Educating students about the process of evolution through natural selection is vitally important because not only is it the unifying theory of biological science, it is also widely regarded as difficult for students to fully comprehend. Anderson and colleagues (2002) describe alternative ideas and misconceptions about natural selection as highly resistant to change. Catley (2006) suggests that the educational emphasis on microevolutionary processes has left both teachers and students with a poor understanding of macroevolution and speciation. In order to effectively develop curriculum for the teaching of evolutionary biology, it is important to know what students think and understand about natural selection. For this study, the authors surveyed 9th grade (freshman) general science and 12th grade (senior) biology students' knowledge and attitudes about natural selection before and after a constructivist sequence targeting specific concepts related to natural selection and the nature of scientific investigation. The authors' goal was to investigate whether students grasped not only the basic theory of natural selection, but also the underlying concepts that support the theory and are needed for a more thorough understanding of natural selection. They identified these fundamental concepts as (1) population variation; (2) mutation and the genetic basis for diversity; and (3) selective pressure in the environment. (Contains 3 figures and 4 tables.)