With more than 300,000 species found across almost every life zone on Earth, plants play a pivotal role in the lives of all organisms on this planet. However, the vast majority of plant species are only found in Earth's equatorial belt. This phenomenon, known as the latitudinal diversity gradient, is poorly understood although recent investigations have identified several key hypotheses. In this project, the PI's will employ direct tests of the main prediction from these hypotheses: that rates of species diversification differ predictably with distance from the equator. This hypothesis will be tested using an exceptionally diverse and ecologically significant group of plants: the sedges.
The results of this research will address how interactions between diversification, geographic contingency, and ecological adaptation have generated current patterns of diversity in plants. This research has the potential to dramatically change the way that society thinks about the evolution of biodiversity, and the conclusions will substantially advance understanding of the processes that generate it. These results will have tangible scientific value to all researchers studying biodiversity, evolutionary ecology, or any aspect of sedge biology. The PI's will also promote teaching, training, and learning in the public community at large during the course of this research.
In this project, researchers from Washington State University worked to better understand how species diversity is distributed across the continents - particularly between the tropics and temperate zones. This research used the large family of palnts known as sedges (Cyperaceae) to study these patterns using a novel method for gathering data from public databases to create large diagrams of evolutionary relationships, a method known as supermatrix phylogeny reconstruction. There are three primary directions in which this work has significant impact on the scientific community. First, the researchers have developed considerably better supported phylogenies of sedges that have been previously published. These data will be of high utility to any taxonomists working within this globally diverse and important family. These impacts are straightforward: improvements to our understanding of evolutionary relationships allow interested researchers to take advantage of our findings in order to develop better hypotheses about the processes that underlie sedge evolution. In addition, these phylogenies are of high utility for downstream evolutionary analysis addressing community composition, the evolution of regional floras, and inquiry into geographic processes of plant diversification, particularly because Cyperaceae constitute a diverse component of a majority of the planet's regional floras. The other primary direction in which these research findings will impact the field of phylogenetic research is through the methodological improvements that we have developed to help inform the reconstruction of large evolutionary trees using sparse data matrices compiled from large sets of frequently non-overlapping DNA sequence data. The taxonomic breadth of publicly-available sequence data is relatively high for most lineages, and the ease of access to these data is constantly improving. However, because of the low levels of overlap among these publicly available datasets, phylogeny reconstruction can pose several non-trivial challenges. These challenges, which primarily include lack of resolution and difficulty in reducing the scope of challenges to a manageable level, are the foci of much of our work to date. The tools and techniques that developed here will help automate computationally intensive tasks and improve the ability of any phylogenetic researcher to 1) identify potential sources of difficulty in the dataset, 2) quantify the utility of any given dataset for phylogenetic reconstruction, and 3) help identify ways to improve sampling of any given dataset toward the end goal of improving the ability of the data to inform phylogenetic reconstructions. These are high-impact, widely applicable results which, in combination with complementary findings from other researchers addressing similar problems, will help transition the field of phylogenetic inquiry toward a stage of development where the reconstruction of very large phylogenies containing vast amounts of the Earth's biodiversity will be possible. The third significant aspect of these findings involves the support of two important hypotheses related to the difference in species richness between the tropics and temperate zone: the tropical conservatism hypothesis of Wiens and Donoghue and the out of the tropics model of Jablonski et al. This research has found strong support that higher tropical net diversification is driven by higher extratropical extinction, so much so in sedges that rates of net diversification in extratropical lineages are actually negative. Because a negative net diversification rate implies extinction rates exceeding speciation, it appears that in the case of sedges, tropical biodiversity reservoirs are primarily responsible for preventing this cosmopolitan family from going entirely extinct. These findings shed more light on a persistent and enigmatic problem of biogeography and global ecology, and do much to further our understanding of the evolution and maintenance of biodiversity at a global scale. Of particular importance is the understanding implied from our results that the tropics act as a global reservoir of biodiversity, and that the ongoing species extinctions there may thus have catastrophic effects on the long-term viability of Earth's ecosystems.
