When two populations of a species are separated for many generations, their genomes start to drift apart as they adapt to their different environments. Eventually they can become two distinct species. But new research now suggests it's not always incompatible DNA that halts interbreeding — sometimes it's incompatible gut microbes.
You've no doubt heard a lot about the microbiome by now. We know that the microbes inside our body outnumber our own cells 10 to 1. We also know that different species have their own makeup of gut microbes, and you can tell a chimpanzee from a person just by analyzing the bacteria in stool samples. Scientists are now beginning to understand the surprising consequences to related species having different gut microbes.