Teaching Tree Thinking to College Students: It’s Not as Easy as You Think

The ability to understand and reason with tree-of-life diagrams (i.e., cladograms), referred to as tree thinking, is an essential skill for biology students. Yet, recent findings indicate that cladograms are cognitively opaque to many college students, leading them to misinterpret the information depicted. The current studies address the impact of prior biological background and instruction in phylogenetics on students’ competence at two foundational tree-thinking skills. In Study 1, college students with stronger ( N = 52) and weaker ( N = 60) backgrounds in biology were asked to (a) identify all the nested clades in two cladograms and (b) evaluate evolutionary relatedness among taxa positioned at different hierarchical levels (two questions) and included in a polytomy (two questions). Stronger-background students were more successful than weaker-background students. In Study 2, a subset of the stronger-background students ( N = 41) who were enrolled in an evolution class subsequently received two days of instruction on phylogenetics. As expected, these students’ tree-thinking skills generally improved with instruction. However, although these students did very well at marking the nested clades, fundamental misinterpretations of relative evolutionary relatedness remained. The latter was especially, although not exclusively, the case for taxa included in a polytomy. These results highlight the importance of teaching cladistics, as well as the need to tailor such instruction to the difficulties students have learning key macroevolutionary concepts.