Background: Evolutionary trees illustrate relationships among taxa. Interpreting these relationships requires developing a set of "tree-thinking" skills that are typically included in introductory college biology courses. One of these skills is determining relationships among taxa using the most recent common ancestor, yet many students instead use one or more alternate strategies to determine relationships. Several alternate strategies have been well documented and these include using superficial similarity, proximity at the tips of a tree, or the fewest intervening nodes in the tree to group taxa. Results: We administered interviews (n = 16) and pencil-and-paper questionnaires (n = 205), and constructed a valid and reliable assessment that measured how well students determined relationships among taxa on an evolutionary tree. Our questions asked students to consider a focal taxon and identify which of two additional taxa is most closely related to it. We paired the use of most recent common ancestor with one of three alternative strategies (i.e., similarity, proximity, or node-counting) to explicitly test students' understanding of the relationships among the taxa on each tree. Conclusions: Our assessment enables us to identify students who are effectively distracted by an alternative strategy, those who use the most recent common ancestor inconsistently, or who are guessing in order to determine relationships among taxa. Our 18-question tool (see Additional file 1) can be used for formative assessment of student understanding of how to interpret relationships on evolutionary trees. Because our assessment tests for the same skill throughout, students who answer incorrectly, even once, likely have an incomplete understanding of how to determine relationships on evolutionary trees and should receive follow-up instruction.
A phylogeny depicts the hypothesized evolutionary relationships among taxa as a nested hierarchical branching diagram. Interpreting the relationships among taxa on a phylogeny is part of a set of skills called tree-thinking. Because published phylogenies are not constructed for the purpose of tree-thinking pedagogy, the information can be difficult for students to interpret and explicit instruction is required for mastery of the tree-thinking skill-set. We present a process to construct customizable assessment questions using published phylogenies, to assess a key tree-thinking skill, determining relatedness among taxa on a phylogeny. We detail how to construct two types of forced-choice questions: binary-choice and four-choice. In both question types, students are presented with a phylogeny and are instructed to determine which taxon from a list of taxa is most closely related to a focal taxon. The list of taxa includes distracters as possible responses explicitly selected based on common alternative strategies (similarity, proximity, node counting), in addition to the correct response. Instructors can select taxa of their own choosing in order to customize assessments. These assessment questions can be utilized during instruction as a formative assessment to enhance learning or in a summative assessment.
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