Many students do not recognize that individual organisms within populations vary, and this may make it difficult for them to recognize the essential role variation plays in natural selection. Also, many students have weak scientific reasoning skills, and this makes it difficult for them to recognize misconceptions they might have. This paper describes a 2-h laboratory for college students that introduces them to genetic diversity and gives them practice using hypothetico-deductive reasoning. In brief, the lab presents students with DNA sequences from Africans, Europeans, and Asians, and asks students to determine whether people from each continent qualify as distinct "races." Comparison of the DNA sequences shows that people on each continent are not more similar to one another than to people on other continents, and therefore do not qualify as distinct races. Ninety-four percent of our students reported that the laboratory was interesting, and 79% reported that it was a valuable learning experience. We developed and used a survey to measure the extent to which students recognized variation and its significance within populations and showed that the lab increased student awareness of variation. We also showed that the lab improved the ability of students to construct hypothetico-deductive arguments.