Stick insects may have done what biologists once thought was impossible: lose something as complicated as a wing in the course of evolution but recover it millions of years later.
That’s not supposed to happen with so-called complex traits, at least according to a long-reigning view of evolution, says Michael F. Whiting of Brigham Young University in Provo, Utah. Yet that’s the story revealed by a new family tree based on DNA data from 37 species of stick insects, he and his colleagues report in the Jan. 16 Nature.
“We were very shocked,” says Whiting. He predicts that if biologists start looking closely, they’ll find complex traits that have vanished and reappeared in other creatures.
Anthony Zera, who studies insect-wing development at the University of Nebraska in Lincoln, says that if the new stick-insect family tree turns out to be correct, the result has wide implications for evolutionary biology. “It would change the way I would think about complex traits,” he says.
Biologists have long theorized that evolution has no rewind button, explains Whiting. When an animal depends for survival on a structure, such as the wing, any mutations that sabotage it get cleaned out of the gene pool quickly. However, if circumstances change so that the animal can get along flightless–and then some genetic quirk stops wing formation–harmful mutations build up unchecked in the rest of the wing-production pathway.
Whiting says that he didn’t set out to challenge this dogma. He and his colleagues are working their way through an ambitious project to use DNA data to construct a new comprehensive family tree of all insects. One group focused on stick insects, which often mimic twigs or other parts of plants. This team found that the stick-insect family tree appeared to have sprouted upside down. The 16 species that provide the lower branches, representing the more ancient lineages, had the supposedly new-fangled characteristic of winglessness. Then, at the top of the DNA tree, wings appeared, seeming to have re-evolved at least four times, after up to 50 million years of winglessness.
“It’s as if a mammalogist found a whale [a former land creature] walking around on legs,” says Whiting.
He proposes that the wings break the old rule because the genetic components of their production pathway may do double duty in other valuable life processes. Thus, these other functions would edit out any harmful mutations.
Whiting and his colleagues have made a big claim, and other scientists are reacting with caution and lots of questions. William Jeffery of the University of Maryland at College Park says that he wants to know more about just how completely the wings had disappeared. He points out that blind cavefish, which he studies, still develop some of the embryonic precursors of eyes. For wings, he says, he’s prepared to be impressed if even developmental traces had disappeared.
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