Program evaluation frameworks that assess organizations as complex adaptive systems can open our eyes to new approaches and understandings. The proposed iterative and adaptive developmental program evaluation framework interrelates developmental evaluation principles with adaptive action approaches. We explain and demonstrate the framework with a mixed methods case study of the Teaching and Researching Environmental Education (TREE) Semester, an undergraduate pre-service environmental and sustainability educator leadership program. To illustrate developmental evaluation rigor, we describe the collection of TREE students' end-of-program quantitative and qualitative self-assessment responses to factors that helped their learning and promoted their learning gains. Analyzing these responses allowed us to continually make adaptive changes to develop a thriving learning community that fosters environmental and sustainability education leadership development. The iterative and adaptive developmental program evaluation framework improved our understanding of TREE's learning community as a complex adaptive system by elucidating five simple, emergent, pattern-forming rules. The rules describe activity selection, enhanced strategies for communication and support, integrating staff and student learning, and program pace. This demonstration of a developmental program evaluation provides a model framework for environmental and sustainability education program evaluators, especially those who work with new, innovative, complex, and re-imagined programs.