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Title: The Systematics and Evolution of Neotropical Bellflowers (Campanulaceae: Lobelioideae)
Abstract: The Andean mountain chains of South America are home to exceptionally high levels of biodiversity, including one-sixth of all plant species. Among them, the 550 species in the Neotropical bellflower clade (Campanulaceae: Lobelioideae) represent one of the single largest Andean radiations. In this dissertation, I perform a series of studies, ranging from alpha taxonomy to phylogenetic comparative biology, to understand their diversification.
In Chapter 1, I resolve phylogenetic relationships of the Neotropical bellflowers from five plastid DNA regions using maximum likelihood and Bayesian inference. This phylogeny greatly improves the resolution within this group and highlights the need for taxonomic revisions. My results demonstrate that the majority of species of Centropogon, Burmeistera, and Siphocampylus form a primarily mainland Neotropical clade, collectively termed the centropogonids. Within the centropogonids, I identify high support for the monophyly of Burmeistera and the polyphyly of Centropogon and mainland Siphocampylus. Caribbean Siphocampylus, however, group with other Caribbean lobelioid species. Ancestral character state reconstruction identifies a dynamic pattern of fruit evolution within the centropogonids, which underscores the difficulty of diagnosing broad taxonomic groups on the basis of fruit type. Finally, we identify that the centropogonids form a Pan-Andean radiation with broad habitat diversity, which sets the stage for a subsequent chapter in my thesis (Chapter 4).
In Chapters 2 and 3, I describe four species of centropogonids as new to science. In Chapter 2, two species of Burmeistera from the Cordillera de Talamanca are described: B. serratifolia Lagom. & D. Santam., endemic to Panama, and B. monroi D. Santam. & Lagom., from both Panama and Costa Rica. These species are included in a dichotomous key to all Central American species of Burmeistera. In chapter 3, I describe two species of Siphocampylus from the Central Andes of Peru and Bolivia: S. antonellii Lagom. & D. Santam., endemic to high elevation grasslands of Calca, Peru, and S. siberiensis Lagom. & D. Santam., endemic to cloud forests of Cochabamba, Bolivia. Both species are robust shrubs that produce tubular pink flowers that are likely pollinated by hummingbirds.
In Chapter 4, I use the Neotropical bellflowers as a model system to understand the triggers of rapid diversification in the Andes. I explore the interaction of abiotic and biotic factors by applying a series of diversification models that incorporate orogeny, climate, and biological trait evolution to a time-calibrated phylogeny of the group. Here, I demonstrate that speciation rates rose with increasing Andean elevation, while extinction decreased during global cooling. Of these, climate appears to have played the more important role, which I attribute to preadaptation to cool climates. Biotic traits that facilitate plant-animal mutualisms, specifically fruit type and especially pollination syndrome, additionally enhanced diversification rates. These results suggest that, while the Andes acted as a species pump for this lineage, a synergistic interaction of abiotic and biotic factors underlies its rapid diversification, which culminated in one of the fastest plant radiations documented to date: the centropogonids. This study underscores the complex interplay of ecological and historical determinants in generating the world’s biodiversity.
In Chapter 5, I investigate the evolution of vertebrate pollination syndromes in the most diverse subclade of the Neotropical bellflowers, the centropogonids. This builds on the finding in Chapter 4 that mutualisms involving pollination by vertebrates (hummingbirds and bats) have contributed substantially to Neotropical bellflower diversification. In this clade, flowers pollinated by hummingbird and bats share many characteristics: tubular corollas, abundant nectar production, highly specific pollen deposition mechanisms, and placement away from the main plant body. Despite these similarities, phylogenetic principal component analyses demonstrate that flowers hypothesized to be pollinated by bats and by hummingbirds fall into distinct regions of morphospace, lending strong support to our use of pollinator syndromes within this group. Character state reconstructions demonstrate that the centropogonids were hummingbird pollinated ancestrally, and that bat pollination has evolved independently fourteen times, with nine reversals to hummingbird pollination. An obligate relationship with sicklebill hummingbirds, characterized by extremely curved bills, evolved once. I further use an Ornstein-Uhlenbeck framework to identify a series of quantitative floral traits whose evolution appears to be most closely tied to the hummingbird and bat pollination syndromes. I investigate the phylogenetic sequence and rate of evolution of these key traits to understand the their potential importance with regard to the dynamic evolution of pollinator associations the group.