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Reproductive isolation mechanism that acts after the point of conception.
Postzygotic reproductive barriers act after the point of production of the zygote, contrasting prezygotic barriers which instead act to prevent conception. This represents a fundamental difference because up to the point of conception no eggs have been wasted nor progeny.
Why do I say wasted? This is because "wasting" progeny is the fundamental basis of the idea of postzygotic reproductive barriers, that is, the production of offspring that have a lower fitness, because they are hybrids, than they would if they had not been hybrids.
In what ways can hybrid fitness be lowered? They can either die young or otherwise produce fewer offspring overall. How does this affect parental fitness? This is a matter of opportunity costs, where the production of hybrid progeny is assumed to have the effect of preventing the production of an equivalent number of non-hybrid progeny.
The technical terms for the various categories of postzygotic barriers are reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Basically these come down to not surviving long enough to produce a typical number of offspring, surviving but nevertheless not producing a typically number of offspring, and producing a typically number of offspring but those offspring display a reduced fitness relative to non-hybrid offspring, respectively.
Importantly, postzygotic barriers drive the evolution of prezygotic barriers, as adaptations, and therefore resulting speciation events. How do you avoid the costs of producing inferior progeny? You avoid conceiving those progeny in the first place, where "avoidance of conception" basically is explicitly what prezyogenic reproductive barriers achieve.
The mule is the classic example of a postzygotic barrier as they almost always display a reduced hybrid fertility, and this is despite the fact that mules are otherwise impressively robust animals.
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