The tree of life provides a fundamental roadmap to understanding biodiversity, yet requires integration across scales of the biological hierarchy and a unique set of tree thinking skills. This combination can be challenging for undergraduates at the introductory level because of their preconceptions regarding distinct fields of biology compounded by the unique structure of phylogenetic trees. To address these two challenges while providing an undergraduate research opportunity, we developed an activity for introductory biology students that integrates molecular, organismal, and evolutionary biology. This activity relies on woody plant identification, comparative morphology, and DNA sequence analysis to teach students how to reconstruct and interpret phylogenetic trees. After building separate phylogenetic hypotheses using morphological characters and molecular data, they compare their results with a master Tree of Trees to identify instances of homology and homoplasy. After delivering this activity, the majority of students scored the activity as “helpful to very helpful” in increasing their understanding of these concepts. Overall, we deliver a framework for developing comparable Tree of Trees–type activities that leverage students' interests in familiar organisms and requires them to span scales of the biological hierarchy while improving their tree thinking skills.

In this classroom activity, students build a phylogeny for woody plant species based on the morphology of their twigs. Using any available twigs, students can practice the process of cladistics to test evolutionary hypotheses for real organisms. They identify homologous characters, determine polarity through outgroup comparison, and construct a parsimonious tree based on synapomorphies (shared derived characters). This activity efficiently demonstrates many systematics concepts, including homology, homoplasy (convergence and reversal), polarity, synapomorphy, symplesiomorphy, autapomorphy, polytomy, and parsimony. It also engages students in inquiry, promotes student collaboration, raises awareness of plant structure, and exposes students to the diversity of common local trees.

This exercise demonstrates the principle of parsimony in constructing cladograms. Although it is designed using mammalian cranial characters, the activity could be adapted for characters from any group of organisms. Students score categorical traits on skulls and record the data in a spreadsheet. Using the Mesquite software package, students generate arbitrary cladograms and measure tree length. They then move taxa around to reduce tree length. The exercise can become competitive when students report out on tree lengths and try to achieve shorter trees than their peers. The resulting cladograms can be compared with a published mammalian phylogeny. The exercise illustrates phylogenetics, the principle of parsimony, and hypothesis testing using morphological data.