# Phylogenetics: Distance Methods. COMP Spring 2015 Luay Nakhleh, Rice University

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1 Phylogenetics: Distance Methods COMP Spring 2015 Luay Nakhleh, Rice University

2 Outline Evolutionary models and distance corrections Distance-based methods

3 Evolutionary Models and Distance Correction

4 The p distance p = D L D L : the number of positions at which two sequences differ : the length of each of the two sequences

5 The Poisson Distance Correction Assume that the probability of mutation at a site follows a Poisson distribution, with a uniform mutation rate r per site per time unit After a time t, the average number of mutations at each site will be rt The probability of n mutations having occurred at a given site during time t is given by the formula e rt (rt) n n!

6 The Poisson Distance Correction We want to derive a formula that relates the p-distance to the actual number of mutations that have occurred Consider two sequences that diverged time t ago The probability of no mutation having occurred at a site is e -rt for each sequence, given the assumption of a Poisson distribution of mutations The probability of neither sequence having mutated at that site is given by the expression e -2rt We also assume that no situation has occurred in which several mutations at a site have resulted in both sequences being identical In this case, this probability can be equated with the observed fraction of identical sites, given by (1-p), where p is the p-distance

7 The Poisson Distance Correction Because each sequence has evolved independently from the common ancestor, they are an evolutionary distance 2rt from each other, which we will write as d This evolutionary distance d is measured in terms of the average number of mutations that have occurred per site, not the time since divergence This leads to the equation 1-p = e -d, from which we can derive the Poisson corrected distance d p = ln(1 p)

8 The Poisson Distance Correction

9 The Gamma Distance Correction A questionable assumption is that of an equal rate of mutation at different positions in the sequence In 1971, Uzzell and Corbin reported that a Gamma distribution (Γ) can effectively model realistic variation in mutation rates Such a distribution can be written with one parameter, a, which determines the site variation

10 The Gamma Distance Correction Using this, it is possible to derive a corrected distance, referred to as the Gamma distance dγ: d Γ = a [ ] (1 p) 1/a 1 Values of a have been estimated from real protein-sequence data to vary between 0.2 and 3.5

11 The Poisson Distance Correction

12 The Jukes-Cantor (JC) Model The models described so far include no information about the chemical nature of the sequences, which means they apply to both nucleotide and protein sequences Some evolutionary models have been constructed specifically for nucleotide sequences One of the simplest such models is that Jukes-Cantor (JC) model It assumes all sites are independent and have identical mutation rates Further, it assumes all possible nucleotide substitutions occur at the same rate α per unit time

13 The Jukes-Cantor (JC) Model A matrix can represent the substitution rates: A C G T A -3α α α α C α -3α α α G α α -3α α T α α α -3α

14 The Jukes-Cantor (JC) Model Suppose that an ancestral sequence diverged time t ago into two related sequences After this time, the fraction of identical sites between the two sequences is q(t), and the fraction of different sites is p(t), so that p(t)+q(t)=1 We can calculate q(t+1), the fraction of identical sites after time t+1 There are two ways of getting an identical site at time t+1: Two aligned sites not mutating: the probability of this event is (1-3α) 2 (1-6α). Since q(t) sites were identical at time t, we expect (1-6α)q(t) remain identical at time t+1 One of two different aligned sites at time t mutate to become identical to the other at time t+1: the probability of this event is 2α(1-3α)p(t) 2αp(t)

15 The Jukes-Cantor (JC) Model Therefore, the fraction of identical sites at time t+1, q(t+1) is q(t+1) = (1-6α)q(t) + 2αp(t) This allows for estimating the derivative of q(t) with time as dq(t)/dt = q(t+1) - q(t) = 2α - 8αq(t) This gives rise to q(t) = 1/4(1+3e -8αt ), which includes the condition that at time t=0 all equivalent sites on the two sequences were identical (q(0)=1) Notice that q =1/4, so this model predicts a minimum 25% identity even on aligning unrelated nucleotide sequences 3αt mutations would be expected during a time t for each sequence site on each sequence At any time each site will be a particular base, which will mutate to one of the other three bases at the rate α

16 The Jukes-Cantor (JC) Model Hence, the evolutionary distance between two sequences under this model is 6αt This corrected distance, djc, can be obtained as d JC = 3 4 ln ( p ) To obtain a value for the corrected distance, substitute p with the observed proportion of site differences in the alignment

17 The Kimura 2-Parameter Model One improvement over the JC model involves distinguishing between rates of transitions and transversions Rates α and β are assigned to transitions and transversions, respectively When this is the only modification made, this amounts to the Kimura twoparameter (K2P) model, and has the rate matrix A C G T A -2β-α β α β C β -2β-α β α G α β -2β-α β T β α β -2β-α

18 The Kimura 2-Parameter Model The K2P model results in a corrected distance, dk2p, given by d K2P = 1 2 ln(1 2P Q) 1 4 ln(1 2Q) where P and Q are the observed fractions of aligned sites whose two bases are related by a transition or transversion mutation, respectively Notice that the p-distance, p, equals P+Q The transition/transversion ratio, R, is defined as α/2β

19 The HKY85 Model Hasegawa, Kishino, and Yano (1985) Allows for any base composition πa:πc:πg:πt Has the rate matrix A C G T A (-2β-α) πa βπc απg βπt C βπa (-2β-α)πC βπg απt G απa βπc (-2β-α)πG βπt T βπa απc βπg (-2β-α)πT

20 Choice of a Model of Evolution Model Base composition R=1? Identical transitio n rates? Identical transversio n rates? Reference JC 1:1:1:1 no yes yes Jukes and Cantor (1969) F81 variable no yes yes Felsenstein (1981) K2P 1:1:1:1 yes yes yes Kimura (1980) HKY85 variable yes no no Hasegawa et al. (1985) TN variable yes no yes Tamura and Nei (1993) K3P variable yes no yes Kimura (1981) SYM 1:1:1:1 yes no no Zharkikh (1994) GTR variable yes no no Rodriguez et al. (1990)

21 Rates Across Sites To allow for varying mutation rates across sites, the Gamma distribution can be applied If it is applied to the JC model with Γ parameter a, the corrected distance equation becomes d JC+Γ = 3 4 a [ ( p ) 1 a 1 ]

22 Models of Protein-sequence Evolution Models that we just described can be modified to apply to protein sequences For example, the JC distance correction for protein sequences is d JCprot = ln ( 1 20 ) 19 p However, the more common practice is to use empirical matrices, such as the JTT (Jones, Taylor, and Thornton) matrix

23 Distance-based Methods

24 Distance-based Methods Reconstruct a phylogenetic tree for a set of sequences on the basis of their pairwise evolutionary distances Derivation of these distances involve equations such as the ones we saw before (distance correction formulas) Problems with distances include Wrong alignment leads to incorrect distances Assumptions in the evolutionary models used may not hold Formulas for computing distances are exact only in the limit of infinitely long sequences, which means the true evolutionary distances cannot always be recovered exactly

25 Additivity A B C D A 1 3 C A B B 2 3 D C 0 6 D 0

26 The Distance-based Phylogeny Problem Input: Matrix M of pairwise distances among species S Output: Tree T leaf-labeled with S, and consistent with M

27 The Least-squares Problem Input: Distance matrix D, and weights matrix w Output: Tree T with branch lengths that minimizes LS(T )= n i=1 j i w ij (D ij d ij ) 2 The distances defined by the tree T

28 Distance-based Methods The least-squares problem is NP-complete We will describe three polynomial-time heuristics Unweighted pair-group method using arithmetic averages (UPGMA) Fitch-Margoliash Neighbor joining

29 The UPGMA Method Assumes a constant molecular clock, and a consequence, infers ultrametric trees Main idea: the two sequences with the shortest evolutionary distance between them are assumed to have been the last to diverge, and must therefore have arisen from the most recent internal node in the tree. Furthermore, their branches must be on equal length, and so must be half their distance

30 1. Initialization The UPGMA Method 1. n clusters, one per taxon 2. Iteration 1. Find two clusters X and Y whose distance is smallest 2. Create a new cluster XY that is the union of the two clusters X and Y, and add it to the set of clusters 3. Remove the two clusters X and Y from the set of clusters 4. Compute the distance between XY and every other cluster in the set 5. Repeat until one cluster is left

31 The UPGMA Method Q1: What is the distance between two clusters X and Y? d XY = 1 N X N Y i X,j Y d ij Q2: When creating a new cluster Z, how do we compute its distance to every other cluster, W? d ZW = N Xd XW + N Y d YW N X + N Y

32 UPGMA: An Example

33 UPGMA: An Example

34 UPGMA: An Example

35 The Fitch-Margoliash Method The method is based on the analysis of a three-leaf tree (triplet) d AB = b 1 + b 2 d AC = b 1 + b 3 d BC = b 2 + b 3 b 1 = 1 2 (d AB + d AC d BC ) b 2 = 1 2 (d AB + d BC d AC ) b 3 = 1 2 (d AC + d BC d AB )

36 The Fitch-Margoliash Method Trees with more than three leaves can be generated in a stepwise fashion similar to that used in UPGMA At every stage, three clusters are defined, with all sequences belonging to one of the clusters The distance between clusters is defined by a simple arithmetic average of the distances between sequences in the different clusters

37 The Fitch-Margoliash Method At the start of each step, we have a list of sequences not yet part of the growing tree and of clusters representing each part of the growing tree The distances between all these sequences and clusters are calculated, and the two most closely related are selected as the first two clusters of a three-leaf tree A third cluster is defined that contains the remainder of the sequences, and the distances to the other two are calculated

38 The Fitch-Margoliash Method Using the equations described, one can then determine the branch lengths from this third cluster to the other two clusters and the location of the internal node that connects them These two clusters are then combined into a single cluster with distances to other sequences again defined by simple averages

39 The Fitch-Margoliash Method There is now one less sequence (cluster) to incorporate into the growing tree By repetition of these steps, this technique is able to generate a single tree in a similar manner to UPGMA The trees produced by UPGMA and Fitch-Margoliash are identical in terms of topology, yet differ in the branch lengths assigned

40 Fitch-Margoliash: An Example

41 Fitch-Margoliash: An Example

42 Fitch-Margoliash: An Example

43 Fitch-Margoliash: An Example

44 Fitch-Margoliash: An Example

45 The NJ Method The basis of the method lies in the concept of minimum evolution, namely that the true tree will be that for which the total branch length, S, is shortest Neighbors in a phylogenetic tree are defined by a pair of nodes that are separated by just one other node Pairs of tree nodes are identified at each step of the method (just like with UPGMA and Fitch-Margoliash) and used to gradually build up a tree

46 The NJ Method: Deriving the Neighbor-joining Equations S = N i=1 b ix = 1 N 1 N i<j d ij S 12 = b 1Y + b 2Y + b XY + N i=3 b ix bef : the length of the branch between nodes e and f

47 The NJ Method: Deriving the Neighbor-joining Equations We need to convert the equation into a form that uses the sequence distances d This can be achieved as S 12 = 1 2(N 2) N i=3 and simplified further into where (d 1i + d 2i )+ 1 N 2 S 12 = 2d sum U 1 U2 2(N 2) N 3 i<j + d 12 2 d ij + d 12 2 U 1 = N d 1i U 2 = i=1 N d 2i d sum = i=1 N i<j d ij

48 The NJ Method: Deriving the Neighbor-joining Equations Every pair of sequences i and j, if separated from the star node, produce a tree of total branch length Sij According to the minimum evolution principle, the tree that should be chosen is that with the smallest Sij This is equivalent to finding the pair of sequences with the smallest value of the quantity δij defined by δ ij = d ij U i + U j N 2

49 The NJ Method: Deriving the Neighbor-joining Equations Once this pair has been found, the distances to the new node Y must be calculated and b iy = 1 2 ( d ij + U i U j N 2 b jy = d ij b iy To calculate the distances from Y to every other sequence k: ) b Yk = 1 2 (d ik + d jk d ij )

50 The NJ Method: Deriving the Neighbor-joining Equations To add more nodes, we now repeat the process, starting with the star tree formed by removing sequences i and j, to leave a star tree with node Y as a new leaf Note that at each step, the value of N in the formulas decreases by 1

51 NJ: An Example

52 NJ: An Example

53 NJ: An Example

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