What is an example of a postzygotic reproductive barrier?

A postzygotic reproductive barrier occurs after fertilization, affecting the viability or reproductive ability of the hybrid offspring. Hybrid sterility is a perfect example of this type of barrier. In cases such as the mating between a horse and a donkey, the resulting mule is typically sterile, meaning it cannot produce offspring. This prevents gene flow between the two parent species, thus maintaining their separate identities despite the initial hybridization.

In contrast, temporal isolation, behavioral isolation, and geographical isolation are all prezygotic barriers, which occur before fertilization. Temporal isolation occurs when species breed at different times, preventing mating. Behavioral isolation involves differences in mating behaviors or rituals that prevent species from recognizing each other as potential mates. Geographical isolation happens when physical barriers (like mountains or rivers) separate populations, preventing them from encountering each other to mate. Each of these prezygotic barriers stops the formation of hybrids altogether, rather than affecting the viability or fertility of hybrids formed.