The Case of the Mule:
* Species Involved: A male donkey (jack) and a female horse (mare).
* Hybrid Offspring: A mule.
* Postzygotic Barrier: Hybrid inviability. Mules are sterile; they cannot reproduce.
* Why it Happens: Horses and donkeys have different chromosome numbers (horses have 64, donkeys have 62). While they can produce offspring (a mule), the mismatch in chromosomes makes it impossible for the mule to form viable gametes (sperm or eggs).
Explanation:
Postzygotic barriers occur *after* fertilization. They prevent the hybrid offspring from surviving or reproducing successfully. Hybrid inviability specifically refers to the death of the hybrid offspring, either before or shortly after birth. In this case, the mule is technically viable, but its sterility is a direct result of the chromosomal mismatch, making it a postzygotic barrier.
Other Examples:
* Hybrid sterility: Ligers (lion-tiger hybrids) are often sterile.
* Hybrid breakdown: Some plant hybrids may be fertile in the first generation, but their offspring are weak or infertile.
