With applications of Tree of Life data becoming ever more prevalent in everyday contexts, tree thinking has emerged as a vital component of scientific literacy. This article reports a study testing the hypothesis that instruction in natural selection, which is the primary focus of US evolution education at the high school and introductory college levels, does little to promote tree-thinking skills and that explicit instruction in understanding evolutionary tree diagrams is required. Testing this contention required the creation of a novel tree-thinking assessment and an instructional intervention, both guided by deep knowledge of evolutionary biology and of science education research. College students (N?=?124) with weaker versus stronger backgrounds in biology were randomly assigned to control versus instructional conditions and were also assessed for their knowledge of natural selection. Although knowledge of natural selection and ability to engage in tree thinking were correlated, a short instructional booklet that provided a basic introduction to evolutionary trees predicted tree-thinking success more strongly than did either knowledge of natural selection or previous college coursework in biology. Clearly, tree thinking and natural selection are dissociable constructs that must both be taught for students to grasp the full gamut of evolutionary patterns and processes.