Scientists have long believed that the difference between two species came down to reproductive incompatibility as a result of adapting to different environments. But researchers recently found that may not be the case — it’s actually in the genes. A research team found that “selfish” genes, or meiotic drive elements, determine whether two species converge or diverge. In a new study using two different species of fruit flies, the researchers show that before the rest of the genetic chromosome develops, sex chromosomes from two species already evolve to be incompatible with one another. “Genes from one species simply can’t talk to genes from another species,” said Daven Presgraves, a dean’s professor of biology at the University of Rochester. But some species are “leaky,” researchers say, meaning a selfish gene can cross over to another species and still be compatible.