Branch-specific rate shift probabilities
marginalShiftProbsTree computes a version of a
phylogenetic tree where each branch length is equal to the marginal
probability that a shift occurred on a particular branch. The
cumulativeShiftProbsTree includes the cumulative probability
that a shift occurred on a given branch. See details.
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An object of class
The marginal shift probability tree is a copy of the target phylogeny, but where each branch length is equal to the branch-specific marginal probability that a rate-shift occurred on the focal branch. For example, a branch length of 0.333 implies that 1/3 of all samples from the posterior had a rate shift on the focal branch.
Note: It is highly inaccurate to use marginal shift probabilities as a measure of whether diversification rate heterogeneity occurs within a given dataset. Consider the following example. Suppose you have a tree with topology (A, (B, C)). You find a marginal shift probability of 0.5 on the branch leading to clade C, and also a marginal shift probability of 0.5 on the branch leading to clade BC. Even though the marginal shift probabilities appear low, it may be the case that the joint probability of a shift occurring on either the branch leading to C or BC is 1.0. Hence, you could be extremely confident (posterior probabilities approaching 1.0) in rate heterogeneity, yet find that no single branch has a particularly high marginal shift probability. In fact, this is exactly what we expect in most real datasets, because there is rarely enough signal to strongly support the occurrence of a shift on any particular branch.
The cumulative shift probability tree is a copy of the target phylogeny but where branch lengths are equal to the cumulative probability that a rate shift occurred somewhere on the path between the root and the focal branch. A branch length equal to 0.0 implies that the branch in question has evolutionary rate dynamics that are shared with the evolutionary process starting at the root of the tree. A branch length of 1.0 implies that, with posterior probability 1.0, the rate dynamics on a branch are decoupled from the "root process".
An object of class
phylo, but with branch lengths equal to
the marginal or cumulative shift probabilities.
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data(whales) data(events.whales) ed <- getEventData(whales, events.whales, nsamples = 500) # computing the marginal shift probs tree: mst <- marginalShiftProbsTree(ed) # The cumulative shift probs tree: cst <- cumulativeShiftProbsTree(ed) #compare the two types of shift trees side-by-side: plot.new() par(mfrow=c(1,2)) plot.phylo(mst, no.margin=TRUE, show.tip.label=FALSE) plot.phylo(cst, no.margin=TRUE, show.tip.label=FALSE)
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