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.

The bacteria selectively kills male Blue Moons before they can hatch

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 developed 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, whose study appears in the journal Science.

Rapid natural selection

Gregory Hurst, a University College researcher who worked with Mr Charlat said: "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 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 butterfly had either reached or were approaching those of females.

The researchers are not sure whether the gene that suppressed the parasite emerged from a mutation in the local population or whether it was introduced by migratory Southeast Asian butterflies in which the mutation existed.

But they said that the repopulation of male butterflies illustrates rapid natural selection, a process in which traits that help a species survive become more prominent in a population.

"We're witnessing an evolutionary arms race between the parasite and the host. This strengthens the view that parasites can be major drivers in evolution," Mr Charlat said.