How do cladistics work? Generating phylogenetic trees using everyday objects.
This activity has benefited from input from faculty educators beyond the author through a review and suggestion process.
This review took place as a part of a faculty professional development workshop where groups of faculty reviewed each others' activities and offered feedback and ideas for improvements. To learn more about the process On the Cutting Edge uses for activity review, see http://serc.carleton.edu/NAGTWorkshops/review.html.
This page first made public: Jun 19, 2009
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Part A – Candy bars: The instructor provides about 12 "taxa" of candy bar. Students are welcome to taste the samples as they work, but make sure some of each bar is left for examination. Working in groups, students assign characters and states to the candy taxa, decide what's synapomorphic, and sketch a cladogram (tree), marking where new characters or states arise. Toward the end of the period, groups share their findings with the rest of the class, and together all students decide on the best tree. As students discuss the validity of their characters and argue about how to arrive at the most parsimonious tree, they will begin to comprehend the rules – and pitfalls – of cladistic analysis. At this point, the instructor might have them actually enter character data into a computer program such as MacClade, and project the tree results for all to see and discuss.
Part B – Get creative!: After the candy bar lab, the students' homework assignment is to construct a cladogram and evolutionary hypothesis about a group of objects of their choosing. Ideas for "taxa" include models of car (characters could include hatchback, 4WD, sunroof, number of seats, engine capacity, etc.), board games (spinner or dice, number of players, cards used, etc.), or rock bands (members, instruments, genre, etc.). Just about anything – biological organisms excluded – with enough characters and states can be used, and students enjoy coming up with unique approaches. Students will hand in a character matrix, hand-drawn tree with character transformations indicated, and an evolutionary narrative explaining the relationships among their group and discussing constraints and biases. Depending on the time allotted, and the instructor's expertise, this could go a step further with students entering character data into PAUP and/or MacClade to obtain computer-generated trees.
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