Experts in many fields not only know more than novices; they organize and use their knowledge very differently than novices. Most of our biology majors begin college as novices, and find it difficult to organize and connect concepts and knowledge. How do they become more expert thinkers and biologists? My research seeks to characterize the changes, especially early changes, that signal shifts to more expert-like thinking for undergraduate biologists. Using the Biology Problem Sorting Task, students sort and categorize biology problems at the beginning and end of single semesters of introductory biology. My team is collecting data from students enrolled in first and second semester courses for majors and non-majors, from systematic observations of classroom environments, and from demographics, in order to develop mechanistic and predictive models about who advances, and under what circumstances. Early results point to the importance of incorporating scientific practices in our undergraduate biology courses. My research is also helping to systematically identify particularly difficult concepts and misconceptions that may inhibit the development of student expertise. This research has applications in systematic and finer-grained curriculum planning, and has the potential to target measures that boost retention and advancement toward mastery.