Butterfly shows evolution at work

Scientists say they have seen one of the fastest evolutionary changes ever observed in a species of butterfly. The tropical blue moon butterfly has developed a way of fighting back against parasitic bacteria. Six years ago, males accounted for just 1% of the blue moon population on two islands in the South Pacific. But by last year, the butterflies had evolved a gene to keep the bacteria in check and male numbers were up to about 40% of the population. Scientists believe the comeback is due to "suppressor" genes that control the Wolbachia bacteria that is passed down from the mother and kills the male embryos before they hatch. "To my knowledge, this is the fastest evolutionary change that has ever been observed," said Sylvain Charlat, of University College London, UK, whose study appears in the journal Science. Gregory Hurst, a University College researcher who worked with Mr Charlat, added: "We usually think of natural selection as acting slowly, over hundreds of thousands of years.

"But the example in this study happened in the blink of the eye, in terms of evolutionary time, and is a remarkable thing to get to observe." The team first documented the massive imbalance in the sex ratio of the blue moon butterfly (Hypolimnas bolina) on the Samoan islands of Savaii and Upolu in 2001. In 2006, they started a new survey after an increase in reports of male sightings at Upolo. They found that the numbers of male butterflies had either reached or were approaching those of females.