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When bugs fly. Wingless insects, like this darkling beetle (Apterotheca costata), don't become genetically isolated more often than their winged counterparts do.

Evolution's No-Fly Zone

Animal populations that become isolated by rivers and other geographic barriers often evolve into new species. So you might expect that wingless insects, which should have trouble surmounting such barriers, would be particularly prone to this kind of speciation. But a new study in this month's Journal of Evolutionary Biology shows that speciation is no more likely for infrequent flyers.

Ever since Alfred Russel Wallace saw separate species of monkeys on either side of the Amazon River in the 19th century, evolutionary biologists have been fascinated by "vicariant speciation." This theory states that when animals get stuck behind geographical barriers like big rivers, valleys, or mountain ranges, adaptation and random genetic changes cause them to drift apart and become new species. As a result, scientists had always thought that vicariant speciation would be more common in less mobile animals.

Systematic entomologist Pat Bouchard of Agriculture and Agri-Food Canada in Ottawa and evolutionary biologist Daniel Brooks of the University of Toronto, Canada, addressed this theory by studying 13 groups of beetles and true bugs (Heteroptera) from a rainforest in northeastern Australia. Of the 79 species studied, 60 were flightless. The researchers compared evolutionary trees for each group, based on the insects' DNA and morphology, with the range of each species.Wings made little difference for speciation. About one-third of both winged and flightless insects had become new species when their range was sliced up by a geographical barrier--in this case, a dry valley. In addition, the absence of wings did not appear to hamper dispersal, as winged and nonwinged insects were each present on an average of two mountain plateaus, all of which were roughly the same size. Apparently, says Brooks, "flightless insects have other means of dispersal than flying, and flying insects do not fly as far as they might." He adds that insects may fly more for finding food and mates than for long-distance travel.Evolutionary biologist Arne Mooers of Simon Fraser University in Vancouver, Canada, who uses evolutionary trees to unravel how and how often species split, says the fact that winged and flightless insects have similar ranges is "intriguing." But he is not convinced that a lack of wings would not promote vicariant speciation. "They haven't sold me yet," he says.

Related sites
Charles Smith's essay on historical biogeography
Daniel Brooks's Web site
Pat Bouchard's Web site
Australian government's page on the Wet Tropics World Heritage site
The true bugs