Phylogenetic trees are essential to understanding evolutionary relatedness, yet undergraduates struggle to interpret these visualizations. This research uses data from students enrolled in a majors introductory biology course to characterize patterns in students’ tree thinking and how students’ reasoning changes over time and in response to instruction.

Background: Phylogenetic trees have become increasingly essential across biology disciplines. Consequently, learning about phylogenetic trees has become an important component of biology education and an area of interest for biology education research. Construction tasks, in which students generate phylogenetic trees from some type of data, are often used for instruction. However, the impact of these exercises on student learning is uncertain, in part due to our fragmented knowledge of what students construct during the tasks. The goal of this project was to develop a more robust method for describing student-generated phylogenetic trees, which will support future investigations that attempt to link construction tasks with student learning.Results: Through iterative examination of data from an introductory biology course, we developed a method for describing student-generated phylogenetic trees in terms of style, conventionality, and accuracy. Students used the diagonal style more often than the bracket style for construction tasks. The majority of phylogenetic trees were constructed conventionally, and variable orientation of branches was the most common unconventional feature. In addition, the majority of phylogenetic trees were generated correctly (no errors) or adequately (minor errors only) in terms of accuracy. Suggesting extant taxa are descended from other extant taxa was the most common major error, while empty branches and extra nodes were very common minor errors. Conclusions:The method we developed to describe student-constructed phylogenetic trees uncovered several trends that warrant further investigation. For example, while diagonal and bracket phylogenetic trees contain equivalent information, student preference for using the diagonal style could impact comprehension. In addition, despite a lack of explicit instruction, students generated phylogenetic trees that were largely conventional and accurate. Surprisingly, accuracy and conventionality were also dependent on each other. Our method for describing phylogenetic trees constructed by students is based on data from one introductory biology course at one institution, and the results are likely limited. We encourage researchers to use our method as a baseline for developing a more generalizable tool, which will support future investigations that attempt to link construction tasks with student learning.

Phylogenetic trees have become an important component of biology education, but their utility in the classroom is compromised by widespread misinterpretations among students. One factor that may contribute to student difficulties is style, as diagonal and bracket phylogenetic trees are both commonly used in biology. Previous research using surveys found that students performed better with bracket phylogenetic trees across a variety of interpretation tasks. The present study builds on prior research by comparing how students interpret diagonal and bracket phylogenetic trees in the context of an introductory biology course and by expanding the style comparison to include construction tasks. Students performed significantly better with bracket phylogenetic trees for some, but not all, interpretation tasks. In addition, students who constructed bracket phylogenetic trees were significantly more accurate compared to those who used the diagonal style. Thus, our results reinforce previous research for interpretations, and the performance gap between styles extended to construction tasks. It remains to be seen, however, if such differences persist after instruction that balances the use of diagonal and bracket phylogenetic trees.

Phylogenetic trees have become increasingly important across the life sciences, and as a result, learning to interpret and reason from these diagrams is now an essential component of biology education. Unfortunately, students often struggle to understand phylogenetic trees. Style (i.e., diagonal or bracket) is one factor that has been observed to impact how students interpret phylogenetic trees, and one goal of this research was to investigate these style effects across an introductory biology course. In addition, we investigated the impact of instruction that integrated diagonal and bracket phylogenetic trees equally. Before instruction, students were significantly more accurate with the bracket style for a variety of interpretation and construction tasks. After instruction, however, students were significantly more accurate only for construction tasks and interpretations involving taxa relatedness when using the bracket style. Thus, instruction that used both styles equally mitigated some, but not all, style effects. These results inform the development of research-based instruction that best supports student understanding of phylogenetic trees.