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Through a collaboration with The Desert Institute, an arm of the Joshua Tree National Park Association, and with the support of the National Science Foundation's CAREER Program, our lab runs a Citizen Science Course each spring in Tikaboo Valley, Nevada. Through participating in data collection as part of an ongoing study of Joshua trees in Tikaboo Valley, students will learn about the natural history of the Joshua tree – yucca moth pollination system, will learn about how coevolution can contribute to speciation, and how hybrid zones can be used to study speciation.

You can read more about our citizen science program in this article in the Las Vegas Journal Review, or listen to a story about the program on KNPR's State of Nevada.

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Research Goals:  A central question in evolutionary ecology is how interactions between different organisms shape their evolution. A particularly important problem in this field is to understand how natural selection resulting from these interactions can create new species. ‘Hybrid zones’ – places where two different species come into contact and interbreed – are considered to be ‘natural laboratories’ for studying evolution and the formation of species. By examining how the features of organisms change across space in a hybrid zone, we can get clues as to which features are being affected by natural selection, and how natural selection may be involved in creating new species.

 

Joshua trees (tree-like plants in the genus Yucca that are endemic to the Mojave Desert) are one half of a remarkable pollination story. Joshua trees (and yuccas in general) are pollinated exclusively by small, drab insects called yucca moths. The moths collect tiny balls of pollen in their mouths, and then move from flower to flower, carefully placing pollen onto the stigmas (the receptive surface of the flower). The moths get something out of the deal too – they lay their eggs inside the developing flower, and their caterpillars will eat some of the Joshua tree seeds. Recently, work by Olle Pellmyr, a scientist at the University of Idaho, showed that there are actually two different species of moth associated with Joshua trees. The two moth species differ in size, and in the length of their ‘ovipositors’ – the organ that they use to deposit their eggs inside the developing flower. This spurred further study of the Joshua trees themselves, and on closer examination it was discovered that trees pollinated by each moth are also different species. Amazingly, it also turns out that the flowers of each Joshua tree species match the differences in the ovipositors of the two moth species.

 

The match between the Joshua trees’ flowers and the moths that pollinate them seems like too much of a coincidence to have arisen purely by chance. The hypothesis that my lab is exploring is that natural selection is responsible for this pattern. To test this hypothesis, we are trying to determine whether features of Joshua tree flowers are evolving through natural selection, and whether differences between the two species of moths are the source of this selection. In this class, we will be studying a hybrid zone, where the two species of Joshua tree and their respective pollinators occur together. We will establish four transects – linear paths along which we will record occurrences of Joshua trees – and collect data about the morphology of the trees in study plots placed at regular intervals along these transects. We will also collect leaf tissue for genetic analysis, and install insect monitoring traps. By analyzing how different features of the Joshua tree vary across these transects, we may be able to better understand whether these plants are evolving through natural selection.