Hypolimnas bolina: Also known as the Great Eggfly or Blue Moon Butterfly. Public domain photo by Comacontro at Wikimedia CommonsScientists have reported witnessing a remarkable and rapid display of evolution through natural selection at work in butterflies in the South Pacific.

Sylvain Charlat and Gregory Hurst along with an international research team stationed in the Samoan Islands observed a huge discrepancy in the ratio of males to females in the Hypolimnas bolina butterfly population. At the beginning of 2006, the scientists noticed that the females on the island of Savii outnumbered the males 99 to 1. It turns out the distorted ratio was caused by the Wolbachia an inherited bacterium which selectively kills the male butterflies before they can hatch. The bacteria are passed down through the mother’s genes.

But by year’s end, Charlat and his colleagues discovered the male population had - within just 10 generations - increased to 39% of the population. The researchers credit the males’ swift recovery to a suppressor gene switching on to counter the bacteria.

"To my knowledge, this is the fastest evolutionary change that has ever been observed," said Charlat, a post-doctoral researcher with joint appointments at the University College London, and 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."

Whether the suppressor gene’s appearance was due to a chance mutation within the local population or introduced by migrating Southeast Asian butterflies where it was already present is not yet known. But Charlat and his team hope to pinpoint the cause in the next three years.

"In essence, organisms must evolve or change to stay in the same place, whether it's a predator-prey relationship, or a parasite-host interaction," said Charlat. "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."

Regardless of which route natural selection took, Gregory Hurst the co-author of the study appearing in today’s issue of the journal Science, thinks the end result is still a stunningly rapid evolutionary response to an environmental change.

"We usually think of natural selection as acting slowly, over hundreds or thousands of years," Hurst said. "But the example in this study happened in a blink of the eye, in terms of evolutionary time, and is a remarkable thing to get to observe."

In 2002, Hurst and his colleagues identified Wolbachia as the cause of the lopsided sex ratio. He holds a current post as a reader in ecology and evolution at University College London.

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UC - Berkely news release

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