Rethinking Tree-Thinking: Cladograms, Ancestors & Evidence

Abstract

I offer comments on two recent articles in The American Biology Teacher by Davenport and colleagues addressing the interpretation and construction of phylogenetic trees. The “tree-thinking” literature suggests that students need to acquire a clear understanding of the meaning of phylogenetic tree diagrams. To this end, I provide clarifications of terminology and address the problematical status of “ancestors.” Cladograms are not genealogies viewed from a distance, but empirical hypotheses of relationship based on the distribution of shared derived character states. I describe an exercise employed in an introductory systematics course that emphasizes the empirical activities of character delimitation and formation of groups based on those characters.