A Samoan butterfly has provided scientists with with one of the most clear and fastest cases of evolution under natural selection .
Researchers from London's University College say a bacteria passed down through the female Blue Moon or Great Eggfly butterfly killed males, meaning the species found on Savaii and Upolu was faced with extinction.
In 2001 only one percent of the butterflies were male, but the rapid development of a genetic mutation that suppresses the bacteria has seen male and female levels almost equalise.
They say their tests have confirmed the recovery was not due to changes in the parasite.
The scientists say they do not know whether the mutation already existed in the local butterflies or was introduced through a South American species.