Efforts to improve university science education continue to emphasize the importance of active learning, including frequent formative assessment and timely feedback that helps students reach desired learning outcomes. Yet, nationwide, many instructors continue to use primarily lecture-based teaching methods, with limited use of formative assessment and feedback. Factors that affect instructor adoption and implementation of new teaching techniques include departmental norms, peer interaction, and classroom environment. In this work, a model of the impacts of departmental teaching and social norms and peer interactions on instructor innovation decision is presented. This model is then used to explore 1) instructor teaching-related interactions within a single biology department, assessing the conditions for innovation diffusion, and 2) instructor perceptions of norms and interactions in that department and their impact on decision-making. Finally, introductory biology instructors’ use of assessment and feedback techniques were characterized in a lecture hall and an active learning classroom to see how innovation adoption translates to specific assessment practice and investigate the impact of the active learning classroom. Results indicate that perceptions and practices vary widely, but that both peer interactions and active learning classrooms may have a positive impact on teaching innovation adoption and practices in a university biology department. In addition, the pattern of interactions within this department allows instructors of varying assessment experience to interact, making it potentially conducive to the spread of teaching ideas. The model and results presented here will assist in understanding the factors involved in instructor decision-making and can be leveraged to help promote the use of formative assessment and other evidence-based teaching practices.