The argument outgroup
can be either character or numeric. In
the first case, it gives the labels of the tips of the new outgroup;
in the second case the numbers of these labels in the vector
phy$tip.label
are given. If outgroup
is of length one (i.e. a single value), then the
tree is rerooted using the node below this tip as the new root.
If outgroup
is of length two or more, the most recent common
ancestor (MRCA) is used as the new root. Note that the tree is really
unrooted before being rerooted, so that if outgroup
is already
the outgroup, then the returned tree is not the same than the original
one (see examples). If outgroup
is not monophyletic, the
operation fails and an error message is issued.