Adding a tip to a sister taxon on the tree

Here's a quick primer on how to attach a tip not in our tree to it's putative sister taxon half-way along the terminal edge length to its sister taxon.

## load phytools & simulate 
library(phytools)
tree<-pbtree(n=26,tip.label=LETTERS)
plotTree(tree)

## add tip "A2" to sister taxon "A"
tip<-"A2"
sister<-"A"
tree<-bind.tip(tree,tip,where=which(tree$tip.label==sister),
    position=0.5*tree$edge.length[which(tree$edge[,2]==
    which(tree$tip.label==sister))])
## add tip "T2" to "T"
tip<-"T2"
sister<-"T"
tree<-bind.tip(tree,tip,where=which(tree$tip.label==sister),
    position=0.5*tree$edge.length[which(tree$edge[,2]==
    which(tree$tip.label==sister))])
plotTree(tree)

This works with ultrametric trees. If our trees are not ultrametric, then we also have to include the terminal edge length that we want to have for our tip to be added in each case.

OK, we've done it!