Untangling the contribution of characters to evolutionary relationships: a case study using fossils,morphology, and Genes

Given the importance of phylogenetic trees to understanding common ancestry and evolution, they are a necessary part of the undergraduate biology curriculum. However, a number of common misconceptions, such as reading across branch tips and understanding homoplasy, can pose difficulties in student understanding. Students also may take phylogenetic trees to be fact, instead of hypotheses. Below we outline a case study that we have used in upper-level undergraduate evolution and ichthyology courses that utilizes shark teeth (representing fossils), body characters, and mitochondrial genes. Students construct their own trees using freely available software, and are prompted to compare their trees with a series of questions. Finally, students explore homoplasy, polytomies, and trees as hypotheses during a class discussion period. This case study gives students practice with tree-thinking, as well as demonstrating that tree topology is reliant on which characters and tree-building algorithms are used.