According to the question, mating between two populations of Rhagoletis produce hybrid flies.
This means that the two population is present in the same habitat or locality, hence they are capable of coming in contact with each other and mate. Hence, the answer cannot be Habitat Isolation.
Mating produces hybrid flies. This is possible when the gametes (egg and sperm) of the two different populations are capable of capable of coming close to each other and undergoing fusion (fertilization). Hence, there is no Mechanical Isolation.
The zygote formed by the above fertilization is capable of developing into healthy hybrid flies with normal life span. Hence, Pre-zygotic Isolation is absent.
But the hybrid flies formed are sterile and not fertile, that is, they are incapable of producing viable gametes which can undergo fertilization and produce a new offspring.Therefore, the eggs laid by the hybrid flies hatch less often. Hence, there is existence of Reduced hybrid fertility.
