Apples were brought to North America, about 400 years ago and around 1850, one fly species, Rhagoletis, went from living on their native hawthorn fruit to living on apples. Rhagoletis flies spend most of their lives underground and dormant. Adults emerge from the ground as their host fruit ripens. They mature, mate, lay eggs, and die within a few weeks. Apples are ripe several days after hawthorn fruit and flies from the apple trees reach adulthood about 30 days later than hawthorn flies. Even though these tflies live in the same area, the adult flies exist at different times, so they do not interbreed any longer. Scientists collected samples of each group of flies. They placed some of the flies, together, in flasks with both apples and hawthorn fruit. Other flasks contained either apple-eating flies or hawthorne-eating flies. Only the flies in the single fruit flasks mated and produced offspring. To confirm their results, the scientists isolated flies from each group and put one of each into a flask with food. Eventually the flies died without mating and scientists concluded that a new species had evolved. Consider the steps of natural selection as they relate to the development of a new species. Which step BEST applies in this case?