The course used evolution as a unifying theme. Evolution was taught as a distinct unit as well as integrated throughout the course. Like the qualitative basics of the kinetic-molecular theory, the theoretical framework of biological evolution is relatively simple:

1. Individuals in a population of any species vary in many heritable traits.

2. Those individuals with traits best suited to the local environment generally leave a disproportionately large number of surviving offspring; thus, these traits appear in larger numbers in successive generations. Natural selection is the editing process that selects individuals with heritable features that favor their reproductive success.

The research employed a qualitative design. Each student participated in a series of 17 structured and open-ended interviews spread throughout the school year. The interview questions were developed using reports of alternate conceptions (misconceptions) about evolution from the literature. Concepts and changes in concepts were identified as each of the three researchers reviewed transcripts of the interviews, notes from classroom observations, and examinations and other written work.

The student participants were selected using a purposeful sampling designed to provide the widest possible diversity in content knowledge, given the limitations imposed by their school population. All three were seniors at a university high school, students only 3 to 12 months removed from college-level introductory courses. Their study of evolutionary theory involved no math or statistics. Instead, they studied a qualitative treatment of the historical development of the theory, evidence for evolution, evolutionary relationships, patterns of evolution, and aspects of evolutionary explanations.

The report identifies four modes of conceptual change in the students studied, two of which are congruent with the conceptual change model proposed by Posner et al. (Science Education, 1982, 66, 211-226) and two others suggesting that additional modes of conceptual restructuring occur. The conceptual change model suggests that the concepts we use to help organize our understanding of the world can only be replaced by new concepts if a new concept (say, for example, a particle theory of matter, or natural selection) is judged to be plausible, more intelligible, and more fruitful than its predecessor (for example, a continuous model of matter, or a "need-to-change" model of evolution). According to the model, an individual conceptual change can be wholesale, with a new concept replacing the old concept, or can be gradual, with the new concept evolving from the old one.

This study documents two additional modes of change that occurred over the course of the year: a cascading change, during which a change in one conception allowed a sequence of conceptual changes to occur, and a dual construction, during which students constructed two logically incompatible conceptions without recognizing the inconsistency. The study also reminds us that changing minds so students accept new ideas can be a slow process, even though the change appears simple to those of us who have already made that change.