Phylogentic Systematics of Valerianaceae
One of my current research foci is the integration of comparative ecology, biogeography, and phylogeny in Valerianaceae (350 spp.; Dipsacales), with the purpose of elucidating processes important in generating diversity during the formation of the páramo habitat in the high Andes of South America. The South American valerians are considered to be among the most striking examples of adaptive radiation in the flowering plants. The primary goal is deciphering relationships within this group as well as understanding its biogeographic history. Of the 250 species of Valerianaceae, approximately 150 occur in the Andes of South America. Species in the Andes are found primarily in the páramo vegetation at the highest elevations, sometimes exceeding 4800 meters, but also occur in other habitats, such as montane cloud forests. These species take on a wide variety of growth forms, including annual herbs, vines, rosettes, cushion plants, and shrubs to small trees. In addition, several different mating systems have evolved, including dioecy and gynodioecy. Phylogenies based on molecular data suggest that the South American Valerianaceae represent a single lineage that arose from a single colonization event. Dates based on molecular clock estimates place the age of this event between 3.5 and 7.0 million years ago. If these estimates are accurate, Valerianaceae represents a remarkable example of explosive adaptive radiation. Given a well-supported phylogenetic hypothesis, I am especially interested in exploring issues surrounding the quantification and characterization of plant diversity patterns in space and time. This will provide an important model for the study of several characters, including reduction in flower parts, the evolution and development of asymmetric flowers, and the evolution of mating systems.
A well resolved/supported phylogeny for this group will also aid greatly in our understanding of the formation and biogeography of alpine flora. Valerianaceae has independently colonized alpine habitats in the Himalayas, Rocky Mountains, Sierra Nevada, Alps, and Andes. The alpine life zone is characterized by a suite of climatic factors and harbors a unique spectrum of growth forms (e.g., rosette and cushion plants). At high altitudes above the treeline, this life zone is fragmented into numerous, more or less, isolated (similar to island chains) regions that are nested within various regional floras. There is great opportunity for the study of the historical biogeography of alpine taxa. Investigations into the composition and origin of the diverse alpine floras of the world have been conducted for more than a century. It is currently thought that alpine floras consist of ancestral elements, immigrants from other alpine areas, and some recently evolved lineages. A major research interest of mine is trying to understand the phylogenetic relationships among alpine species and their lowland relatives to provide a basis for addressing questions concerning the historical structuring, timing, and diversification of alpine floras. Recently, my lab has started using ecological niche modeling approaches to investigate these questions. Ultimately, I would like to be able to compare different mountain systems (especially the Andes, Himalayas, Sierra Nevada, and Rocky Mountains) in terms of their similarities and differences in the composition and formation of their alpine floras in a phylogenetic context. In the recent decades, alpine regions around the globe have seen the influences of potential global climate change (e.g., retreat of glacial ice in the Alps) and environmental degradation due to human activity (e.g., burning & grazing of páramo habitat in the Andes).
In collaboration with several other researchers, I am compiling a world-wide species list for Valerianaceae. Once a good estimate of the number of species that are out there, we will attempt to collect sequence data for all of them. As of today, about 100 species (a little less than a third) have been collected and sequenced for a variety of molecular markers. A completely sampled phylogeny will allow me to answer several intriguing questions concerning character evolution and biogeography
Preliminary results suggest that many of the traditionally recognized taxa are not monophyletic (see publications). Hopefully, when all is said and done, this research will lead to a world-wide taxonomic revision of Valerianaceae