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Doug Raiford Lesson 9
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3 Approaches Distance Parsimony Maximum Likelihood Have already seen a distance method 12/18/20152Phylogenetics Part II
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What’s wrong with UPGMA? Let’s revisit the example Can this be? Doesn’t the derived tree imply that B is equidistant from C and D 12/18/2015Phylogenetics Part II3 ABCD ABCD A0767 B045 C03 D0
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UPGMA averaged the two and put them both (branches for C and D) at 1.5 What if don’t have equal rates of evolution after a divergence 12/18/2015Phylogenetics Part II4 ABCD ABCD A0767 B045 C03 D0 4.5 12 2.5
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Differing rates of evolution can sometimes cause problems with UPGMA Especially if very similar (small distances) 12/18/2015Phylogenetics Part II5 ABC A043 B03 C0 ABC 1 21 1 This treeYields this matrixYields this tree BCA
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Also called minimum evolution method Definition of parsimony: 1 a : the quality of being careful with money or resources : thrift b : the quality or state of being stingy 2 : economy in the use of means to an end; especially : economy of explanation in conformity with Occam's razor Ockham's razor: the simplest explanation is usually the best 12/18/20156Phylogenetics Part II
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Looks at each column of an MSA and attempts to find a tree that describes Builds a consensus tree atgccgca-actgccgcaggagatcaggactttcatgaatatcatcatgcgtggga-ttcag acctccatacgtgccccaggagatctggactttcacc---tggatcatgcgaccgtacctac t-atgg-t-cgtgccgcaggagatcaggactttca-gt--g-aatcatctgg-cgc--c-aa t--tcgt-ac-tgccccaggagatctggactttcaaa---ca-atcatgcgcc-g-tc-tat aattccgtacgtgccgcaggagatcaggactttcag-t--a-tatcatctgtc-ggc--tag 12/18/20157Phylogenetics Part II
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What do we mean when we say “attempts to find a tree that describes” Attempts to fit all possible trees in each column and choose best How determine all possible trees? How determine which one has the best fit? Assume that majority nucleotide represents ancestor AGCT AACT One possible tree AAAG A 0 0 A or a G 0 if A 1 if A 12/18/20158Phylogenetics Part II Total mutations that explain this tree = 1 Pretty darn good
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When there are two organisms there is only one possible tree AB 12/18/20159Phylogenetics Part II
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What about when there are three Third could go… AB 12/18/201510Phylogenetics Part II
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For each of the previous 3 trees, could add 4 th to any of its branches (or could form a new root) Each of the possible trees had 4 branches so could add to one of 4 locations (or splice in at top) So total number of trees with 4 leaves: 3*5=15 12/18/2015Phylogenetics Part II11 AB If this were the tree
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N i is number of trees given i taxa B i is the number of branches in a tree given i taxa B i =B i-1 +2, also i x 2-2 N i =N i-1 *(B i-1 +1) plus 1 due to possible new root N 2 = 1 B 2 =2 12/18/2015Phylogenetics Part II12 TaxaBranchesTrees 221 343 4615 58105 610945 71210,395 814135,135 9162,027,025 101834,459,425 1120654,729,075 Defined by a recurrence relation so … That’s right, as usual, exponential Defined by a recurrence relation so … That’s right, as usual, exponential What does this growth rate look like?
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Rooted vs. un-rooted Wherever the root is, un-kink it 12/18/2015Phylogenetics Part II13
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Always bifurcated Can never have 3 branches “from” a single node What are the odds? 12/18/2015Phylogenetics Part II14 A BC D
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Three possible trees 12/18/2015Phylogenetics Part II15 A BC D A DC B A CB D Are there any other combinations?
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For each of the three trees (having 4 taxa) could add a branch to any of the 5 branches 3*5=15 trees 12/18/2015Phylogenetics Part II16 A BC D
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Outgroup Include an organism that is known to be further away from all taxa than they are from each other 12/18/201517Phylogenetics Part II A BC D If outgroup goes here… outgroup ABCD
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N i is number of trees given i taxa B i is the number of branches in a tree given i taxa B i =B i-1 +2, also i x 2-3 N i =N i-1 *(B i-1 ) No need for a “plus 1” for a possible new root because there are no roots N 2 = 1 B 2 =2 12/18/2015Phylogenetics Part II18 TaxaBranchesTrees 331 453 5715 69105 711945 81310,395 915135,135 10172,027,025 111934,459,425 1221654,729,075
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Noticed that for un-rooted trees: B i =2i-3 (for i 2) Also noticed N i =N i-1 *B i-1 And reduced to (2n-5)(2n-7)(2n-9)…(3)(1) where n is number of taxa Shorthand: (2n-5)!! For rooted N i =N i-1 *(B i-1 +1) Reduced to (2n-3)!! 12/18/201519Phylogenetics Part II Ni=B i-1 *N i-1 =(2(i-1)-3)N i-1 =(2i-5)N i-1 =(2i-5)(2i-7)N i-2 Till the N term gets to 3 Double factorial: each successive number reduced by two
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Radical reduction in the number Still only bought one additional taxa 12/18/2015Phylogenetics Part II20 TaxaUn-rooted treesRooted trees 313 4315 5 105 6 945 7 10,395 8 135,135 9 2,027,025 102,027,02534,459,425 1134,459,425654,729,075 12654,729,07513,749,310,575
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Even brighter mathematicians 12/18/201521Phylogenetics Part II Can you see why?
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Not really a candidate for dynamic programming Don’t repeat a bunch of sub- problems over and over Each sub-problem is a tree, and they are all unique 12/18/2015Phylogenetics Part II22 Still exponential
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Discard large subsets of possible solutions Use heuristics or predictions 12/18/2015Phylogenetics Part II23 Don’t bother
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Calculate a reasonable upper bound using a fast algorithm like UPGMA (hierarchical clustering) Incrementally grow potential trees Any branch that any that go over threshold stop investigating 12/18/2015Phylogenetics Part II24 A BC D X X X Don’t bother, over threshold
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Some columns all same Add no meaning All trees minimum Columns that are all different Also add no meaning Must have minimum 2 nt’s (or aa’s) that are the same Useful in one respect If all the same infer makeup of ancestor 12/18/2015Phylogenetics Part II25 AGCT AACT ACCT AAAA A 0 0 A A 0 0 00
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Each column yields a tree If all agree done If some different use majority rule If sample too small perform bootstrapping randomly draw sequences from MSA Generate more trees labeled branches with the percentage of bootstrap trees in which they appear Used as a measure of support (repeatability) 12/18/2015Phylogenetics Part II26
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Still have maximum likelihood Also, some inferential stuff, but that’s all in the next lecture 12/18/2015Phylogenetics Part II27
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12/18/201528Phylogenetics Part III
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