∞ generated and posted on 2016.12.23 ∞
Reduced fitness of the progeny of the offspring generated by a mating between different species.
| With Hybrid Breakdown the breakdown occurs in terms of the functionality not of the children of the hybridization but instead of the children of those children. |
Hybrid breakdown is a reduction in fitness as seen in the grandchildren to the two individuals from separate gene pools that participated in the hybridization event to begin with. Hybrid breakdown, in other words, is something that happens to the offspring of hybrids rather than to hybrids themselves.
That is, even if reduced hybrid viability and reduced hybrid fertility are avoided, one additional postzygotic barrier to forming hybrid progeny still exists, and that is a reduced fitness associated not with the progeny of the interspecific cross but instead the progeny of those progeny.
It is not the children but the grandchildren that are less functional than the individuals that managed to mate with a member of a different species in the first place!