Several studies have examined the alternative conceptions that students possess about the process of natural selection. The goal of this study was to explore the nature of the changes in students' explanations of evolutionary scenarios. Fifty names were randomly selected from a pool of over 200 high school students who took a pretest prior to and a posttest following instruction about evolution. Teleological and Lamarckian explanations accounted for over half of the students' explanation on the pretest, but dropped to less than 20% on the posttest. Most of the students that, on the pretest, attributed evolutionary change to individual need for a trait or extended use or disuse of some part of the body shifted, on the posttest, to explanations that described the role of a population's variation to the evolutionary process. Explanations that included the idea of spontaneous genetic mutations increased, but this totalled less than 10% of all posttest responses.