BIS2C+SectA+Lect2-3+Outline

BIS2C+SectA+Lect2-3+Outline - Distinguishing between...

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BIS2C W2009 Lectures 2-3 (7 January 2009) 1 Fun with trees (cont d) Today’s lecture s (7 January) Tree terminology and concepts (cont’d) Evidence needed for building trees Building phylogenetic trees with parsimony Film: “Discovering the Great Tree of Life” Tree terminology and concepts Bifurcating trees , polytomies , and uncertainty . Monophyletic group (=clade) : a group of organisms consisting of their most recent common ancestor (MRCA) and all its descendants Non-monophyletic groups: paraphyletic and polyphyletic groups. Monophyletic groups (clades) and synapomorphies . Convergence , reversal and homology . Phylogeny and classification Phylogeny = the evolutionary relationships among taxa Classification = the naming of taxa and their placement in a hierarchical arrangement Species: compound ( binomial ) names Higher taxa: single (uninominal) names. Ranked classifications. Sister groups , ingroups and outgroups. Derived traits and ancestral traits . Distinguishing between
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Unformatted text preview: . Distinguishing between cladograms , phylograms and chronograms . BIS2C W2009 Lectures 2-3 (7 January 2009) 2 How do we reconstruct phylogenetic trees? Trees are inferred from the similarities that are observed among taxa. Trees as hypotheses. Characters , character states , and coding of characters. Kinds of characters. Character-taxon data matrix . The principle of parsimony: searching for a tree that requires the fewest steps. What is an unrooted tree ? Basic procedure for finding/searching for the most parsimonious tree. 1. Ignore parsimony-uninformative characters. 2. Draw possible trees. 3. For each informative character count the minimum number of steps required on each tree. 4. Sum over all characters to get the “score” (number of steps) for each tree. 5. The most parsimonious tree(s) is/are the one(s) with lowest score. 6. Root the tree along the branch leading to the outgroup (if known). Example: four-taxon problem....
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