Answer:
Natural selection.
Step-by-step explanation:
Natural selection can be defined as a biological process in which species of living organisms having certain traits that enable them to adapt to environmental factors such as predators, competition for food, climate change, sex mates, etc., tend to survive and reproduce, as well as passing on their genes to subsequent generations.
Simply stated, natural selection entails the survival of the fittest. Therefore, the species that are able to adapt to the environment will increase in number while the ones who can't adapt will die and go into extinction.
In this scenario, the mechanism of evolution responsible for this was natural selection because after about 40 generations the evolution of reproductive isolation was under way for the fruit fly. Thus, the more the offsprings that are reproduced by the parent organism, the more likely are they to survive.
Additionally, there is a better chance of having good or beneficial traits being passed from the parent organism to her offsprings.