The diagram documents a famous experiment by Diane Dodd. Dodd took an initial population of Drosophila fruit flies and separated them into two different containers. The first group was fed food containing mostly starch. The second group was fed food containing mostly maltose. After eight generations apart (about four months), the flies were re-introduced and could not longer interbreed. They had become two new species.

What is the most likely reason the flies evolved into two species?

A) At least one of the groups of flies became sterile and unable to lay eggs.

B) The two flies were really separate species to begin with, but it went unnoticed.

C) The two different food sources introduced many new DNA mutations very quickly and the flies changed drastically.

D) Separation with different food sources gave an advantage to mutant flies that could only digest starch or digest maltose. They survived and the species changed in these directions.