Which of the following is an example of a postzygotic barrier?

Hybrid infertility is a clear example of a postzygotic barrier because it occurs after fertilization has taken place. In this case, even though the sperm and egg may successfully combine to form a zygote, the resulting offspring are unable to reproduce successfully. This could manifest as the offspring being sterile, as seen in mules (the hybrid offspring of horses and donkeys), which cannot produce viable gametes. This barrier effectively prevents gene flow between the parent species by ensuring that even if hybridization occurs, the hybrids do not contribute to the gene pool of either parent species.

On the other hand, the other options represent prezygotic barriers, which impede the processes leading up to fertilization. Temporal isolation refers to species that reproduce at different times, thereby preventing mating. Behavioral isolation involves differences in mating behaviors or rituals that inhibit different species from interbreeding. Mechanical isolation occurs when differences in reproductive structures prevent successful mating. All these mechanisms occur before the formation of a zygote, distinguishing them from postzygotic barriers like hybrid infertility.