Hybrid sterility is an example of:
Postzygotic barriers act after fertilization reducing hybrid viability or fertility, including hybrid inviability where embryos abort, hybrid sterility where mule with 63 chromosomes cannot produce functional gametes due to mismatched pairing, and hybrid breakdown where second generation has low fitness. These arise from Dobzhansky-Muller incompatibilities accumulating after divergence. Mule exemplifies sterility. Therefore Postzygotic isolation represents postzygotic isolation maintaining species boundaries after mating. This concept integrates genetics, ecology and molecular evidence, freque
Ref: Coyne & Orr, Speciation, Chapter 1: Species Concepts and Isolation.