This article reports on a study that assessed knowledge of and misconceptions about natural selection in second-semester biology majors in two classes characterized by different instructional strategies. The active-learning class achieved significant postcourse gains in the number and diversity of key concepts of natural selection employed in evolutionary explanations and exhibited significant decreases in misconception use. Compared with the traditionally taught class, the active-learning class was characterized by fewer misconceptions and greater mean key-concept diversity scores. Nevertheless, both classes demonstrated inadequate postcourse levels of evolutionary understanding: After a year of college biology, 70 percent of students in the active-learning group and 86 percent in the traditionally taught group employed one or more misconceptions in their evolutionary explanations. Faculty in upper-division courses must be prepared to address students' misconceptions and provide additional opportunities for improving student understanding of natural selection.