The fluttering wing of an insect is a gossamer marvel capable of such power and precision that it is thought to be one of the chief innovations that allowed them to become the soaring rulers of this age.
So intricate are the nerves and muscles of these aviational workings, not to mention the wing itself, that entomologists have long assumed that when a lineage of insects evolves to become wingless, its descendants can never again regain this complex machinery. Instead, the assumption went, the entire lineage would be grounded into evolutionary perpetuity.
So it has been with great surprise that researchers have greeted a paper in the current Nature in which an international team of researchers reports evidence that wingless stick insects have re-evolved wings at least four times in the history of the group.
"When I first described these results to entomologists," said Dr. Michael F. Whiting, an evolutionary biologist at Brigham Young University and the lead author of the paper, "the response was, `Impossible, impossible, impossible.' "
Some scientists, Dr. Whiting recalled, told him that it was such an absurd proposition that no matter what evidence he might muster, they could never accept it.
Beyond perplexing entomologists, the new study is intriguing evolutionary biologists, because it suggests that not only wings, but also any complex characters like limbs or eyes that can be lost over evolutionary time can likewise re-evolve.
"The thinking has always been that these losses are an evolutionary dead-end," Dr. Joel Kingsolver, an evolutionary biologist at the University of North Carolina, said. "But maybe it's not so hard to regenerate the structure, after all."
Researchers have always assumed that when ancient winged insect ancestors gave rise to the earliest lineages of stick insects that these most primitive species were winged and that stick insects lost their wings only later in lineages that would become evolutionary blind alleys. But when the researchers used DNA sequence information to create a family tree of the stick insects, they found the opposite. All the oldest groups were wingless, meaning that the winged stick insects had actually arisen later by re-evolving wings.
"I had an undergraduate working on the project," Dr. Whiting said about Taylor Maxwell, who is a co-author along with a graduate student, Sven Bradler. "I was sure he'd messed everything up."
But when researchers checked his work, they found that the results were solid.
Researchers find the study hard to accept because dogma has held that after wings are no longer made, the genetic machinery to build wings should no longer be under natural selection.
So, the thinking went, those genes should mutate freely, obliterating their ability to produce the equipment needed to fly.
Nevertheless, the re-evolution result has held up, and the team has expanded its study, from the 37 species in the new paper to 150 and counting. The authors note that the new study does not speak to the biggest question about such complex features — their origin. That is, no cycles of losing and regaining of wings can explain how insects first evolved their wings.
Researchers said the findings suggested that the genetic machinery remained intact but latent in stick insects, possibly because that machinery is involved in developing features that the wingless insects continue to grow like legs. The team noted that the same could be true of other genetic machinery for other complex features.
The one group of researchers that has taken the new finding in stride is developmental biologists. They are used to dealing with mutants on which a leg will sprout where a mouth part usually grows or a pair of wings will appear on an insect's back where two golf-club-like navigational organs are usually found.
Well aware of the potential for complex structures to appear where they have not appeared for some long evolutionary time, or perhaps ever, such researchers are not particularly shocked to see any such feature, even the hallowed wing, flash on and off easily over evolutionary time.
Why might the earliest stick insects have lost their wings? Researchers speculate that for a creature that is always trying to look like an immobile twig, flying may not be the most advantageous behavior.
Wings could have re-evolved later in the evolution of the group, scientists suggest, when it became more advantageous for the stick insects to abandon their botanical disguises on occasion, perhaps to fly off to find new food sources or to seek out distant mates.
