An object of class \code{phylo} with branch lengths.
}
\author{Julien Dutheil \email{julien.dutheil@univ-montp2.fr} and
- Emmanuel Paradis \email{Emmanuel.Paradis@mpl.ird.fr}}
+ Emmanuel Paradis}
\references{
Grafen, A. (1989) The phylogenetic regression. \emph{Philosophical
Transactions of the Royal society of London. Series B. Biological