Post-zygotic Barriers to Reproduction
In some cases, no barriers exist to mating between members of different species. In these cases, the zygote formed is called a hybrid. However, even after a hybrid zygote forms, reproduction may still not be successful. When that reproduction is successful, living hybrids are usually themselves unable to reproduce. The production of hybrid offspring is costly: the energy of mating and producing offspring has still been spent by the parents, with no future inheritance of their genetic material in return.
Hybrid Zygote Abnormality
While gametes from different species can sometimes fuse to produce a hybrid zygote, these zygotes are frequently abnormal. Most do not survive to birth or germination. Those that do often do not develop normally and never reach sexual maturity. In these cases, the mating of different species is said to be unsuccessful even though offspring were produced because those offspring are incapable of passing on their genes.
Among those hybrid offspring that do develop normally and reach sexual maturity, most are sterile, meaning they do not produce viable gametes. For example, mules result from the mating of a horse with a donkey. They are born and develop into normal healthy adult animals, but they cannot produce offspring of their own.
Low Hybrid Viability
Unlike mules, most hybrid offspring that survive the zygote stage to be born are not healthy. Most die before reaching reproductive age. As with the production of hybrids that cannot mature into reproductive adults, reproduction that results in unhealthy hybrids that die before reaching sexual maturity is considered unsuccessful.