Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

If new species evolve in the same territory or in neighboring territories, crossing can normally occur between their members and the occurrence of interspecific crosses would blur the boundaries between the species. This process can be prevented by various internal prezygotic and postzygotic reproductive isolation mechanisms.As a consequence of these mechanisms, internal postzygotic and prezygotic reproductive barriers exist between emerging species. Any ecological, ethological, physiological or biochemical factor reducing the probability of the formation of a hybrid zygote can be considered to be a prezygotic barrier. Any such factor that reduces the probability of development of these zygotes into adults capable of reproduction is considered to be a postzygotic barrier.