Total Cophenetic Index
TotalCopheneticIndex() calculates the total cophenetic index
(Mir et al. 2013) for any tree, a measure of its balance;
TCIContext() lists its possible values.
TotalCopheneticIndex(x) TCIContext(x) ## S3 method for class 'numeric' TCIContext(x)
x |
A tree of class |
The Total Cophenetic Index is a measure of tree balance – i.e. whether a (phylogenetic) tree comprises symmetric pairs of nodes, or has a pectinate 'caterpillar' shape. The index has a greater resolution power than Sackin's and Colless' indices, and can be applied to trees that are not perfectly resolved.
For a tree with n leaves, the Total Cophenetic Index can take values of
0 to choose(n, 3).
The minimum value is higher for a perfectly resolved (i.e. dichotomous) tree
(see Lemma 14 of Mir et al. 2013).
Formulae to calculate the expected values under the Yule and Uniform models
of evolution are given in Theorems 17 and 23.
Full details are provided by Mir et al. (2013).
TotalCopheneticIndex() returns an integer denoting the total cophenetic index.
TCIContext() returns a data frame detailing the maximum and minimum value
obtainable for the Total Cophenetic Index for rooted binary trees with the
number of leaves of the given tree, and the expected value under the Yule
and Uniform models.
The variance of the expected value is given under the Yule model, but cannot
be obtained by calculation for the Uniform model.
Martin R. Smith (martin.smith@durham.ac.uk)
Mir A, Rosselló F, Rotger LA (2013). “A new balance index for phylogenetic trees.” Mathematical Biosciences, 241(1), 125–136. doi: 10.1016/j.mbs.2012.10.005, https://doi.org/10.1016/j.mbs.2012.10.005.
cophen.index() in the package
'CollessLike'
provides an alternative implementation of this index and its predecessors.
Other tree characterization functions:
CladisticInfo(),
Stemwardness
# Balanced trees have the minimum index for a binary tree; # Pectinate trees the maximum: TCIContext(8) TotalCopheneticIndex(PectinateTree(8)) TotalCopheneticIndex(BalancedTree(8)) TotalCopheneticIndex(StarTree(8)) # Examples from Mir et al. (2013): tree12 <- ape::read.tree(text='(1, (2, (3, (4, 5))));') #Fig. 4, tree 12 TotalCopheneticIndex(tree12) # 10 tree8 <- ape::read.tree(text='((1, 2, 3, 4), 5);') #Fig. 4, tree 8 TotalCopheneticIndex(tree8) # 6 TCIContext(tree8) TCIContext(5L) # Context for a tree with 5 leaves.
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