In this study, we compare rDNA substitution rates for a group of closely related plant species in the western Pacific that exist in different biomes. The results of this comparison indicate higher rates of substitution for species living in habitats with greater biologically available energy. We interpret that finding as potentially important in understanding evolution because of its implication that substitution rate may be a function of biologically available energy and its correlate, productivity. The relevance of this research is twofold. First, contrasting closely related species across different biomes allows for a comparison between rates of molecular evolution across different energetic/productivity regimes while controlling for phylogenetically influenced variation. Second, the research indicates some of the design parameters for future studies that are required to explore the importance of this relationship among different groups of related organisms. If higher rates of molecular evolution where there is greater available energy are found to be widespread this might bring an additional dimension to the understanding of macroevolutionary pattern and process.