Dissertation Research: The Roles of Edaphic Shifts and Long-Distance Dispersal in the Evolution of the American Desert Genus Tiquilia (Boraginaceae)

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A grant has been awarded to Dr. Robert Jansen and Mr. Michael Moore at the University of Texas to study how the differing soil tolerances of individual plant species have influenced plant evolution. Plant biologists have long known that boundaries between different soil types often promote the evolution of new species. However, they have rarely examined the importance of such shifts in soil tolerance on the evolutionary history of whole groups of related plant species. Jansen and Moore will specifically address this larger scale evolutionary question by assessing the influence of shifting soil tolerances during the evolution of the genus Tiquilia. The 27 species of small desert shrubs in Tiquilia are ideally suited for this study because they differ remarkably in soil preference despite occurring in the climatically similar deserts of North and South America. Individual species of Tiquilia grow in one of three substrate regimes: deep sand, gypsum, or both gypsum and limestone. To determine the evolutionary pattern of substrate shifts in this genus, DNA sequences of several genes will be collected for all species of Tiquilia. These sequences will be utilized to develop an evolutionary tree detailing the relationships among these species. This tree will then be used as a basis to infer the history of soil preference in the genus. This project represents one of the first studies to assess the significance of substrate shifts in patterning the evolutionary history of an entire plant group. Considering that soil type ranks second behind climate in determining the geographic distribution of plant species, it is surprising that only a handful of studies have addressed the relationship between evolutionary history and soil preference among closely related plant species. This study therefore promises to advance substantially our understanding of an underappreciated and potentially important influence on the evolution of plant groups.