This study investigated the participant structure and content of discourse in five high-school science classrooms and their connection to scientifically literate practices for talking, reasoning, and evaluating claims. Through a detailed exploration of the way teachers introduced classes to the topic of evolution, I was able to examine how teachers used language to build a social framework for participation, examined the opportunities and challenges stemming from their various approaches, and explored how the structure and content of classroom talk contributed to framing science. This study used techniques from interactional sociolinguistics and conversational analysis to examine videos of interaction in five secondary biology classrooms on the day teachers introduced their students to lessons focused on the topic of evolution. Implications of this study focus on how teacher's discourse moves could open or close a discussion to student knowledge contributions, and emphasize how open discussions offer both challenges and opportunities to teachers wishing to facilitate scientifically literate discourse practices in their classroom. / text