∞ generated and posted on 2016.12.24 ∞
Lowered reproductive success in comparison to parents that serves as a postzygotic barrier to reproduction.
This is particularly given a relative absence of reduced hybrid viability. That is, reduced hybrid fertility for reasons other than reduced hybrid viability. Such reduced hybrid fertility can be due, for example, to problems arising within the hybrid during meiosis.
The classic example of this phenomenon is the mule, a quite exceptional animal that nevertheless only tends to reproduce about "once in a blue moon". Indeed, in ancient Roman days this same sentiment instead was stated as something along the lines of "only about as often as a mule foals".