This paper explores how a learning theory enriched a collaborative teacher inquiry discourse where lesson study was adopted as the educational action research model to promote teacher professional development. Four Grade 9-10 biology teachers in Singapore drew from variation theory to collaboratively plan and teach new genetics content as part of teacher research. As a researcher-facilitator, I have drawn from the teachers' experiences of infusing variation theory into their teaching practice, of developing their own theories of teaching and learning, and of developing student-focused pedagogies to highlight the roles variation theory played. These included providing concrete examples of patterns of variations teachers could use, increasing the teachers' attention and sensitivity to students, and developing the teachers' theoretical lens in their approach to teaching. Reflecting on the teachers' experiences, I made an appeal for teachers' development of theoretical lenses in teacher inquiry, and bridging of theory and research to teaching practice.