German researcher Christian Rabeling was digging up ant colonies on a college campus in Brazil when he found something unexpected—certain ants appeared smaller and shinier and had wings. Rabeling soon realized that those strange ants belonged to a previously undocumented species, a parasite that was feeding off the nutrients of the already familiar ants. In a study published today in the journal Current Biology, Rabeling and a group of scientists, including Ted Schultz, curator of ants at the National Museum of Natural History, claim that the very existence of the parasite ants provides new evidence supporting a controversial theory on evolution.

At the center of the new findings is an evolutionary concept called sympatric speciation, the possibility “for a species to split into two species without any geographic separation,” Schultz says. “That’s usually been criticized and usually been rejected. It’s a very hard thing to prove.”

But Rabeling and Schultz believe they’ve done it.

The ant colony they studied was situated under a group of eucalyptus trees at São Paulo State University in Brazil. The familiar ant, Mycocepurus goeldii, is a fungus-farming species, meaning it grows fungus and relies on it for nutrients. This ant has been observed throughout Brazil and in neighboring countries. But within that one colony on the university campus exists a parasite ant, Mycocepurus castrator. Rather than help grow fungus, the parasites spend their lives eating the food reserves and reproducing. Sometimes they go undetected; other times, mobs of the farmer ants identify and kill them.

Most new species develop in geographic isolation from the original species, a concept called allopatric speciation. It is rare for a species like the parasite ants to evolve from another species within the same nest.

In the 1930s, 70 years after Charles Darwin published On the Origin of Species, biologist Ernst Meyer began talking about allopatric speciation. He was adamant about it; Rabeling describes him as “the biologist equivalent of a Prussian general.” But one of Meyer’s students, Guy Bush, challenged that concept and spent his career seeking evidence of sympatric speciation. Decades later, the scientific community continues to hotly debate those two possibilities for how species evolve.

Rabeling and Schultz are confident that the parasite ants evolved without geographic separation. “This example seems to be one of the best cases yet for sympatric speciation,” Schultz says. The team used genetic analysis to prove that the parasites were unique from—but also descendants of—their hosts. They estimate that the genetic division happened some 37,000 years ago, a short period in evolutionary history.

While critics of sympatric speciation may not come after Rabeling and Schultz with pitchforks and torches, they are likely to be skeptical of the findings. “If you look at what we call sister species,” says Jerry Coyne, a professor at the University of Chicago and one of the authors of the seminal book Speciation, “you almost invariably find…that they’re geographically isolated from one another."

“Unless they have evidence that this phenomenon actually occurred in geographic isolation…then the case is not convincing," says Coyne, who has not yet seen Rabeling and Schultz’s newest findings. "Just saying that you have sister species of ants and that one is parasitic on the other, is evidence of sympatric speciation, is not correct.”

But Rabeling and Schultz are the latest in a line of researchers going back to Guy Bush who have sought to prove that species don’t need geographic separation in order to evolve. Researchers have conducted similar studies near Lake Victoria in eastern Africa and in the waterways in Nicaragua.

“We believe that we caught evolution during the act of speciation,” says Rabeling, who also served as a postdoctoral fellow at the Natural History Museum and is an assistant professor at the University of Rochester. It’s a “kind of evolution-in-progress,” he said in a press release.

“Evolution is largely about speciation, one species becoming two,” Schultz says. “So if you can understand the general rules about how speciation occurs, you have made a big giant step towards understanding evolution in general.”