Overall, these initial results suggest that a relaxed phylogenetic approach may be the most appropriate even when phylogenetic relationships are of primary concern and the rooting and dating of the tree are of less interest. The molecular clock assumption can be relaxed in a variety of ways [ 13 - 151749 - 52 ]. To convert this tree from units of time to molecular evolutionary units, the rates are either assigned to branches [ 1517 ] or to nodes [ 5354 ]. The first such model to be described [ 15 ] assigned rates to the midpoints of branches and the assumed lognormal prior distribution relating the midpoint of the ancestral branch to the midpoint of the derived branch. Another interesting model is the exponential distribution model of Aris-Brosou and Yang [ 17 ], which employed an exponential prior distribution on rate r with a mean and therefore standard deviation equal to the ancestral rate r Aand with no dependence on the time between the two rates. This second model represents a more punctuated view of change in evolutionary rate, so that only the number of branching events, and not the length of time between events, determines the amount of change in evolutionary rate.