Post-zygotic Isolation

This type of isolation occurs after mating between members of two different species, forming a zygote which leads to the formation of a hybrid. These hybrids are often infertile, hence unable to reproduce. This type of production of hybrid isn’t considered a successful mating and are sexually immature.   

• Zygote mortality and non-viability of hybrids: When two species combine to form a zygote, this happens when the zygote perishes before becoming a viable embryo. As an alternative, the hybrid embryo might develop but not live to adulthood. For instance, a mule- a cross between a horse and a donkey—is sterile and incapable of procreation.
• Hybrid sterility: When a hybrid is born, but is unable to produce live gametes, it is unable to procreate. For instance, a hybrid mouse between two species could be produced, but it might not be able to produce healthy sperm or eggs.
• Multiple mechanisms: To stop interbreeding, several reproductive isolation mechanisms may occasionally cooperate. For instance, two plant species may have incompatible chromosomes, distinct pollinators, and different flowering dates, all of which hinder interbreeding.
• Hybrid sex: According to Haldane’s rule, the sex that is absent, infrequent, or sterile in a hybrid between two species is more likely to be the heterogametic sex (the sex with two different sex chromosomes). Male offspring from a hybrid between two species with distinct sex chromosomes may be infertile because, for instance, in mammals, men are XY and females are XX.