution Carries Major Ecological Implications
[A photograph for newspaper use is available at http://www.clarkson.edu/news/photos/flies.jpg.]
Potsdam, N.Y. – When Charles Darwin formulated his theory of evolution, he emphasized the importance of the way local environment affects how an organism changes over time. But often such change— or “natural selection”— occurs so slowly that it is impossible to observe.
Now, however, breakthrough research by Clarkson University Professor George Gilchrist and scientists from the University of Washington and the University of Barcelona suggests that such change can occur much faster than previously believed— and therefore carries enormous implications for ecological issues such as species invasion. Their work comparing the evolution of European and North American fruit flies appears in the Jan. 14 issue of Science.
The team of evolutionary biologists studied a native European fruit fly, Drosophila subobscura, which was accidentally introduced to the west coasts of North and South America 20 years ago. As the North American invasion spread over an area from southern California to British Columbia, the flies living farther north evolved larger wings, a pattern similar to one found among the flies living in Europe since the last ice age. Biologists measure wing length because it is an easily measured and repeatable index of body size.
In 1997, Gilchrist and his colleagues collected flies at 11 sites from Santa Barbara to the northern tip of Vancouver Island. The next year they captured flies in similar latitudes in Europe at 10 sites from southern Spain to Denmark. They then bred the flies in the laboratory, keeping each population of the European and North American flies separate, but giving them similar living conditions and food.
After six generations, the biologists measured the flies’ wing length. The results were striking. They found a gradual increase in wing size of the flies collected from south to north in both the European and North American flies— despite the brief time the North American flies had been on the continent.
“Virtually all animal species exhibit an increase in size as you collect toward the poles,” says Gilchrist, “but we assumed this pattern had evolved over thousands of years. To see this magnitude of genetic change occur in just two decades is astonishing. It suggests that evolution may be important in determining the impact of other invading species such as zebra mussels and purple loosestrife on native populations.”
Although the precise environmental factors that favor large body size at high latitudes are still not clear, the fact that it occurred twice under similar natural conditions makes a good argument for the importance of natural selection and the repeatability of evolution. Gilchrist and his colleagues have just returned from collecting Drosophila subobscura in Chile and, in a few months, will see if rapid evolution of a similar pattern has occurred in the southern hemisphere.
Although the general pattern of size change is repeated across the two continents, the details of how the wings have elongated reveal a surprising difference. The fly wing is divided in half by a horizontal vein, in the way that the human arm divides at the elbow, into upper arm and forearm. In Europe, most of the latitudinal variation involves elongation of the “upper arm”, whereas in North America most of the change is in the “forearm.”
“It will be especially interesting to see if the South American flies show the same pattern of elongation as the North American flies,” notes Gilchrist.
Species introductions are currently reshaping much of Earth’s biological diversity. “We know a lot about the ecological factors affecting biological invasions, but very little about the evolutionary changes taking place,” says Gilchrist. “Globally, species introductions are pushing many native species towards extinction. The rapid changes we see in invading Drosophila subobscura suggest that conservation programs may need to consider how evolution may affect competition between native and introduced species.”
Photo caption: This image shows the difference in wing size between two female European Drosophila subobscura. The specimen on the left came from Valencia, Spain, and the one on the right was taken from Aarhus, Denmark. (Photo by George Gilchrist, Clarkson University)