THE first genetic evidence of a different way species evolve has been discovered by studying a vicious ant.
The findings mean that it may have been possible for a species to split into two, even when living side by side. It happens when two groups develop powerful tastes and social taboos that seem to put them off interbreeding.
Until now the major force driving speciation was believed to be geographic. For instance a group might become isolated from the rest of its species when stuck on an island. In time, this group becomes genetically different from the original group and the two can no longer interbreed.
But this week in Nature, scientists say they have used genetic techniques to show that social barriers can be sufficient to prevent different groups in the same species from interbreeding.
They studied the fire ant, a vicious creature from south-east America. Some fire ants live in communities in which there is one queen - monogyne communities. Others live in polygyne communities in which there are several queens.
The two communities live side by side and are the same species - but interbreeding appears to be minimal.
Prof Kenneth Ross, an entomologist at Rochester University, New York, has shown that if a monogyne queen presumes to enter a polygyne nest, she is killed by the males there because she is too fat and robust.
"She is executed by the polygyne workers - they execute their own queens if they are too robust or fat as well," he said.
Meanwhile, no new queens, monogyne or polygyne, are accepted by the males in a monogyne camp, which cuts off another route for interbreeding.
A third route for interbreeding - between male polygyne ants and female monogyne ants - is also cut off, the genetic evidence shows. This is because the culture in polygyne communities appears to be to mate in the nest rather than to go out hunting for a mate, said Prof Ross.
He and his co-author Dr DeWayne Shoemaker have shown that the DNA flows between the two groups in just one way, from monogyne males going out and mating with polygyne females. So, while some interbreeding does occur, there is plenty of reproductive isolation between the groups.
The work may have implications for the evolution of all social animals, such as wasps, mammals and even humans. It may also help to explain why there are 25,000 species of ant.
