AnswersIntroCladistics - 2.8 GSI Introduction to Cladistics...

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GSI Introduction to Cladistics - 1 GSI Introduction to Cladistics Labs This Exercise is done in Discussion Section and read before the lab. GSIs can pick one or two exercises. Purpose What do we want students to get out of this lab? Perhaps to answer a few generalized questions: How is data collected for a cladistic analysis? What type of data is collected? How are characters mapped onto a cladogram, and what do these characters mean? How is the best cladogram selected from the data, using the principle parsimony? What does the cladogram mean? (And what doesn’t it mean?) What are the sources of error in the cladistic method? (A comparison with phenetics might fit in here). What kinds of interesting questions can be answered by cladistics? In other words, so what: who cares? Cladograms: What Do They Mean (and Not Mean)? In this course we try to make a distinction between a cladogram and other types of phylogenetic diagrams. Hardcore cladists cannot even agree on what cladograms mean, but we are sticking to one definition, rather than confusing them with the debate. By all means we should all try to be consistent; you should not present a different view just because you think it is better (in the past, there have been problems with GSIs attempting to use definitions other than those used by the lecturers). Before the students begin, some main points that you might emphasize/review about cladograms, with a few sketches on the board, are: synapomorphies are evidence for groups within the cladogram synapomorphy and plesiomorphy are relative terms; a particular character mapped onto a cladogram may be a synapomorphy for one group of taxa, but a pleisiomorphy for another group 2.8
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GSI Introduction to Cladistics - 2 only monophyletic groups are properly recognized as taxonomic groups a taxon may be part of more than one monophyletic group nodes do not represent ancestral taxa; they only represent the diversification (cladogen- esis) of one lineage into two lineages branch lengths and internode lengths do not represent an absolute time scale however, the relative timing of speciation events (nodes) can be determined; we can decide if taxa A and B have a more recent common ancestor than A and C, but we cannot say how much more recently from the cladogram alone cladograms indicate the direction in time of character changes convergence in characters is revealed by the most parsimonious cladogram The Bottom Line: Keep It Simple Keep it simple. If anything, there is a tendency for GSIs to try to become too sophisticated in their explanations. Avoid any jargon not in lecture or the manual. And don’t let them get bogged down in collecting their data (about 10 characters and half a dozen to a dozen taxa are plenty). Remem- ber that students will probably get the most out of constructing their trees and thinking about what the tree means.
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AnswersIntroCladistics - 2.8 GSI Introduction to Cladistics...

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