Fossils are routinely used as the most defensible method for calibrating phylogenetic hypotheses to absolute time. Calibration is often required before ideas can be tested, such as timing of a prehistoric event or rate of change of a trait. This project examines a fossil rich plant group, the Cryptanthinae (Boraginaceae), to test two recently proposed but untested methods for incorporating fossils in phylogenetic analyses. After evaluating these methods, the resulting time-calibrated hypothesis of phylogenetic relationships will be used to examine the New World amphitropical disjunction, a distribution pattern seen in the Cryptanthinae and many other groups of plants that is likely the result of long-distance dispersal by birds between North and South America.
This project will result in three main contributions to science. First, evaluation of novel methods for incorporating fossil data into phylogenetic studies will benefit all future studies using fossil data. Importantly, these methods can be generalized to any study using morphological data. Second, New World amphitropical disjunction is common yet understudied. This study will test current hypotheses regarding this biogeographic pattern. Finally, this project will generate a hypothesis of relationships in the Cryptanthinae that will inform taxonomy and conservation in this large and ecologically important plant group.
This NSF-supported dissertation research has focused on understanding the evolutionary history and biogeography of a common, ecologically important group of annual plants, the "popcorn flowers" (Plagiobothrys and relatives, Boraginaceae). The popcorn flowers are interesting in part due to their biogeographic distribution in the New World, with species occurring in both temperate North America and South America but lacking in the intervening New World tropics. The popcorn flowers are species-rich on both continents, a very rare condition that has facilitated the development and testing of hypotheses relating to the origin of this interesting biogeographic pattern. A further exciting aspect of the group is that, contrary to most other groups of annual plants, the popcorn flowers have a robust fossil record. These Miocene fossils can be used to place the inferences made about the evolutionary history of the popcorn flowers in the context of geologic time. It has been unclear how best to incorporate fossil data into evolutionary studies, so this research has applied several current methods to do so. This project began with the use of DNA sequence data to infer relationships among the genera and species of popcorn flowers. This aspect of the project demonstrated that one of the largest genera of popcorn flowers, Plagiobothrys, currently comprises four separate evolutionary lineages. Two of these lineages are being recognized as new genera, one reverts to an older genus name, and the last retains the name Plagiobothrys. Using the most robust estimate of evolutionary relationships, the biogeographic history of Plagiobothrys and relatives was studied. This analysis demonstrated that the popcorn flowers originated in North America, with at least 5 dispersals to South America in Plagiobothrys alone. By incorporating the fossils in this group, the evolutionary tree was scaled to geologic time, thereby permitting the assessment of the absolute ages of the North America to South America dispersals. These dispersals between the continents occurred at many different times, from shortly after the middle Miocene (e.g., 11 million years ago) to potentially as recently as the Pleistocene (e.g., 300,000 years ago). This timing is consistent with dispersal by birds, which migrate between the continents via the Pacific Flyway. Interestingly, the dispersals do not seem to correlate with the shape and size of the fruits as one might expect with animal dispersed fruits. In summary, this project has resulted in a deeper understanding of the evolution of a common and ecologically important member of the flora of North America. It permitted the evaluation of a significant biogeographic pattern, New World amphitropical disjunction. Use of fossil data permitted all evolutionary inferences to be scaled to absolute time.
