Introductory biology students have a lot of trouble reading evolutionary trees. On last fall’s final exam, I picked up an image that I found online (see right; it is figure 15 in this article), labeled the species in it, and asked my students this true/false question: “The evolutionary tree at right shows that a human is more closely related to a sea lily than it is to a sea urchin.” A majority of the students incorrectly answered “True,” and from previous conversations with students I think I know why. They count the nodes in the tree, then incorrectly infer that more nodes means more evolutionary steps and, therefore, a more distant evolutionary relationship. The concept of common ancestry, which I do try to emphasize in my class, clearly is not translating to the reading of evolutionary trees (which I have not explicitly taught).
I am not alone. A recent paper by Dees et. al (2014) confirmed that student difficulties in reading evolutionary trees are stubbornly resistant to instruction. At multiple times throughout the semester, the authors asked small groups and individual students to answer questions about taxa relatedness using an evolutionary tree. Importantly, after each question, students had to explain the reasoning for their choice. This approach allowed the authors to test both student “correctness” and their understanding of evolutionary trees.
Students did improve throughout the course. At the beginning of instruction, 8% of students correctly answered a question about taxa relatedness, but no student who answered correctly provided the correct reasoning. On the final exam, after repeated targeted instruction on evolutionary trees, 38% of students correctly answered a question about an evolutionary tree and provided correct reasoning. Many other students who provided correct reasoning answered incorrectly, suggesting that students are memorizing how to interpret evolutionary trees without understanding how to apply their knowledge. Still others answered correctly but provided incorrect reasoning. Since answers often did not align with reasoning, the authors concluded that multiple choice questions are not sufficient for testing student interpretations of trees.
The students who interpreted the trees correctly used either of two strategies: most recent common ancestry and monophyletic grouping. The most common incorrect methods were counting the number of nodes between two groups (that’s what I think my students were doing on last semester’s final), comparing branch tip proximity, and counting synapomorphies.
In direct response to this article, digital author Matt Taylor has created a couple of learning resources to try to help students learn the correct ways to read evolutionary trees. Both are posted below. The first is a video, which is set up like a tutoring session, with the narrator working step-by-step through possible incorrect and correct ways to interpret an evolutionary tree. The second is an interactive exercise that allows students to test their understanding. It provides targeted feedback that explains why some methods of reading the tree are incorrect.
Let us know what you think, and feel free to add ideas of your own. How do you teach your students to interpret evolutionary trees?
Dees, J., Momsen, J., Niemi, J., & Montplaisir, L. (2014). Student interpretations of phylogenetic trees in an introductory biology course. CBE—Life Science Education, 13, 666–676. DOI: 10.1187/cbe.14-01-0003
Gregory, T.R. (2008). Understanding Evolutionary Trees. Evo Edu Research, 1, 121–137. DOI: 10.1007/s12052-008-0035-x