A report published today in the journal Nature, however, describes the case of two closely related wasp species for which that barrier is not a river, nor a mountain range, but rather a microbial infection. The new research suggests that a bacteria known as Wolbachia pipiens spurred the evolution of these wasps. Wolbachia is a particularly wily bacteria. In order to assure that as many wasps as possible pass the microbe on to the next generation, Wolbachia , which is passed down through the maternal line, alters the sperm of its male host such that it can only breed with an infected female. Any offspring from that pairing will thus inherit the microbe.
Uninfected offspring can only result from matings between uninfected wasps.
After studying the many genetic barriers that can keep the species apart, the researchers found that the presence of Wolbachia was the most formidable barrier to interbreeding, and thus likely the oldest. That supports the argument that Wolbachia can promote host speciation.