Cutting Edge > Paleontology > Teaching Activities > How do cladistics work? Generating phylogenetic trees using everyday objects.

How do cladistics work? Generating phylogenetic trees using everyday objects.

Hilary Lackey
,
Cal Poly - Pomona; Pomona College
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  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
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This page first made public: Jun 19, 2009

Summary

This lab activity is intended to introduce students to concepts of homology and evolutionary characters, and to help them understand how cladistic analyses are performed. The activity begins with generating a phylogeny of candy bars in class, and ends with a homework write up in which students create their own phylogeny of any group of objects.

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Context

Audience

For use in an intermediate to advanced paleontology or historical geology course.

Skills and concepts that students must have mastered

Students should already have encountered the basics of systematics and the Linnaean hierarchy. Students should have been introduced to the concept of shared, derived characters (synapomorphy), analogy and homology, and have seen/discussed examples of cladograms and phenograms in order to be comfortable with the concept of evolutionary trees.

How the activity is situated in the course

This is a stand-alone lab activity.

Goals

Content/concepts goals for this activity

Phylogeny, cladistic analysis, characters and states

Higher order thinking skills goals for this activity

critical thinking (evaluation of phylogenetic methods); generating hypotheses (about evolutionary relationships)

Other skills goals for this activity

Description of the activity/assignment

Students of paleontology, evolutionary biology and Earth history will be best able to critically analyze readings about evolution if they have first hand experience generating their own evolutionary hypotheses. In this lab exercise, students apply the principles of cladistics to create evolutionary trees of fun, familiar objects. The candy bar lab concept – expanded here - came from Dr. Robert Gaines of Pomona College.

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.

Determining whether students have met the goals

Students will hand in a lab write-up that includes their evolutionary trees and a written evolutionary hypothesis. Additionally, they will answer follow-up questions about the nature of homology and analogy, recognizing synapomorphy, and the bias inherent in assigning characters and character states. Because each student's work will be different and partially subjective, the grading rubric will not include an answer key, but should evaluate the student's comprehension and application of the rules of cladistic analysis.

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