In 2001, male Hypolimnas bolina butterflies on the Samoan islands of Savaii and Upolu were extremely rare. Just 1 percent of these butterflies -- known commonly as Blue Moon or Great Eggfly -- were male. They were under attack by the Wolbachia bacteria, a parasite passed down through the female that kills off male butterflies before they can hatch.

Last year, the numbers of males had either reached or were approaching those of females. They were helped by the development of a genetic mutation that suppresses the bacteria, sparing the males and allowing them to quickly repopulate.

"This is one of the most clear and fastest cases of evolution under natural selection," said Sylvain Charlat of University College London, whose study appears in the journal Science.

To test whether the male butterflies' resurgence was due to genetic changes in the butterflies or changes in the parasite, he and colleagues bred infected female butterflies with butterflies from a different island that did not have the genetic mutation.

The butterfly offspring of this pairing were then bred with butterflies from a non-infected island to dilute the gene that suppressed the parasite.

"After we did that for three generations, we came back to complete male killing," Charlat said in a telephone interview.

"That demonstrated that the observed pattern was due to suppression and not due to another phenomenon," he said.

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

What is clear, they said, is 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.

Natural selection typically moves very slowly, sometimes over hundreds of years, they said, but when under severe attack, this process was accelerated.

"It is the speed of the process that demonstrates the intensity of the selection," Charlat said.