Hoskin and colleagues argue that because southern females have the most to lose in such cross-breeding, there may have been selection pressure to evolve a mating strategy to minimize dead-end mating with northern males. This appears to have occurred in the contact region where a population of the southern lineage had become isolated from the rest of its lineage and had developed a preference for certain male calls. The male frog call in this population has diverged significantly from both the northern and southern lineage calls.
"If females have a reason not to get the mating wrong, and they have some way of telling the males apart - the call - the theory is that this should create evolutionary pressure for the female choice to evolve so that they pick the right males," Moritz said.
This so-called reinforcement has been controversial since the time of Charles Darwin, with some biologists claiming that it requires too many steps for evolution to get it right.
"Some have argued that it's just too complicated and that it is not really necessary, and there have been few convincing demonstrations. In their view, differences between populations arise because of natural selection or genetic drift or mutation or some combination of those three, and reproductive isolation is just some glorious accident that arises from that," Moritz said. "We do have very compelling evidence. We have addressed various lines of evidence and conclude that there has been reinforcement and that has given rise to a new species based on very strong female choice."
As a comparison, they looked at a second contact zone on the border between north and south, where frogs were not isolated from either lineage.