Postzygotic Isolation

Prezygotic and postzygotic are important when attempting to understand how it is possible for two species that may live in the same area to coexist without mating. If two species do happen to mate and produce offspring, isolation still occurs. Postzygotic isolation occurs after an egg has been fertilized and an offspring has been produced. There are two major forms of postzygotic isolation: hybrid inviability and hybrid sterility. Hybrid inviability refers to the extremely low fitness of an offspring from two different species. This low fitness often leads to the death of the offspring, inhibiting the continuance of this potential new species. Similar to hybrid inviability, offspring is still produced with hybrid sterility. This offspring, however may have high overall fitness, but cannot mate because of sterility. This type of isolation can be seen in mules, the offspring of a horse and a donkey. While mules can survive perfectly fine, two mules cannot create an offspring.