Hybrid sterility in the case of a mule.
* What happens: A male donkey (jack) and a female horse (mare) can mate and produce a hybrid offspring called a mule. Mules are strong and hardy, often used for work, but they are sterile. They cannot reproduce with each other or with their parent species.
* Why it's postzygotic: The isolation occurs *after* the formation of a zygote (the fertilized egg). The mule is viable, meaning it can be born and survive, but it is reproductively isolated from both horses and donkeys.
* What causes it: The chromosomes of horses and donkeys differ in number. Horses have 64 chromosomes, while donkeys have 62. This difference in chromosome number causes problems during meiosis (cell division that produces sperm and eggs) in the mule, leading to non-functional gametes (sperm and eggs) that cannot be used to create viable offspring.
This example demonstrates postzygotic isolation because the reproductive isolation occurs after the formation of a hybrid offspring, preventing the hybrid from contributing to future generations and further separating the parental species.
