The Framework for K-12 Science Education set an ambitious goal of broadening participation in science learning for all students. Meeting this vision will involve supporting teachers in making meaningful connections with the cultural and linguistic resources their students bring to school; in essence, developing pedagogies that frame these resources as assets important to learning. In this manuscript, we present a qualitative case study of one community of high school science teachers who participated in a year-long professional learning focused on formative assessment co-design related to natural selection. Findings show that the process of formative assessment co-design surfaced both deficit- and asset-based statements about students' contributions. Teachers were more likely to share deficit-based statements as compared to facilitators, whose statements were more asset-based. This was particularly true with reference to students' prior knowledge and linguistic resources. At the same time, our analysis suggests that teachers were more likely to share more asset-based framings of learners when practicing for and reflecting on enactment of formative assessment tasks. These findings suggest that supportive co-design environments can encourage teachers to take more asset-oriented views of learners. We discuss the implications of these findings for professional learning and science classroom practice.