It has long been thought that different species did not mate because of geographical isolation; that they would adapt differently depending upon their environment. However, new research shows that “selfish genes,” or meiotic drive elements, and their exchange of genetic material, or gene flow, among species may dictate whether two species are able to mate. The research show that sex chromosomes evolve to be genetically incompatible between species faster than the rest of the genetic chromosomes, and that they propagate themselves at the expense of other genes. They manipulate reproduction so that they can transmit themselves to more than their fair share of the genome. For example, in hybrid male fruit flies, meiotic drive elements usually kill any sperm that don’t carry them, leaving mostly sperm that do carry the meiotic drive elements. The research was published in the journal eLife.