What commonly prevents hybrids from being viable offspring?

Hybrids can often face viability issues due to postzygotic isolating mechanisms. Once fertilization occurs and a hybrid zygote is formed, these mechanisms can lead to problems that prevent the hybrid from developing into a healthy, fertile adult. Postzygotic barriers include factors like hybrid inviability, where the developing hybrids do not survive to a mature stage, and hybrid sterility, where adults may be born but are not capable of reproducing (as seen in mules, the offspring of a horse and donkey). These barriers happen after fertilization and effectively reduce the chances of successful reproduction between distinct species, despite initial mating and fertilization.