What is an example of hybrid breakdown?

Hybrid breakdown is a form of postzygotic reproductive isolation that occurs when hybrid offspring between two species are viable and fertile in the first generation but produce inviable or sterile offspring in subsequent generations. An example of this can be seen in certain species of frogs, such as the Rana species, where hybrids between two closely related species may survive but exhibit reduced fitness and fertility in their offspring, ultimately leading to a decline in hybrid population viability over generations.