Learning progressions, or sequences of how ideas and practices develop within domains, are increasingly a focus of science education research. Recently, researchers have called for these progressions to be used as interpretive frameworks for teachers' instructional planning and assessment practices. In this study, we explore data from two high schools collected in two studies. In the first study, we engaged with teachers to develop and refine a learning progression for natural selection alongside formative assessments. In the second study, we took this learning progression to teachers at a different school, and used it to co-develop formative assessments and plan units. We adopt a communities of practice perspective to frame two case studies of these schools, taking the learning progression as a boundary object that not only maintained its meaning across the two different communities, but also took up different meanings within each community. We found that the learning community that helped to develop the learning progression used it as an opportunity to bring previously disparate units into sync, and to develop and enact a common sequence of formative assessments within their unit. In contrast, at the second school, teachers struggled to make sense of the learning progression within the accountability context of their school, as well as other tools provided them by the school and district. These results indicate that teachers could potentially benefit from the opportunity to co-develop learning progressions with researchers that capture their ideas that are shared within the community; however, if learning progressions are not in sync with other tools provided to teachers to structure their planning, they will not be taken up in the same way.