If the apple maggot flies preference for the apple is decreased so much that it has equal preference between the two trees, then the incipient species will begin to interbreed. This is because there is now no reason for them to not mate, the previous prezygotic barrier is now abolished. They are able to mate freely resulting with a viable offspring and no natural selectiuon against them. They will no longer remain separated, they will over time either grow into one species as they continue to interbreed, or another barrier will be formed through mutation or a behavoirial change and reinforced. This allows for the separtion back into the two trees they prefer.