KEY POINTS:
A butterfly found in Samoa has displayed the fastest known rate of evolution as a result of a dramatic "arms race" with a microscopic parasite that kills only males of the species.
Biologists have witnessed how the butterfly has fought back against the parasite by spreading a gene that confers resistance against a type of bacteria that kills male embryos before they hatch.
The scientists said the rapid spread of the gene is an example of the Red Queen principle of evolution - named after the character in Lewis Carroll's Through the Looking Glass who ran faster and faster only to stay in the same place.
Normally the sex ratio of the blue moon butterfly - hypolimnas bolina - on the Samoan islands of Savaii and Upolu is the usual 50:50, but because of attacks by the bacterial parasite the proportion of males fell to below 1 per cent, with females making up more than 99 per cent.
But in 2006 field workers did a survey of the butterfly and found the sex ratio had suddenly bounced back up to near normal.
Sylvain Charlet of University College London and the University of California, Berkeley, said they found males now accounting for about 40 per cent of the total population.
"To my knowledge, this is the fastest evolutionary change that has ever been observed," Dr Charlet said.
"This study shows that when a population experiences very intense selective pressures, such as an extremely skewed sex ratio, evolution can happen very fast."
The scientists did laboratory experiments and found the parasite was a bacterium known as wolbachia which is passed from the mother and selectively kills males embryos before they have time to hatch.
The researchers hatched eggs from females captured during 2006 and found the males were able to survive wolbachia infection to produce sex ratios that were almost normal.
They believe the resistance was the result of a "suppressor" gene that had spread among the population, which had allowed males on the island of Savaii to increase from 1 per cent to about 40 per cent in less than a year.
Dr Charlet said it was not yet known whether the suppressor gene emerged as a result of a chance mutation within the local population of butterflies or had been introduced by butterflies migrating from other parts of Southeast Asia where 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," Dr Charlet said.
"The take-home message is that evolution can be really, really fast."
Biologists have known about heavily skewed sex ratios in certain butterfly species since the 1920s, but it was not until 2002 that scientists led by Gregory Hurst of University College London first identified wolbachia bacteria as the culprit.
When sex ratios are altered significantly from the 50:50 norm, natural selection can exert intense pressure on individuals to bring the ratio back to normal.
This is why the blue moon butterfly evolved so quickly to get back to the same place, as the Red Queen principle dictates.
"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. In the case of H. bolina, we're witnessing an evolutionary arms race between the parasite and the host," Dr Charlet said.
Dr Hurst said there were few examples of evolutionary change that could be observed over the human lifespan, yet this study, published in the journal Science, demonstrated that evolution did occur rapidly over a period of months.
"We usually think of natural selection as acting slowly, over hundreds or thousand of years. 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."
- INDEPENDENT
