Speciation is the divergence of two or more lineages from a common ancestor. For speciation to occur, there must be some reproductive isolation that occurs between formerly reproducing members of the original ancestor species. While most of these reproductive isolations are prezygotic isolations, there are still some types of postzygotic isolation that leads to making sure the newly made species stay separate and do not converge back together.
Before the postzygotic isolation can happen, there must be an offspring born from a male and female of two different species. This means there were no prezygotic isolations, like the fitting together of the sex organs or incompatibility of the gametes or differences in mating rituals or locations, that kept the species in reproductive isolation. Once the sperm and the egg fuse during fertilization in sexual reproduction, a diploid zygote is produced. The zygote then goes on to develop into the offspring that is born and hopefully will then become a viable adult.
However, offspring of two different species (known as a "hybrid") are not always viable. Sometimes they will self-abort before being born. Other times, they will be sickly or weak as they develop. Even if they make it to adulthood, a hybrid will most likely be unable to produce its offspring and therefore reinforce the concept that the two species are more suited to their environments as separate species as natural selection works on the hybrids.
Below are the different types of postzygotic isolation mechanisms that reinforce the idea that the two species that created the hybrid are better off as separate species and should continue with evolution on their own paths.
The Zygote Is Not Viable
Even if the sperm and the egg from the two separate species can fuse during fertilization, that does not mean the zygote will survive. The incompatibilities of the gametes may be a product of the number of chromosomes each species has or how those gametes are formed during meiosis. A hybrid of two species that do not have compatible chromosomes in either shape, size, or number will often self-abort or not make it to full term.
If the hybrid does manage to make it to birth, it often has at least one, and more likely multiple defects that keep it from becoming a healthy, functioning adult that can reproduce and pass down its genes to the next generation. Natural selection ensures that only the individuals with favorable adaptations survive long enough to reproduce. Therefore, if the hybrid form is not strong enough to survive long enough to reproduce, it reinforces the idea that the two species should stay separate.
Adults of the Hybrid Species Are Not Viable
If the hybrid can survive through the zygote and early life stages, it will become an adult. However, it does not mean that it will thrive once it reaches adulthood. Hybrids are often not suited for their environment the way a pure species would be. They may have trouble competing for resources such as food and shelter. Without the necessities of sustaining life, the adult would not be viable in its environment.
Once again, this puts the hybrid at a distinct disadvantage evolution-wise, and natural selection steps in to correct the situation. Individuals that are not viable and not desirable will most likely not reproduce and pass down its genes to its offspring. This, again, reinforces the idea of speciation and keeping the lineages on the tree of life going in different directions.
Adults of the Hybrid Species Are Not Fertile
Even though hybrids are not prevalent for all species in nature, there are many hybrids out there that were viable zygotes and even viable adults. However, most animal hybrids are sterile at adulthood. Many of these hybrids have chromosome incompatibilities that make them sterile. So even though they survived development and are strong enough to make it to adulthood, they are not able to reproduce and pass down their genes to the next generation.
Since, in nature, "fitness" is determined by the number of offspring an individual leaves behind and the genes are passed on, hybrids are usually considered "unfit" since they cannot pass down their genes. Most types of hybrids can only be made by the mating of two different species instead of two hybrids producing their own offspring of their species. For instance, a mule is a hybrid of a donkey and a horse. However, mules are sterile and cannot produce offspring, so the only way to make more mules is to mate more donkeys and horses.