We present a laboratory exercise that leverages student interest in genetics to observe and understand evolution by natural selection. Students begin with white-eyed fruit fly populations, to which they introduce a single advantageous variant (one male with red eyes). The superior health and vision associated with having the red-eye-color allele confers a fitness advantage, and the students can watch the spread of the allele within the population. The increasing numbers of red-eyed flies they observe over generations demonstrate evolution by natural selection. The students concurrently learn genetic principles, including basic inheritance and X-linkage.