A team of biologists says it has found what is quite likely the first-ever documented case of "re-evolution," suggesting that nature does indeed offer second chances -- a species can evolve a new characteristic, lose it and then regain it.
That's a radical idea, because for most of modern times, scientists have taught that evolution, at least in part, functions on the principle of "use it or lose it." This is one reason seals no longer have paws, moles see badly and humans lack heavy fur.
The team is challenging that assumption based on its analysis of DNA from 37 species of the insect order Phasmatodea -- commonly known as "walking sticks" -- which showed that they evolved from winged to wingless and back again. In fact, walking sticks made the shift four times.
The discovery calls into question one of the tenets of evolutionary biology: that if a species loses a complex characteristic, the gene or genes that express it will subsequently mutate so much that the function can never be recovered.
"We were shocked," said team leader Michael F. Whiting, an evolutionary biologist from Brigham Young University. "Even though there is no empirical evidence, it has been dogma for two centuries that something like flight requires so many complicated systems that it could only be evolved once, and would be very difficult to reinvent."
The study shows that "somehow this whole developmental problem can be switched on and switched off," said Pennsylvania State University biologist James Marden. "That's cool, and not just cool for insects. That's cool across the board."
Although walking sticks exist around the world, including in the Washington area, Whiting said he did most of his research in New Guinea, home to a large selection of the insects. Walking sticks number more than 3,200 species worldwide and come in both winged and wingless varieties.
They are big bugs. The smallest is about the size of a person's pinkie, while the largest -- about 18 inches long -- is the longest insect in the world. Walking sticks survive by using natural camouflage that makes them look like sticks, leaves, tree bark, shoots of grass or reeds. Whiting said the 18-inch walking stick hangs from a branch and sways like a dead stick.
The walking sticks' closest relative is the "web spinner," which sprays webs from its front feet -- like Spiderman. Other near relations include cockroaches, termites, mantises, grasshoppers and earwigs.
In its analysis, the team examined three genes from winged and wingless walking sticks. The analysis enabled the team to rank those species from most primitive to least primitive. The DNA from the most primitive species most closely matched that of the web spinners, pointing toward a common ancestor for both insects.
"The thought was that the insects that did not have wings were probably the most advanced," Whiting said. Insects can lose their wings for several reasons, he explained. For parasites, such as fleas or lice, wings are superfluous and awkward. And cold-weather insects shed their wings to decrease the surface area of their bodies.
For walking sticks, the theory held, being wingless meant that the females, at least, could devote greater energy to egg production. Also, while primitive walking sticks would have needed wings to get away from predators, more advanced species would have developed such good disguises that they would no longer need to fly.
The first surprise was that all of the most primitive walking sticks, unlike the web spinners next to them in the evolutionary tree, were wingless, Whiting said. Much further along, however, winged species reappeared. Subsequently, winged species disappeared and reappeared three more times.
"The inference is that the wingless ones evolved from a common ancestor that had wings," Whiting said. "The primitive species lost their wings, but 50 [million] to 100 million years later, more advanced species regained their wings." Whiting said walking sticks probably made their first appearance 300 million to 350 million years ago.
The findings did not come as a surprise to evolutionary biologist Piotr Naskrecki, of the nonprofit Conservation International. Naskrecki said he had observed similar behavior in other species and found it "very difficult to believe that this research is the first documented case of a complex feature being lost and recovered."
The more interesting implication of the study, he suggested, is that "there is a mechanism that preserves the genetic ability" to create wings, without becoming hopelessly corrupted through mutation.
"There must be something," Naskrecki said. "I just can't imagine what that mechanism would be."
Penn State's Marden suggested that the wing genes might avoid mutation if they were used for a related function. "Most of the genes involved in wing development are also involved in leg development," he said. Perhaps making legs kept the wing genes from atrophying, he said, enabling them to be summoned for a rerun 50 million years later.