What is Postzygotic Isolation in Evolution?

 What is Postzygotic Isolation in Evolution?

Species formation is the divergence of two or more lines from a common ancestor. For species formation to occur, there must be some reproductive isolation that occurs between formerly reproducing members of the original ancestor species. While most of these reproductive isolates are prezygotic isolates, there are still certain types of postzygotic isolates that lead to ensuring that the newly created species remain separate and do not converge again.

Before postzygotic isolation can take place, there must be an offspring born from a male and female of two different species. This means that there were no prezygotic isolations, such as the coupling together of the genitals or incompatibility of germ cells or differences in mating rituals or sites, which kept the species in reproductive isolation. When sperm and egg fuse during fertilization in sexual reproduction, a diploid zygote is produced. The zygote then continues to develop into offspring that are born and hopefully then become a viable adult.

But offspring of two different species (known as a "hybrid") are not always profitable. Sometimes even will interrupt before they are born. Other times, they will be sickly or weak as they develop. Even if they do so into adulthood, a hybrid will probably not be able to produce its offspring and thus reinforce the idea that the two species are more suitable for their environments as separate species as the natural selection works on the hybrids.

Below are different types of postzygotic isolation mechanisms that reinforce the idea that the two species that created the hybrid are better as separate species and should continue to evolve in their own ways.

The zygote is not feasible

Although sperm and eggs from the two separate species digest during fertilization, this does not mean that the zygote will survive. Incompatibilities of the germ cells can be a product of the number of chromosomes each species has or how these germ cells are formed during meiosis. A hybrid of two species that does not have compatible chromosomes in any shape, size or number often itself interrupts or does not make it full term.

If the hybrid does not succeed in making it to birth, it often has at least one, and more likely several shortcomings, that keep it from becoming a healthy functioning adult who can reproduce and pass down its genes to the next generation. Natural selection ensures that only people 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 notion that the two species should remain separate.

Adults in Hybrid species are not viable

If the hybrid can survive through the zygote and early life stages, it will mature. However, this does not mean that it will thrive when it reaches adulthood. Hybrids are often not suitable for their environment how a pure species would be. They may find it difficult to compete for resources such as food and housing. Without necessities of subsistence, adults would not be profitable in their environment.

Once again, this puts the hybrid at a clear disadvantage evolution-wise, and natural selection steps in correcting the situation. Individuals that are not viable and undesirable are unlikely to reproduce and pass on their genes to their offspring. This, again, reinforces the idea of ​​species formation and keeping the lines of the tree of life going in different directions.

Adults in Hybrid species do not Fertile

Although hybrids are not more common for all species in the wild, there are many hybrids out there that were viable zygotes and even viable adults. Most animal hybrids are sterile in adulthood. Many of these hybrids have chromosome incompatibilities that make them sterile. So even though they survived evolution and are strong enough to make it into adulthood, they are not able to reproduce and pass down their genes to the next generation.

Because in nature, "fitness" is determined by the number of offspring an individual leaves behind and the genes are passed on, hybrids are usually considered "unsuitable" because they cannot pass down their genes. Most types of hybrids can only be made by mating two different species instead of two hybrids producing their own offspring of their species. For example, a mule is a hybrid of a donkey and a horse. But mules are sterile and can not produce offspring, so the only way to make more mules is to mate several donkeys and horses.

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