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.