The central goal for ecologists is to understand the patterns of species diversity that we see in nature. The basic mechanisms promoting species coexistence are remarkably few, but predicting coexistence has proven difficult. We know that potentially competing species can coexist either because they are different from one another, allowing for the use of different niches, or because they are very similar and respond to the environment in similar ways. Advances in our ability to read the genetic codes of species have allowed biologists great insight into the evolutionary relationships among species. I will be using the evolutionary relatedness between species to predict how similar or different two species have to be to coexist together. The ability to predict which species should coexist, will inform how we restore habitats and manage biological diversity in a changing world.