Tree thinking involves using cladograms, hierarchical diagrams depicting the evolutionary history of a set of taxa, to reason about evolutionary relationships and support inferences. Tree thinking is indispensable in modern science. College students' tree-thinking skills were investigated using tree (much more common in professional biology) and ladder (somewhat more common in textbooks) cladogram formats. Students' responses to questions assessing five tree-thinking skills provided evidence for several perceptual and conceptual factors that impact reasoning (e.g., the Gestalt principles of good continuation and spatial proximity, prior knowledge). Instructional implications of the results include using the tree format for initial instruction and clarifying that most recent common ancestry determines evolutionary relatedness. Broader implications for designing scientific diagrams and promoting diagrammatic literacy are considered. (Contains 7 figures, 4 tables, and 6 notes.)