Dissertation Research: Evolution in a Recently Arisen Species Complex: Phylogeography and Genetic Diversity in Manihot esculenta Crantz (Euphorbiaceae)

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Schaal 9801213 The process by which populations within a species diverge over time to form new species is poorly understood. Schaal and Olsen propose to study this process of genetic divergence, using two species in the plant genus Manihot as a study system. They will examine patterns of genetic differentiation within and among the three subspecies of M. esculenta, as well as between M. esculenta and the closely related species M. pruinosa. Using a combination of DNA sequence data and other molecular genetic markers, they will study how present-day population genetic structure reflects historical relationships among populations in the study system. Their analysis is specifically designed to discriminate between the effects of gene flow (migration) between populations and the random sorting of genetic variation (lineage sorting). Understanding the process of genetic divergence at the boundary between populations and species is crucial to understanding speciation and evolutionary diversification. This issue is thus of fundamental importance in understanding the process of evolution. The study proposed by Schaal and Olsen offers several advantages for studying this speciation process. Because their study system involves populations within a species as well as between species, their analysis explicitly encompasses the population-species interface. Moreover, because interspecific hybridization has been proposed to occur between the two species of Manihot in the wild, the potential influence of gene flow is relevant at both above and below the species boundary. In addition, one of the three subspecies of M. esculenta is the cultivated crop cassava, which is the fourth most important starch crop in the world and a staple food for over 500 million people. The proposed study will help to shed light on the evolutionary and geographical origins of cassava, and will therefore have important implications for approaches to conserv ing genetic diversity in this crop.