Document Type



Doctor of Philosophy



Date of Defense


Graduate Advisor

P. F. Stevens, Ph. D.


Elizabeth A. Kellogg

Amy E. Zanne

Allison J. Miller


The long-isolated island of Madagascar provides a suitable setting for studying species diversification, with most groups of organisms there both radiating and showing a high level of endemism. Noronhia is one of these groups and represents the most successful radiation of the olive family (Oleaceae) in Madagascar, with ca. 80 species. In this study, using plastid and nuclear DNA sequences obtained from a comprehensive sampling both within Noronhia and the family, I show that Noronhia, together with Indian Ocean species of Chionanthus, form a monophyletic clade sister to African Chionanthus. The diversification of Noronhia followed a likely Cenozoic dispersal from Africa. Within Noronhia, phylogenetic relationships are mostly unresolved despite the species showing considerable ecological and phenotypic diversity. In most cases, analyses of bioclimatic, molecular and morphological data, interpreted in phylogenetic and geographic contexts, show support for my initial species hypotheses and offer new insights into species boundaries. Morphological data provide the strongest support while bioclimatic ones are the least informative, suggesting that the broad-scale variation in bioclimatic data does not adequately capture the ecological processes driving the diversification of this genus. However, attempts to understand spatial patterns of richness and coexistence among species of Noronhia show that mountainous areas in the island harbor the highest concentrations of species and the highest endemism. Habitat heterogeneity likely explains how diversity is promoted and maintained in these topographically complex regions. Furthermore, analyses focused on a smaller spatial scale, the Montagne d’Ambre massif, again indicate that habitat heterogeneity plays an important role. Different groups of species grow in different habitats on the mountain, suggesting environmental filtering associated with rainfall and soil nutrient gradients. Overall, the integrative approach applied in this study highlights the importance of using different kinds of data analyzed at various scales to understand species diversification.

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