This study explores one teacher's instructional method for teaching life sciences using argumentation and argumentative writing rather than simple templates for writing claims and evidence. The microethnographic discourse analytic case study reported here included the teacher and 26 "advanced" eighth-grade students in a suburban middle school. Nine consecutive video-recorded lessons and related data were analyzed, focusing on how the teacher and students constructed the theory of evolution during instructional conversations about evidence and reasoning and about the content of students' written arguments on the theory. The teacher created a context in which students developed arguments with teacher support to ensure that they were learning to use argumentation as a heuristic to understand concepts and to engage in argumentative practice central to doing science. The modes of participation of two case study students are contrasted to explore two different trajectories and to examine particular cases of writing a lab report.