Three examples of postzygotic barriers include hybrid inviability, hybrid sterility and hybrid breakdown. Postzygotic barriers are reproductive mechanisms that reduce gene flow after fertilization between closely related species.
Hybrid inviability is the inability of a hybrid zygote to fully develop. An inviable hybrid generally dies in the early stages of development and is not carried to full term. Hybrid sterility is when the hybrid develops into a viable healthy offspring but is unable to reproduce; the sterility is often due to a difference between number of chromosomes in the hybrid's parent species. Hybrid breakdown is when the hybrid develops into a healthy offspring that carries the capability to reproduce. The breakdown refers to the inability of the hybrid's offspring to reproduce, as they are most often sterile.
The most famous example of a postzygotic barrier is the mule. A mule is a hybrid between a horse and a donkey. Mules nearly always display hybrid sterility.
Reproductive barriers are present to reduce interbreeding between similar species that have overlapping habitats. Reproductive barriers encourage speciation. There are both prezygotic and postzygotic barriers. After fertilization, a zygote is formed that develops into a hybrid. The purpose of postzygotic barriers is to reduce the fitness of the hybrid.