 These parameter-rich models reflect a better understanding of molecular evolution (e.g., that transversion and  transition[5]  rates between nucleotides are rarely equivalent) and the combination of lowered costs and increasing accessibility to growing computational power that permits more parameter rich analyses. The large number of models available leads to an interesting statistical problem in its own right known as the problem of model selection, i.e., which model of evolution do we use in our phylogenetic tools? For discussions of this problem in the context of phylogenetics, see (Posada &amp; Crandall 1998; Sullivan &amp; Joyce 2005; Posada 2008).