Case studies of two biology teachers, Cathy and David, from the same minority-majority, urban U.S. high school, provide insights into their instructional practices while they engaged in long-term professional development (PD). Findings suggest why science teachers engaged with PD may, or may not, adopt more adaptive pedagogical approaches in the service of reform-based teaching. Gee's institution- and affinity-identity constructs were used as analytic lenses regarding teachers' perceptions of teaching, learning, and agency in the dual contexts of their school's institutional environment and PD community. Over time, Cathy adopted more inquiry-based instructional practices she learned through PD seminars in building a scientific classroom discourse community with her majority Latinx students. Her professional identity and teaching became more aligned with the more progressive teaching philosophy and instructional practices promoted by the PD affinity group. While David understood and enjoyed the PD, ultimately, he minimally adopted new strategies, adhering mainly to his pre-PD mode of direct instruction, staying within the strict culture of accountability of his school's administrative priorities to raise state test scores. These cases demonstrate why some teachers of diverse students are adaptive adopters of reform-based instruction through new affinity group membership, while others demonstrate greater allegiance to their institution-aligned identities.