BIS2C_Lect2_Ward_Tree_Concepts_ppt

BIS2C_Lect2_Ward_Tree_Concepts_ppt - BIS2C Winter 2012...

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1 BIS2C Winter 2012 (Ward) Lecture 2. Fun with trees (cont d) Today s second lecture (9 Jan) more tree terminology and concepts phylogeny and classification Wednesday (11 Jan) methods for building trees terminal taxa u v x y z root, root node internal nodes internal branches w Internal nodes represent hypothetical ancestral taxa t So far we have been discussing bifurcating trees. These are trees in which each internal node gives rise to two descendant branches. Sometimes we draw trees in which some nodes have three or more descendant branches. Such nodes are called polytomies and they usually reflect uncertainty about phylogenetic relationships. polytomy An extreme case of ignorance: a completely unresolved bush polytomy
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2 There are three possible resolutions of this three branched polytomy (assuming that the true tree is bifurcating). polytomy (1) The beetle is more closely related to (fly + moth) than the wasp is. polytomy (1) The beetle is more closely related to (fly + moth) than the wasp is. polytomy (2) The wasp is more closely related to (fly + moth) than the beetle is. (2) The wasp is more closely related to (fly + moth) than the beetle is. polytomy What is the third possibility?
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3 (3) Wasp and beetle are each other s closest relatives. How many possible resolutions of an 8 way polytomy? polytomy Monophyletic group : a group of organisms consisting of their most recent common ancestor (MRCA) and all its descendants Clade = a monophyletic group Paraphyletic group : a group of organisms consisting of their MRCA but excluding some of its descendants Polyphyletic group : a group of organisms that excludes the MRCA Textbook Life 9e Section 22.4 Clades (monophyletic groups) abcdehgf Clades (monophyletic groups) Most recent common ancestor of h & g Clades (monophyletic groups)
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4 abcdehgf Clades (monophyletic groups) Clades (monophyletic groups) Clades (monophyletic groups) Clades (monophyletic groups) Clades (monophyletic groups) Clades (monophyletic groups) What is the relationship between the number of monophyletic groups and the number of internal nodes in a tree?
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5 What is the relationship between the number of monophyletic groups (x) and the number of internal nodes (y) in a tree? A. x < y B. x > y C. x = y D. depends on the shape of the tree E. depends on the number of terminal taxa in the tree abcdehgf Clades (monophyletic groups) If a tree has n internal nodes, then there are n monophyletic groups in that tree (excluding terminal taxa).
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BIS2C_Lect2_Ward_Tree_Concepts_ppt - BIS2C Winter 2012...

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