The primary goal of initial teacher education programmes is to develop pre-service teachers' pedagogical content knowledge (PCK). In science education, there have been calls for programmes to help pre-service teachers develop this knowledge at a topic-specific level. In this study, I investigated how lesson planning using a specific guideline called the rationale for lesson design (RfLD) can be used as a tool to develop science pre-service teachers' topic-specific pedagogical content knowledge (TSPCK). This study is positioned within the collective PCK realm of the refined consensus model of PCK and the five components of TSPCK. The participants were six third-year Life Sciences pre-service teachers. Data were collected through lesson plans on the topic of evolution and narrative interviews. A 4-point scale rubric was adapted to score the quality of pre-service teachers' PCK in terms of the five components of TSPCK in each section of the RfLD. Results indicate that the pre-service teachers showed higher levels of PCK especially, in terms of learner prior knowledge and curricular saliency. I argue these scores can be attributed to the RfLD guidelines. I recommend that science pre-service teacher programmes need to expose students to this guideline as a way of developing their PCK in teaching methodology modules. Furthermore, I argue for the value of the RfLD in tracking pre-service teachers' PCK.