This paper presents an activity that engages students in model-based reasoning, requiring them to predict the behavior of the trp and lac operons under different environmental conditions. Students are presented six scenarios for the trp operon and five for the lac operon. In most of the scenarios, specific mutations have occurred in genetic elements of the system that alter the behavior from the norm. Students are also challenged to relate their understanding of operon behavior to the “Big Ideas” of homeostasis, evolution, information, interactions, and emergent properties. By using operons to teach students to reason with models of complex systems and understand broad themes, we equip them with powerful skills and ideas that form a solid foundation for their future learning in biology.

The release of A Framework for K–12 Science Education: Practices, Crosscutting Concepts, and Core Ideas ( NRC, 2012 ) provides the basis for the next generation of science standards. This article first describes that foundation for the life sciences; it then presents a draft standard for natural selection and evolution. Finally, there is a discussion of the implications of the new standards for biology programs in general and curriculum, instruction, and assessment in particular.