Collaborative Research: Replicated evolution of leaf form in a neo-tropical radiation of Viburnum (Adoxaceae)

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Much remains unknown about the genesis of biological diversity, including the origin of new species, yet such knowledge is critical to understand evolution and to make predictions about responses to rapid environmental change. This research will investigate a striking case in a group of trees and shrubs (Viburnum) in which rapid differentiation appears to have occurred repeatedly into two radically different plant forms. Major differences in the leaves of these plants appear to have resulted from adaptation to adjacent forest types that differ subtly in environmental variables. The evident replication of this process in several different mountain regions throughout the New World tropics, provides a rare opportunity to critically test a model of rapid ecological speciation. Detailed studies of adjacent plant populations will be carried out in different mountain regions in Mexico with the aim of identifying the environmental factors and the nature of the variation within populations that have favored the rapid evolution of these leaf traits. It should also be possible to identify genes that underlie these key plant characteristics by taking advantage of natural hybridization between these species. These studies will yield general insights into speciation but also into the adaptive significance of the leaf traits that differentiate related species throughout the flowering plants. This research will be carried out in collaboration with Mexican scientists and will involve multiple undergraduate and graduate students. Outreach activities have been designed to foster interactions between participants from the United States and Mexico. This project focuses specifically on understanding the drivers of speciation in three species pairs in the mountains of Eastern and Southern Mexico. The two species in each pair differ dramatically in leaf size and shape (large and round versus small and elliptical), in leaf margins (toothed versus smooth), and in leaf pubescence (densely hairy versus lacking hairs). The research will focus on quantifying variation within individual plants, within populations, and between the species pairs, and on determining the abiotic and biotic factors that differ between their forest habitats. Hypotheses on the adaptive value of the leaf variables will be tested using transects between adjacent habitats and a reciprocal transplant experiment. In areas that have long been disturbed by indigenous agricultural practices, extensive hybridization has resulted in a wide array of intermediate leaf forms. Molecular analyses of these hybrid swarms should allow the identification of the major genes that underlie differences in leaf form. This project is jointly funded with the Office of International Science and Engineering.