In the case of postzygotic reproductive isolation, individuals from two species are capable of producing a zygote, however, these offspring (or their offspring) are incapable of normal growth and/or reproduction. Postzygotic barriers to hybridization include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.
In reduced hybrid viability, hybrids between two species either fails to develop or do develop but lack vigor and rarely, if ever, reach sexual maturity. These offspring are considered unviable because ultimately, they are unable to reproduce.
In reduced hybrid fertility, interbreeding between species occurs and hybrids are formed, however, the hybrids themselves are usually sterile. Reduced fertility is observed in mules, which are sterile hybrids of donkeys and horses.
In hybrid breakdown, hybrids are capable of reproducing but their offspring have either reduced fertility or reduced viability so a hybrid population cannot persist.
If all of these isolating mechanisms do not function, there are a number of possible outcomes. If the hybrids are able to produce fertile offspring among themselves and with the parent species and the offspring have the same fitness, the two parent species may ultimately coalesce back into a single species. The hybrids may persist in a stable hybrid zone (geographic area) between the two parent species (there are many examples of this in the scientific literature). If the hybrids are more well adapted to the environment than either parent species, the two parent species could ultimately disappear and be replace by the hybrid (this has been documented but is very rare).
Genetics Selection and Speciation Part 5 VoiceThread Transcript