This case study is based on Dr. Richard Lenski’s ongoing studies of evolution in E. coli. Students are introduced to prokaryotic biology and to Lenski’s studies based on serial broth cultures of E. coli, which have been monitored for genotypic and phenotypic changes for over 25 years. One subculture evolved the ability to use citrate as food source under conditions where it previously could not. Students explore the cell biology, molecular genetics and ecology of this system by engaging in modules that tie into one another using evolution as an overarching theme. The modules can be used in any order across the biology curriculum or can be used to revisit and/or summarize previously learned topics. This integrative approach employs both problem-based learning techniques and directed questions as students engage in the modules. The case study is appropriate for an introductory biology course, an AP or Honor’s high school biology course or an upper level course in evolution. The research phase of problem-based learning is supported by a series of PowerPoint slides that can be downloaded, edited and used according to instructors’ needs.