Active-learning instructors are more effective when they use pedagogical content knowledge (PCK) to anticipate, interpret, and respond to student thinking. PCK is topic-specific and includes knowledge of student thinking (e.g., common difficulties) and knowledge of instructional strategies (e.g., effective learning tasks). Currently, we know little about how instructors develop PCK. We documented how 11 early-career undergraduate life science instructors developed PCK over multiple semesters by eliciting knowledge as instructors planned, implemented, and reflected on instruction. Qualitative content analysis indicated that instructors' PCK about student thinking was not necessarily grounded in evidence from students and their PCK about instructional strategies varied in whether and how it considered student thinking. We adapted a rubric to test hypotheses about PCK development trajectories. Participants' PCK about student thinking tended to become more grounded in evidence from students and their PCK about instructional strategies tended to focus more on student thinking over time. However, teaching experience did not necessarily lead to PCK development. Case study analysis revealed that pedagogical knowledge and specific practices supported PCK development. We propose a hypothetical model to explain how teaching knowledge and practices support PCK development. We also suggest reflections and actions for instructors who want to develop their PCK.