Extracting the terminal edge lengths for a set of tips

A phytools user asked the following:

Is there a good way to extract the branch length for a given set of tips? For a given tree, we would like to calculate the mean and variance for the terminal branch lengths leading to a specific set of tips. Is there a way in phytools to target a set of tips and extract those branch lengths?

If I understand the question properly, then what we have is a tree:

tree

## 
## Phylogenetic tree with 26 tips and 25 internal nodes.
## 
## Tip labels:
##  A, B, C, D, E, F, ...
## 
## Rooted; includes branch lengths.

and a set of tips:

tips

##  [1] "K" "V" "P" "S" "J" "Q" "X" "B" "H" "D"

and we want to extract the set of terminal edge lengths associated with these tips. This is easy.

## first get the node numbers of the tips
nodes<-sapply(tips,function(x,y) which(y==x),y=tree$tip.label)
## then get the edge lengths for those nodes
edge.lengths<-setNames(tree$edge.length[sapply(nodes,
    function(x,y) which(y==x),y=tree$edge[,2])],names(nodes))

We can check as follows:

## our edge lengths
edge.lengths

##          K          V          P          S          J          Q 
## 0.26489330 0.76661579 0.04553788 0.15279683 0.13592584 0.04553788 
##          X          B          H          D 
## 0.05511474 0.59774131 1.28135176 0.83764106

plotTree(tree)
edgelabels(round(tree$edge.length,3),cex=0.7)

That's all there is to it.

The data used for this exercise were simulated as follows:

library(phytools)
tree<-pbtree(n=26,tip.label=LETTERS)
tips<-sample(LETTERS,10)