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.
Hybrid Infertility
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.
