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Butterfly shows evolution at work
The bacteria selectively kills male blue moons before they can hatch
Scientists say they have seen one of the fastest evolutionary changes ever observed in a species of butterfly.
The tropical blue moon butterfly has developed a way of fighting back against parasitic bacteria.
Six years ago, males accounted for just 1% of the blue moon population on two islands in the South Pacific.
But by last year, the butterflies had evolved a gene to
keep the bacteria in check and male numbers were up to about 40% of the
population.
Scientists believe the comeback is due to “suppressor” genes that control the Wolbachia bacteria that is passed down from the mother and kills the male embryos before they hatch.
“To my knowledge, this is the fastest evolutionary
change that has ever been observed,” said Sylvain Charlat, of
University College London, UK, whose study appears in the journal
Science.
Rapid natural selection
Gregory Hurst, a University College researcher who
worked with Mr Charlat, added: “We usually think of natural selection
as acting slowly, over hundreds of thousands of years.
“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.”The team first documented the massive imbalance in the sex ratio of the blue moon butterfly (Hypolimnas bolina) on the Samoan islands of Savaii and Upolu in 2001.
In 2006, they started a new survey after an increase in reports of male sightings at Upolo.
They found that the numbers of male butterflies had either reached or were approaching those of females.
The researchers are not sure whether the gene that
suppressed the parasite emerged from a mutation in the local population
or whether it was introduced by migratory Southeast Asian butterflies
in which the mutation already existed.
But they said that 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.
“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,” Mr Charlat said.
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