IMAGINE if a whale sprouted legs and crawled back onto the land. It鈥檚 not as crazy as it sounds: the lowly stick insect has proved that even more complex anatomical features can disappear and reappear over the course of evolution.
On a number of occasions in the past 300 million years, stick insects have lost their wings, then re-evolved them. Entomologists have described it as a 鈥渞evolutionary鈥� finding that will force them to rethink one of the rules of evolution.
Michael Whiting, an evolutionary biologist from Brigham Young University in Provo, Utah, and his team stumbled upon the finding while examining the DNA of 37 different phasmids, the stick and leaf insects famous for camouflaging themselves against plants, in a bid to work out their family tree.
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Entomologists have assumed that wings only evolved once in insects. The received wisdom is that a winged ancestor produced the winged phasmids we see today. The 60 per cent of stick insects that don鈥檛 sport wings will, this thinking goes, have jettisoned them along their evolutionary journey so they could expend more energy on reproduction and less on flying.
But Whiting鈥檚 analysis shows that the very first stick insect, which appeared 300 million years ago, had already lost its wings and that stick insects re-evolved the structures at least four times (see Graphic). The study covers only 14 of the 19 known sub-families of phasmids, so it鈥檚 possible that wings reappeared even more often (Nature, vol 421, p 264).
Researchers assumed wings couldn鈥檛 come back once lost as the genes needed to create them would mutate beyond repair once the wings disappeared. But Whiting says there is evidence from the fruit fly Drosophila that the same genes contain instructions for forming wings and legs. If the same were true for stick insects, there would be an evolutionary pressure to stop wing genes from mutating, even in the insects that didn鈥檛 have wings. Those genes could then be turned back on in the future.
Whiting says, however, that while wing re-evolution may seem unlikely to insect researchers, the basic idea of switching regulatory genes off and on is well accepted. Even a single gene can sometimes switch on the growth of a complex structure 鈥� studies indicate that a master gene called Pax-6, for example, might control the development of eyes in all creatures that have them. So Whiting suggests that eyes too could have disappeared and reappeared in animals over time. 鈥淚 remember sitting down with entomologists and hearing them say 鈥榠mpossible, impossible, impossible鈥�,鈥� he says. But 鈥渞e-evolution is probably more common than we thought鈥�.