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How weblike is the tree of life?

One of the most iconic symbols of evolution—the tree of life, a visual metaphor for the branching ancestry of species—has recently become one of its most controversial. The idea of a tree dates back to Charles Darwin himself. In January, a cover of New Scientist featured the tree emblazoned with the words “Darwin was Wrong,” referring to the past decade’s discoveries that single-celled organisms exchange genetic material in ways other than reproduction. Some scientists have suggested that this process, called lateral gene transfer, makes our tree of life really more of a “web of life.”

That New Scientist cover made repeated appearances at the University of Chicago’s 29-31 October “Darwin 2009” conference, where multiple speakers agreed that whatever the extent of lateral gene transfer, it’s not enough to obscure the overall treelike shape of evolution. “If the history of bacteria and eukarya were really a web, then the enterprise of finding a tree would fail, but it hasn’t,” said entomologist Philip Ward of the University of California, Davis. New research supports his case, including a forthcoming Nature paper by Martin Wu et al. that will show tree-based selection to be an effective way of identifying novel protein families, indicating that lateral gene transfer has likely redistributed genes only among closely related branches.

However, anti-Darwinians have seized on this controversy as prime ammunition in their attacks on evolution itself. “The creationists will drive you nuts—they’ll take this controversy to say there’s no universal common ancestry, therefore there’s no tree, therefore there’s no evolution,” said Eugenie Scott, executive director of the National Center for Science Education. Ironically, as Scott and other speakers pointed out, even as creationists claim the tree of life is dead, they have simultaneously adopted the model for themselves: Current dogma touts a “Creationist Orchard” of many separate trees, each with a trunk representing one of Noah’s pairs of passengers.