For many years, scientists have defined speciation as an evolutionary event by which a new species arises due to genetic drift. However, a new study suggests that species may diverge because of the microbes in their gut, not their DNA. Biologists Seth Bordenstein and Robert Brucker of Vanderbilt University studied this phenomenon in three different species of parasitic jewel wasps, tiny insects that drill into fly pupae and allow their eggs to feed on the host. 

Two of the wasp species, Nasonia giraulti and N. longicornis, are closely related whereas the third, N. vitripennis, diverged from the other two species about one million years ago. When N. giraulti and N. longicornis were mated, most of the offspring survived. However, when N. vitripennis was mated with either N. giraulti or N. longicornis, almost all of the second-generation larvae died.

Since Bordenstein and Brucker knew that the gut microbes in N. vitripennis differed from the other two species, they conducted another experiment to test their theory that the wasp’s mortality went beyond incompatible DNA. This time, all three species of Nasonia were raised without gut microbes. When N. vitripennis was mated with either N. giraulti or N. longicornis, almost all of the second-generation offspring survived. However, when the gut microbes were reintroduced, most of the second-generation wasps died.

This study introduces a new way to look at speciation. Now, biologists may have to consider how parental genes could be incompatible with the offspring’s microorganisms. While some scientists are skeptical about whether gut microbes have a great effect on speciation, others believe that examples of gut microbes separating species will become more common.