Scientists upset insect orthodoxies.

A new study on insect evolution has set scientists buzzing. Its findings challenge the conventional wisdom that insect diversity exploded about 125 million years ago in response to the rapid spread of flowering plants. "This is going to be a hotly discussed paper, the opening shot in a good contention," asserts Leo J. Hickey, a paleobotanist at Yale University 

Insect diversity preceded that of flowering plants, called angiosperms, by 120 million years, according to the report. In fact, "insects may have spurred angiosperm diversity rather than the other way around," says coauthor Conrad C. Labandeira, a paleoentomologist at the Smithsonian Institution in Washington, D.C. He and J. John Sepkoski Jr., a paleontologist at the University of Chicago, report on this and other new twists in insect evolution in the July 16 SCIENCE.

Labandeira and Sepkoski traced the evolution of 1,263 insect families based on data gleaned from published studies around the world. The picture they assembled begins 390 million years ago with the first wingless insects. Then, about 325 million years ago, insect evolution took off. Winged groups evolved rapidly., as did insect families with other innovative body parts and the ability to metamorphose. These insects were evolving along with the primitive flora that predated flowering plants, says Labandeira. But the process was cut off 240 million years ago, at the end of the Permian period, by the worst mass extinction in Earth's history It wiped out eight of 27 major orders of insects.

Following the crisis, however, the remaining insect groups "took off like compound interest in the bank" and continued to diversify exponentially until 125 million years ago, says Sepkoski. At that time, when flowering plants began to multiply, insect diversity slowed down. "That's what was so surprising," he says.

Furthermore, the scientists conclude that today's remarkable diversity of insects results from low extinction rates and not, as generally thought, from high evolution rates.

The two researchers also examined the history of how insects feed. The scientists grouped insect mouthparts into 34 classes, such as rasping, hooking, and sucking. They discovered that insects were poised to exploit the new food sources provided by angiosperms: 85 percent of the mouthpart types had evolved prior to the advance of flowering plants.

The view that the coevolution of insects and plants began with the proliferation of angiosperms 120 million years ago needs to be revised, Labandeira says. "The basic building blocks of coevolution go back at least to the Permian extinction," he contends.

Some scientists see even deeper implications in the study. To Hickey, the study implies that the driving force of insect diversity was "a simple increase in the number of species, controlled by a slow rate of extinction relative to evolution. They increased exponentially through time until coming to saturation. The appearance of flowering plants was incidental." This contrasts with the view that diversity is the result "of the complexity of the entire [system] of organisms present," he says.

The new data "should encourage everyone to rethink the implications of coevolution," argues David M. Raup, a paleontologist at the University of Chicago. It may be that "many of the larger patterns in evolution are due to chance" rather than to specific interactions between organisms, he adds.

Some take issue with this view, in part because of the study's design. They point out that the research analyzed diversity at the family level, a more general tax-onomic grouping than species. "Families are a systematic convention invented for human convenience. But in nature, species interact with other species," says May R. Berenbaum, an entomologist at the University of Illinois at Urbana-Champaign.

Steven M. Stanley, a paleobiologist at Johns Hopkins University in Baltimore, is not surprised that the spread of flowering plants failed to produce new insect families. "What it undoubtedly produced was large numbers of species on a global scale. No one should extrapolate this pattern to the species level," he says.

Sepkoski says the question is not whether coevolution occurred, but "whether it has been of fundamental influence in the overall diversification of insects."