University of Rochester
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This page first made public: Jun 4, 2009
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This is the third in a series of exercises in Cladistics for students. The first two are simpler, because they are planned to "work out." This one requires students to grapple with choosing characters (and character states) as well as deal with homology.
The course is "Principles of Paleontology" and is meant for students at the sophomore to junior level in a geology major. Students entering the class should have had Physical Geology and/or Historical Geology. Biology is not a prerequisite. This course is required for students in the Geobiology track of the Geology major.
Skills and concepts that students must have mastered
Students at this point should know the terminology of cladistics: Characters, homology, character states, steps, parsimony, monophyly. Students should understand the difference between taxonomy and phylogeny.
How the activity is situated in the course
Students have already constructed (in class) at least two cladograms, both of which were set up to have a single solution with no homology. It usually takes place near mid-semester.
Content/concepts goals for this activity
To understand what cladograms and cladistics are as a research tool. Any paleontologist needs to know how to construct a cladogram.
Higher order thinking skills goals for this activity
Understanding subjectivity in determination of characters and character states and the weighting of characters.
Other skills goals for this activity
Description of the activity/assignment
Students were provided with drawings of snowflakes and were asked to construct a cladogram of the snowflakes. The drawings were provided by the students themselves during a quiz a few weeks earlier. (The quiz was 1: write your name, 2: draw a snowflake). There were 17 students and therefore 17 snowflakes. Students are asked to construct a character table and determine various character states. Then students construct a second table showing each snowflake and its state for each of the characters. Finally, students are asked to use this to construct a cladogram, as best they can, from their character matrix. Students note homologies and count the steps in their cladograms.
Determining whether students have met the goals
I do not 'grade' these exercises. I simply mark whether they did the exercise or not (some students couldn't be bothered with it). In class when exercises are turned in, we compare and discuss the characters that students used as well as the total steps each cladogram used. We talked about which characters are meaningful and which are not. I used this to show students on one hand the utility of cladistics (some relationships almost always appeared) and also its subjectivity. Students seemed to really enjoy this exercise.
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