3 * Jalview - A Sequence Alignment Editor and Viewer ($$Version-Rel$$)
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23 <title>Tree Calculation</title>
27 <strong>Calculation of trees from alignments</strong>
30 Trees are calculated on either the complete alignment, or just the
31 currently selected group of sequences, via the <a href="calculations.html">calculations dialog</a> opened from the <strong>Calculate→Calculate
32 Tree or PCA...</strong> menu entry. Once calculated, trees are displayed in a new <a
33 href="../calculations/treeviewer.html">tree viewing
34 window</a>. There are four different calculations, using one of two
35 distance measures and constructing the tree from one of two
39 <strong>Distance Measures</strong>
41 <p>Trees are calculated on the basis of a measure of similarity
42 between each pair of sequences in the alignment :
44 <li><strong>PID</strong><br>The percentage identity
45 between the two sequences at each aligned position.
47 <li>PID = Number of equivalent aligned non-gap symbols *
48 100 / Smallest number of non-gap positions in either of both
49 sequences<br> <em>This is essentially the 'number of
50 identical bases (or residues) per 100 base pairs (or
54 <li><strong>BLOSUM62, PAM250, DNA</strong><br />These options
55 use one of the available substitution matrices to compute a sum of
56 scores for the residue pairs at each aligned position.
58 <li>For details about each model, see the <a
59 href="scorematrices.html">list of built-in score
63 <li><strong>Sequence Feature Similarity</strong><br>Trees
64 are constructed from a distance matrix formed from Jaccard
65 distances between sequence features observed at each column of the
68 <li>Similarity at column <em>i</em> = (Total number of
69 features displayed - Sum of number of features in common at <em>i</em>)
70 <br />Similarities are summed over all columns and divided by
71 the number of columns. <br />Since the total number of
72 feature types is constant over all columns of the alignment,
73 we do not scale the matrix, so tree distances can be
74 interpreted as the average number of features that differ over
75 all sites in the aligned region.
78 </ul> Distances are computed based on the currently displayed feature
79 types. Sequences with similar distributions of features of the
80 same type will be grouped together in trees computed with this
81 metric. <em>This measure was introduced in Jalview 2.9</em></li>
83 <li><strong>Secondary Structure Similarity</strong><br>Trees are
84 generated using a distance matrix, which is constructed from Jaccard
85 distances that specifically consider the secondary structure features
86 observed at each column of the alignment.
88 <li>For secondary structure similarity analysis, at any given column
89 <em>i</em>, the range of unique secondary structures is between 0 and 2,
90 reflecting the presence of helices, sheets, coils and gaps.
91 <br>The similarity at column <em>i</em> = Total
92 number of unique secondary structures (which can range from 0 to 2)
93 - Sum of the number of secondary structures in common at column
94 <em>i</em> (which can be either 0 or 1)<br>The similarity scores are
95 summed across all columns and then divided by the total number of
96 columns to calculate an average similarity score.
99 Distance calculations are based on the secondary structures
100 currently displayed. Sequences with similar distributions of secondary
101 structures will be grouped together in trees.<br>
102 <em>The distance between two sequences is maximum when one
103 sequence has a defined secondary structure annotation track and the
104 other does not, indicating complete dissimilarity between them.
105 Whereas, the distance between two sequences is minimum when both of
106 the sequences within the comparison do not have a defined secondary
107 structure annotation track.</em>
111 <strong>Tree Construction Methods</strong>
113 <p>Jalview currently supports two kinds of agglomerative
114 clustering methods. These are not intended to substitute for
115 rigorous phylogenetic tree construction, and may fail on very large
118 <li><strong>UPGMA tree</strong><br> UPGMA stands for
119 Unweighted Pair-Group Method using Arithmetic averages. Clusters
120 are iteratively formed and extended by finding a non-member
121 sequence with the lowest average dissimilarity over the cluster
124 <li><strong>Neighbour Joining tree</strong><br> First
125 described in 1987 by Saitou and Nei, this method applies a greedy
126 algorithm to find the tree with the shortest branch lengths.<br>
127 This method, as implemented in Jalview, is considerably more
128 expensive than UPGMA.</li>
131 A newly calculated tree will be displayed in a new <a
132 href="../calculations/treeviewer.html">tree viewing
133 window</a>. In addition, a new entry with the same tree viewer window
134 name will be added in the Sort menu so that the alignment can be
135 reordered to reflect the ordering of the leafs of the tree. If the
136 tree was calculated on a selected region of the alignment, then the
137 title of the tree view will reflect this.
141 <strong>External Sources for Phylogenetic Trees</strong>
144 A number of programs exist for the reliable construction of
145 phylogenetic trees, which can cope with large numbers of sequences,
146 use better distance methods and can perform bootstrapping. Jalview
148 href="http://evolution.genetics.washington.edu/phylip/newick_doc.html">Newick</a>
149 format tree files using the 'Load Associated Tree' entry of the
150 alignment's File menu. Sequences in the alignment will be
151 automatically associated to nodes in the tree, by matching Sequence
152 IDs to the tree's leaf names.