Phylogenetics+Lab+exercise+03

Note that many vertebrate lineages are excluded from

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Unformatted text preview: le, we would group the 14 beetle species with six segments together — but would not group the seven species with five segments together because this is the original character state. Correct: grouped by derived characteristics Incorrect: grouped by ancestral characteristics 5. Work out conflicts Work out conflicts that arise by some clearly stated method, usually parsimony (more on this later). 6. Build your tree. Follow these rules: • All taxa go on the endpoints of the tree, never at nodes. • All nodes must have a list of derived characteristics, which are common to all taxa above the node (unless the character is later modified). • All derived characteristics appear on the tree only once, unless the character state was derived separately by evolutionary parallelism. Remember that this phylogeny is a hypothesis. It is supported by the available data, but new data or new interpretations of old data could change it! To be confident about your hypothesis, you must scrutinize your data by asking questions like these: • Could a supposed derived characteristic be the result of convergent evolution? • Do your characters make sense from an evolutionary perspective? • Should you consider other characters? • Should you consider additional taxa? Reconstructing trees: A simple example Now we'll go through a simple example based on the steps just described. 1. Choose the taxa. You decide to study the major clades of vertebrates shown in the leftmost column of the table below. (Note that many vertebrate lineages are excluded from this example for the sake of simplicity.) 2. Determine the characters. After studying the vertebrates, you select a set of traits, which seem to be homologies, and build the following data table to record your observations. (Note that many relevant vertebrate characters are excluded from this example for the sake of simplicity.) 3. Determine the polarity of characters. From studying fossils and outgroups closely related to the vertebrate clade, you hypothesize that the ancestor of vertebrates had none of these features. 4. Group taxa by derived characteristics. Since we have a good idea of what the ancestral characters are (see above), this is not so hard. We might start out by examining the egg character. We focus in on the group...
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