What role do postzygotic barriers play in species reproduction?

Postzygotic barriers play a crucial role in reproductive isolation by preventing the development of fertile and viable offspring after fertilization has occurred. These barriers come into play after the sperm has successfully fertilized the egg, leading to the formation of a zygote.

Variations of postzygotic barriers can include hybrid inviability, where the hybrid offspring fails to develop properly or is unable to survive to reproductive maturity, and hybrid sterility, where the hybrids are sterile (like mules, which are the result of breeding between a horse and a donkey). In both cases, these barriers effectively limit gene flow between distinct species, thereby maintaining species identity and promoting speciation.

This understanding helps clarify the concept of reproductive isolation: postzygotic barriers ensure that even if mating occurs between different species, the outcome does not result in successful reproduction, thereby reinforcing species boundaries.