What is the relationship among Hernandiaceae, Lauraceae, and Monimiaceae, and why is this question so difficult to answer?

Molecular and morphological phylogenetic studies in the Laurales have found that Hernandiaceae, Lauraceae, and Monimiaceae sensu stricto form a monophyletic group. Because of the paucity of phylogenetically informative substitutions, however, relationships among families within this clade remain unclear. In general, molecular phylogenies may conflict because of a variety of factors, including substitution rate variation among sites and lineages, taxon sampling, outgroup choice, and base compositional biases. We analyzed a total of 2846 aligned nucleotides from a plastid intron, three spacers, and a portion of the nuclear 26S rRNA gene in a sample of Hernandiaceae, Lauraceae, and Monimiaceae; we used four outgroups with differing substitution rates. Despite obtaining single best topologies with maximum likelihood, minimum evolution, and parsimony approaches, family relationships remained as poorly supported as they were in the previous molecular studies. Exploration of the data indicates that varying substitution rates across lineages or sites, insufficient taxon sampling, fast-evolving outgroups, or biased base composition are unlikely to explain the difficult reconstruction. Exclusion of some of the longest branched taxa (the hemiparasite Cassytha, selected Hernandiaceae, and two of the four outgroups) had no effect on topologies. To resolve relationships among the three families one could now complement existing five-gene data sets by adding the basal genera of Lauraceae, Monimiaceae, and Hernandiaceae, which are newly sampled here, or, our preferred strategy, by sequencing low-copy nuclear genes for the key genera to obtain different kinds of data.