A scientist shines low levels of ultraviolet radiation on a dish containing colonies of ruby-red bacteria. He then makes several replicate plates from this dish over many generations. He finds that in addition to the ruby-red color, there are also pink, orange, and yellow bacteria. Which of the following is the best explanation for these findings?

A. Mutations occurred in the gene for color of the bacteria, which were passed on to successive generations.
B. The scientist selected new types of bacteria by making plates of successive generations over time.
C. One bacterial colony changed its color to better blend in to the media.
D. Natural selection produced the changes in the bacterial color.