Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules.
Afhængigt af hvor langt to arter har afveget siden deres seneste fælles forfader, kan det stadig være muligt for dem at producere afkom, som med heste og æslers parring til at producere muldyr.