Reproductive isolation mechanisms that occur after the formation of a hybrid zygote are known as postzygotic barriers. These mechanisms reduce the viability or reproductive capacity of hybrid offspring. For example, hybrid inviability occurs when the interaction of parental genes impairs the hybrid’s survival, whereas hybrid sterility results when the hybrid offspring is viable but infertile, often due to chromosome number differences between the parent species.
These barriers are critical in the process of speciation because they prevent gene flow between diverging populations even if mating and fertilization occur. The presence of such isolating mechanisms reinforces reproductive divergence, ultimately leading to the evolution of distinct species. Historically, their identification and study have been instrumental in understanding the mechanisms driving evolutionary change and the formation of biodiversity.
