big idea, so that focusing on them is a means of condensing standards. Ideally, a big idea is revisited throughout schooling, so that a student’s knowledge is progressively refined and elaborated. This practice potentially simplifies the alignment between curriculum and assessment because both are tied to the same set of constructs.

The team also advocates that big ideas be chosen with prospective pathways of development firmly in mind. They note that these are sometimes available from research in learning, but typically also draw on the opinions of master teachers as well as some inspired guesswork to bridge gaps in the research base.

Second, standards are aligned with the big ideas, so that they can be considered in the context of more central ideas. This practice is another means of pruning standards, and it is a way to develop coherence among individual standards.

Third, standards are elaborated as learning performances. As described earlier, learning performances describe specific cognitive processes and associated practices that are linked to achieving particular standards, and thus help to guide the selection of situations for gathering evidence of understanding as well as clues as to what the evidence means.

The team illustrates its approach by developing a cartography of big ideas and associated learning performances for evolutionary biology for the first eight years of schooling. The cartography traces the development of six related big ideas that support students’ understanding of evolution. The first and most important is diversity: Why is life so diverse? The other core concepts play a supporting role: (a) ecology, (b) structure-function, (c) variation, (d) change, and (e) geologic processes. In addition to these disciplinary constructs, two essential habits of mind are included: mathematical tools that support reasoning about these big ideas, and forms of reasoning that are often employed in studies of evolution, especially model-based reasoning and comparative analysis. At each of three grade bands (K–2; 3–5, 6–8), standards developed by the National Research Council (1996) and American Association for the Advancement of Science (1993) are elaborated to encompass learning performances. As schooling progresses, these learning performances reflect increasing coordination and connectivity among the big ideas. For example, diversity is at first simply treated as an extant quality of the living world but, over years of schooling, is explained by recourse to concepts developing as students learn about structure-function, variation, change, ecology, and geology.

The team chose this topic because of its critical and unifying role in the biological sciences and because learning about evolution requires synthesis and coordination among a network of related concepts and practices, ranging from genetics and ecology to geology, so that understanding evolution is likely to emerge across years of schooling. Thus, learning about evolution will be progressive and involve coordination among otherwise discrete disciplines (by contrast, one could learn about ecology or geology without considering their roles in evolution). Unlike other areas in science education, evolution has not been thor-

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