"We were surprised by how aggressive the females could be. In some cases, the attacks went so far that the male was killed," said Kaj Hulthén, a biology researcher at Lund University, in a press release.
The study looked at different populations of mosquitofish, which live in underwater caves off the coast of the Bahamas. It found that the female fish developed a strong resistance to males from other caves and that pairs of fish from different environments had a significantly lower chance of fertilization than pairs from the same environment.
The consequence is that they stop mixing their genes. Over time, they become so genetically different that they are no longer considered the same species.
