Arguing from evidence is one of eight key science practices in which students should engage. It is an essential component of science, yet students have difficulties with this practice. We describe a scaffolded claims-evidence-reasoning (CER) argumentation framework that is embedded within a new eight-week, freely available curriculum unit developed by the Genetic Science Learning Center – Evolution: DNA and the Unity of Life. The scaffold provides high school students with practice in both developing and evaluating written arguments. It is designed to incrementally build student skill week-by-week, starting with an introduction to the CER components of an argument, and ending with students evaluating data and constructing a supported written argument. We also present evaluation findings from field testing the argumentation scaffold in the context of the complete Evolution unit in dozens of classrooms. And we discuss how this integrated, scaffolded approach to argumentation influenced both student and teacher learning.