\code{alpha}, and \code{theta}. This may be interesting for the last
one to model varying phenotypic optima. The exact updating formula
from Gillespie (1996) are used which are reduced to BM formula if
- \code{alpha = 0}.
+ \code{alpha = 0}.}
\item{A function:}{it must be of the form \code{foo(x, l)} where
\code{x} is the trait of the ancestor and \code{l} is the branch