Ants hint at how social species evolve.

Two populations of the same species that live far apart and don't mate can develop such different traits that they become separate species, researchers believe. Moreover, sympatric speciation-in which new species evolve from populations that live close together but don't mate-may also occur.

New findings now support a controversial version of that model stating that different social behaviors in groups of a social species, such as ants, living close together may also give rise to new species. D. DeWayne Shoemaker of the University of Rochester (N.Y.) and Kenneth G. Ross of the University of Georgia in Athens describe the study in the Oct. 17 Nature.

"As far as we are aware, Shoemaker and Ross are the first to demonstrate that social behavior influences sympatric speciation," assert Ross H. Crozier of La Trobe University in Bundoora, Australia, and Pekka Pamilo of the University of Uppsala in Sweden in an accompanying commentary.

Shoemaker and Ross studied differences in the social behavior and genetic makeup of two populations of fire ants (Solenopsis invicta) in northern Georgia. Specifically, they looked at segments of mitochondrial DNA, which only mothers pass on to their offspring, and segments of two nuclear genes. One group of ants, called the monogyne form, has one queen per nest. The polygene nest may contain 200 or more.

The genetic differences that the researchers found support their observations of who is-or more important, who is not-mating with whom, they contend. The only genetic link between the two populations is that monogyne males sometimes mate with polygyne females, they assert.

Many roadblocks bar the two groups from mixing their genes. Workers reject queens from the different form, so queens stay away from the other group's nests. Few fertile polygyne males exist, and they appear to mate only with females in a nest.

If monogyne males cease mating with polygyne females altogether, the two types may become distinct species. This could happen if, for example, the females begin mating only in the comfort of their nests and never in the open skies, where they now meet monogyne males. Such a change of heart has occurred among other species of ants, the authors note.