Learning progressions, or representations of how student ideas develop in a domain, hold promise as tools to support teachers' formative assessment practices. The ideas represented in a learning progression might help teachers to identify and make inferences about evidence collected of student thinking, necessary precursors to modifying instruction to help students advance in their learning. The study reported in this article took the novel approach of using a learning progression for natural selection to support teachers' enactment of formative assessment. Sources of data include interviews and videotapes of six high school biology teachers leading assessment conversations around the same formative assessment questions. Results indicate that while teachers picked out and made inferences about student ideas related to the learning progression during assessment conversations, they did not use all parts of the learning progression in the same way. Furthermore, several of the teachers seemed to use the learning progressions simply as catalogs of misconceptions to be "squashed" rather than drawing upon the developmental affordances offered by a learning progression. Results are framed in terms of the utility of learning progressions as supports for classroom practice. (Contains 10 tables and 3 figures.)