In some areas, a bacteria has been killing off the male population for some time. The Wolbachia bacteria is passed down from the mother and selectively kills males before they have a chance to hatch.
But now the males are making a comeback. An international team of scientists who have been studying the species, say it is due to the rise of a gene the butterflies now have which makes them resistant to the bacteria. Last year, on the Samoan islands of Upolu and Savaii, the males went from just 1 percent of the Blue Moon butterfly population to 39 percent in just ten generations and less than a year due to the gene. They confirmed that the bacteria is still present, but the gene suppresses it. It is the fastest known example of natural selection at work.
A full article is linked to the title above. The scientist's reported their findings in the journal "Science".