Studies of diversification rate shifts require knowledge of both phylogeny and time. Yet, even with these components, the picture remains incomplete if the goal is to determine causal factors driving such shifts. The present study uses biogeographic, ecological and biological data within one phylogeny of a paleotropic endemic moss, Mitthyridium to dissect causal factors driving its relatively recent radiation. Mitthyridium, a monophyletic and distinct group, has a relatively narrow geographic distribution including several island endemics and may represent a model radiation. Relationships of lineages below the traditional species level in the group were discovered through phylogenetic analysis of faster evolving regions in the nucleus (Glyceraldehyde-3 phosphate dehydrogenase (GPD)), and chloroplast (3 spacer regions of rbcL and rps4) using samples from across most of the distribution of Mitthyridium. GPD is a single copy nuclear gene that provides appropriate variation for phylogeography. The present study introduces this gylocolytic gene as new to studies of moss systematics and demonstrates its utility for phylogenetic reconstruction, with special emphasis on the population structure of Mitthyridium. Results show that subspecific lineages are biologically distinct and island specific. A particular biological feature distinguishing the lineages--variability in sexual expression (from fully sexual to vegetative)--was mapped to the phylogeographic structure to test the influence of reproductive mode on rate shifts. Preliminary results indicate trends towards increasing asexuality among island populations of Mitthyridium that are correlated with geography. The shifts in reproductive mode, when localized along the phylogeny, are significantly concentrated with changes in rates of diversification, both in anagenetic and cladogenetic contexts. These results allow for a rigorous examination of causal mechanisms driving rapid evolutionary radiation on oceanic islands.

Key words: diversification rate shifts, Glyceraldehyde-3 phosphate dehydrogenase, Mitthyridium, phylogeography