Nature Steers a Predictable Course

Elizabeth Pennisi

Science 14 January 2000: 207-209.

In Darwin's original formulation of his theory of evolution, he emphasized the importance of the local environment in shaping how organisms change through time. Over the past 2 decades, however, his assumption that natural selection, as it is known, is invariably the driving force of evolution has fallen somewhat out of favor. Some evolutionary theorists have argued that "genetic drift," random gene changes that accumulate over time, underlies the evolution of new species. Thus, even with natural selection, evolution's course should be rather unpredictable and not likely to be repeated time and time again, they concluded. But results reported in this issue by two independent teams indicate that natural selection seems to be as important as Darwin had thought, often overriding the randomness of genetic drift.

Both teams took advantage of nature's own evolutionary laboratory. Raymond Huey of the University of Washington, Seattle, and his colleagues studied a European fruit fly, Drosophila subobscura, that was introduced into California some 20 years ago. As the researchers report on page 308, they found that over the south-to-north range of the flies, the insects have evolved larger wings, a change that parallels what happened to this species in Europe.

Dolph Schluter of the University of British Columbia (UBC) in Vancouver and his colleagues studied a very different species, a stickleback fish living in three isolated lakes on British Columbia's Pacific coast. In work described on page 306, the researchers report that the same two species have formed in all three lakes. Each lake contains one with hefty, bottom-dwelling individuals and one with streamlined individuals that feed in the open water. Both studies provide strong evidence confirming "the importance and strength of natural selection as the major agent of evolutionary change," says Douglas Futuyma, an evolutionary biologist at the State University of New York, Stony Brook.

Even the entomologists who first noticed the distinctively black European fruit flies in California almost 20 years ago thought this species provided an opportunity to see evolution in action. But Huey and George Gilchrist, now an evolutionary biologist at Clarkson University in Potsdam, New York, and their colleagues were the first to test whether the flies evolved the same way in the New World as they had in the old. In 1997, they collected D. subobscura flies from 11 spots ranging from just north of Santa Barbara, California, to north of Vancouver. The following year, Huey and Spanish colleagues trapped the flies over roughly the same range of latitudes in Europe, traversing the continent from southern Spain to the middle of Denmark.

The team then raised the different populations of flies, providing the same food and living conditions for them all. After allowing a half-dozen generations to go by, the researchers measured the wing lengths--an indicator of overall body size--of flies from each locale. The results were striking, particularly in the females, says Gilchrist.

He and his colleagues saw an increase in wing size--to a 0.1-millimeter difference, or 4%--in the European flies collected from south to north. And they saw the same increase in the fruit flies from North America, even though the species had spent only a brief time on the continent. Indeed, Andrew Hendry of the University of Massachusetts, Amherst, who has recently completed a survey of evolutionary rates, says that the change "is as fast as I have ever seen. I think this will shake up a lot of people." The adaptive significance of the change is unclear. Still, says evolutionary biologist Jeff Mitton of the University of Colorado, Boulder, the fact that it occurred twice in similar environments makes for "a very clean and compelling story" in favor of natural selection.

The genetic basis of the change may be different in the European and North American versions of D. subobscura, however. Huey and his colleagues found that the European populations lengthened the part of the wing closest to the body, while those in North America extended the outer segment. The work shows that "there can be different ways of attaining the same outcome," notes Futuyma, and thus some aspects of evolution may still be random and unpredictable.

Schluter's team found that the sticklebacks they studied represent an even more dramatic case of parallel evolution. Originally of marine origin, the fish were trapped in coastal lakes formed some 10,000 years ago by a retreating glacier. The lakes are isolated from one another--indeed, two are located on separate islands along the coast--yet each of the three lakes wound up with the same two noninterbreeding varieties of stickleback, the bulky benthic type and the actively swimming limnetic type.

To understand the basis of the reproductive isolation, UBC's Laura Nagel, Janette Boughman, and Howard Rundle tested the mating preferences of the fish. They found that females choose males that look like themselves. For example, benthics mated with benthics, both from their own lake and the others, while shunning all limnetics. "Whatever it is that makes the benthics dislike the limnetics, it's happened over and over again," Schluter explains. That finding, adds Mitton, was "a real surprise" and shows that natural selection can yield new species.

The more researchers probe the corners of nature's laboratory, the more evidence they are likely to find supporting the importance of natural selection, Mitton says. For example, he sees repeated patterns of evolution in some traits of the pinyon pines that he studies. These examples "say that natural selection can cause a population to change very quickly and hint that speciation could [occur] very quickly," he notes. And that makes him even more sure that Darwin was right after all.

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