This article evaluates a validity argument for the degree to which assessment tasks are able to provide evidence about knowledge that fuses information from a progression of core disciplinary ideas in ecology and a progression for the scientific practice of developing evidence-based explanations. The article describes the interpretive framework for the argument, including evidence for how well the assessment tasks are matched to the learning progressions and the methods for interpreting students' responses to the tasks. Findings from a dual-pronged validity study that includes a think-aloud analysis and an item difficulty analysis are presented as evidence. The findings suggest that the tasks provide opportunities for students at multiple ability levels to show evidence of both successes and struggles with the development of knowledge that fuses core disciplinary ideas with the scientific practice of developing evidence-based explanations. In addition, these tasks are generally able to distinguish between different ability-level students. However, some of the assumptions in the interpretive argument are not supported, such as the inability of the data to provide evidence that might neatly place students at a given level on our progressions. Implications for the assessment system, specifically, how responses are elicited from students, are discussed. In addition, we discuss the implications of our findings for defining and redesigning learning progressions. (Contains 7 figures, 5 tables, and 5 notes.)