Phylogenetics and Morphological Evolution of Euphorbia subgenus Euphorbia.

Abstract

Euphorbia subg. Euphorbia is the largest subgenus in Euphorbia and contains every growth form found across the genus. These including typical growth forms but also an array of xerophytic forms including an unusual growth form termed ‘pencil-stem’ plants. These leafless, species belong to multiple clades within the subgenus. The combination of divergent and convergent evolution makes this a particularly good group in which to investigate the evolution of different growth forms. Heretofore, the composition of, and relationships among, established subclades has been unclear. In this dissertation, the largest taxonomic sample from E. subg. Euphorbia to date was compiled, and DNA sequences from one nuclear and two chloroplast regions were used to infer a phylogeny of the group. This phylogeny was then used in combination with estimates of the climatic components of species’ niches to investigate the role of abiotic ecological parameters in the repeated evolution of the pencil-stem growth form. Finally, five gene regions were used to produce the most comprehensive phylogeny to date for the largest section in the subgenus, sect. Euphorbia. This phylogeny, along with geographic distribution data and estimates of species’ “climatic niches”, was then used to test the hypothesis that divergence of growth form within this clade is the result of an historical expansion of the group’s range from Asia into Africa, and a consequential reduction in the size of the plant body during colonization of increasingly arid habitats. I found that E.subg. Euphorbia is composed of four main clades distinguished by their geographic distribution. Further, on average, pencil-stem species in E. subg. Euphorbia occupy significantly drier sites than their leafy relatives, but this pattern may break down at the individual species level. Within E. sect. Euphorbia, the divergent evolution of growth forms is the result of adaptation to seasonal drought. Finally, current species distributions in E. sect. Euphorbia are the result of a complex pattern of range partitioning and a moderate number of dispersal events. The three chapters of this dissertation significantly improve our understanding of the phylogenetic relationships, historical biogeography, and morphological evolution of this speciose and morphologically diverse group of plants.