Samoan butterfly population shows evolution at work: study

The dramatic comeback of a tropical male butterfly, which was almost wiped out of existence by an invasive parasite, shows just how fast natural selection can work in practice, researchers said Thursday.

When researchers sampled the numbers of the Blue Moon butterfly species on the South Pacific island of Savaii at the beginning of 2006, the males accounted for just one percent of the population.

By the end of the year, a period that is equivalent to 10 generations of Blue Moon butterflies, that figure had jumped to almost 40 percent.

Investigators believe the comeback is due to the proliferation of "suppressor" genes that hold in check the Wolbachia bacteria that is passed down from the mother and kills male embryos before they can hatch.

"To my knowledge, this is the fastest evolutionary change that has ever been observed," said Sylvain Charlat, lead author on the study and a post-doctoral researcher at the University of California, Berkeley.

"This study shows that when a population experiences very intense selective pressures, such as an extremely skewed sex ratio, evolution can happen very fast."

"We usually think of natural selection as acting slowly, over hundreds of thousands of years," added Gregory Hurst, a senior author on the paper and a researcher in evolutionary genetics at University College London.

"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."

Charlat and his colleagues first documented the massive imbalance in the sex ratio of the butterfly species on Savaii and the neighbouring island of Upolu in 2001. At that point, the male butterfly was extremely rare, making up just one percent of the total population.

In 2006, the team embarked on a new survey after an increase in reports of male sightings at Upolu.

They found that the sex ratio among the latest crop of insects, (scientific name Hypolimnas bolina) was 1:1 on Upolu and approaching parity on Savaii, even though the female insects were still infected with the Wolbachia parasite, and it was still capable of killing the male of the species.

It is not yet clear whether the suppressor gene emerged from a chance mutation from within the local population, or if it was introduced by migratory Southeast Asian butterflies in which the mutation had already been established.

"But regardless of which of the two sources of the suppressor gene is correct, natural selection is the next step. The suppressor gene allows infected females to produce males, these males will mate with many, many females and the suppressor gene will therefore be in more and more individuals over generations," Charlat explained.

Overall, the waxing and waning fortunes of the male Blue Moon butterfly shows that not only how fast species can evolve, or adapt, but just how important parasites can be as evolutionary drivers, the authors said.

"In the case of H. bolina, 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," said Charlat.