A researcher conducted an experiment on a fruit fly species in which a single population of the flies was split into two separate groups. One group was fed with a starch-based food source, and the other group was fed with a maltose-based food source. The populations remained separated with their different food sources for many generations. When the two groups were finally reunited with each other, it was observed that the fruit flies still preferred to mate with members of their original population over members of the other population that had been fed a different food source.

What process is illustrated by this experiment?

A. coevolution

B. convergent evolution

C. gene flow

D. speciation