Although the core work of science is oriented toward constructing, revising, applying, and defending models of the natural world, models appear only rarely in school science, and usually only as illustrations, rather than theory building tools. We describe the rationale and structure for a learning progression to understand the development of modeling under supportive forms of instruction. In this case, elementary and middle school students are modeling "big ideas" in the life sciences that hold the promise of serving as a conceptual foundation for reasoning about the theory of evolution later in their education. In this conceptual paper, we sketch changes from grades K through 6 in representational and modeling practices across three interlocking constructs that, considered collectively, comprise the aforementioned conceptual foundation: "Change" (in individuals and populations), "Variation," and "Ecosystems." The paper closes by delineating pedagogical principles for supporting the development of modeling across grades of instruction.