A couple of months ago beetles were demoted. Biologists had long thought these insects were life's most diverse order, but according to a new study in Philosophical Transactions of the Royal Society B, that honor now goes to flies. The finding has led to tension within the taxonomy community—part of an ongoing debate about how to define a species.
The fly's new designation took place after scientists at the University of Guelph in Ontario analyzed more than one million insects using DNA bar coding, a computerized taxonomic method that identifies a genetic profile from a bit of an organism's DNA. That profile is then given a bar-code index number, or BIN, which represents a species. The scientists found that one family of flies had 16,000 BINs, a 10-fold increase from previous estimates. If extrapolated worldwide, this finding could “reverse a long-held view of what life is like on our planet,” says lead author Paul D. N. Hebert.
But many traditional taxonomists disagree with the idea that a BIN is equivalent to a species. This paper “highlights a truly fundamental difference in how they categorize biodiversity, such that their numbers and ours can differ by more than an order of magnitude,” says Doug Yanega, an entomologist and traditionalist at the University of California, Riverside. “That's a really astonishing scale of difference. It's like we're looking at different planets.”
The traditionalists, who largely classify species by examining and comparing physical specimens, argue that bar coding can help place organisms in taxonomic orders and families but that it lacks the resolution for categorizing species on its own. In fact, the taxonomy community has already embraced molecular phylogeny, which often uses DNA to figure out evolutionary relationships. “DNA bar coding is not a substitute for traditional taxonomy,” says Andrew V. Z. Brower, a professor of biology at Middle Tennessee State University. “All it does is flag problems that need to be investigated by taxonomists.”
Still, bar coders—a small but influential group in the field—say the method is accurate and point to studies in which the number of species determined by DNA bar coding matches prior counts. Such is the case with European beetles. Some scientists have also embraced the technology because it allows huge volumes of DNA to be analyzed quickly and cheaply, dramatically increasing knowledge about biodiversity at a time when human activities imminently threaten still undiscovered species. “We can't afford to wait [for traditional taxonomic methods],” Hebert says. “We're at a big risk of burning the book of life before we even read it.”
The overall consensus is that bar coding has raised legitimate questions. But many taxonomists remain reluctant to correlate BINs to individual species until the technology is refined further. “It's a powerful tool for assessing a big biodiversity picture, but when you see DNA bar codes as species, you run into problems,” says DeeAnn Reeder, a biologist at Bucknell University. “Bar coding is still a brave new world.” For now, perhaps the only certain things in this debate are death and taxa.