What is postzygotic isolation?

Postzygotic isolation refers to mechanisms that take effect after fertilization has occurred, specifically aimed at preventing hybrid offspring from developing into viable, fertile adults. This type of isolation is critical in the process of speciation because it helps maintain the distinctiveness of species, ensuring that even if mating occurs between different species, the resulting offspring are unable to thrive or reproduce successfully.

Examples of postzygotic isolation include the production of hybrids that either do not survive to adulthood or are sterile, such as the mule, which is the offspring of a horse and a donkey. Even though the mating can occur, the hybrid might be infertile, thus preventing gene flow between the parent species.

In contrast, isolation before fertilization, during mating, or through behavioral traits pertains to prezygotic barriers that prevent different species from even attempting to mate. Thus, the definition of postzygotic isolation specifically relates to events and mechanisms occurring after the zygote has formed and focuses on the viability and fertility of the resulting hybrids. This understanding underscores the significance of postzygotic barriers in evolutionary biology and the maintenance of species boundaries.