Biological diversity arises through speciation—the evolution of two species from a single common ancestor. Under the standard (allopatric) speciation model, new species begin as geographically isolated populations that evolve independently, gradually accumulating genetic differences that render them reproductively isolated from one another. Should such diverging populations later come into secondary geographic contact, incompatible courtship signals could prevent them from forming hybrids (prezygotic isolation) or, if hybrids are formed, incompatible genetic interactions could cause them to be sterile or inviable (postzygotic isolation). Both pre- and postzygotic forms of reproductive isolation prevent genetic exchange, allowing the recently diverged species to remain distinct.