Lec 2 - BIS2C Winter 2009 (Ward) Lecture 2. Fun with trees...

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BIS2C Winter 2009 (Ward) Lecture 2. Fun with trees (cont’d) Today’s lecture (7 Jan) more tree terminology and concepts phylogeny and classification Later this afternoon: Building phylogenetic trees Film: “Discovering the Great Tree of Life”
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root, root node terminal taxa internal nodes internal branches u v w x y z Internal nodes represent hypothetical ancestral taxa t
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So far we have been discussing bifurcating trees. These are trees in which each internal node gives rise to two descendant branches.
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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
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An extreme case of ignorance: a completely unresolved bush polytomy
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There are three possible resolutions of this three-branched polytomy (assuming that the true tree is bifurcating). polytomy
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(1) The beetle is more closely related to (fly + moth) than the wasp is. polytomy
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(1) The beetle is more closely related to (fly + moth) than the wasp is.
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polytomy (2) The wasp is more closely related to (fly + moth) than the beetle is.
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(2) The wasp is more closely related to (fly + moth) than the beetle is.
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polytomy What is the third possibility?
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(3) Wasp and beetle are each other’s closest relatives.
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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 8e) pp. 555
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Clades (monophyletic groups) a b c d e h g f
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a b c d e h g f Clades (monophyletic groups) Most recent common ancestor of h & g
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups)
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a b c d e h g f Clades (monophyletic groups) What is the relationship between the number of internal nodes in a tree and the number of monophyletic groups?
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a b c d e h g f Clades (monophyletic groups) If a tree has n internal nodes, how many monophyletic groups are there in that tree (excluding terminal taxa)?
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a b c d e f g h What are examples of non-monophyletic groups?
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Non-monophyletic groups are groups that include some but not all descendants of the most recent common ancestor of that group a b c d e f g h
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Lec 2 - BIS2C Winter 2009 (Ward) Lecture 2. Fun with trees...

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