Abstract

The genus Weinmannia is a dominant yet systematically challenging component of Andean montane forests, with an evolutionary history shaped by mountain uplift, climatic change, and recurrent gene flow. Weinmannia provides an ideal study system for investigating the processes that generate and maintain the extraordinary biodiversity of the tropical Andes. This dissertation integrates phylogenomics, population genetics, morphometrics, and demographic modeling within a hypothesis-driven framework to examine these processes across macroevolutionary, microevolutionary, and systematic scales. First, we reconstructed the biogeographic history of Weinmannia using a 2bRAD-seq approach to generate a time-calibrated phylogeny for the genus, providing evidence for northward dispersal from temperate regions into the tropical Andes. Our phylogenetic results placed extratropical species as sister to the remainder of the genus, with younger clades distributed towards more equatorial latitudes, supporting a south-to-north colonization route. Although Weinmannia exhibited low niche conservatism in elevation and latitude, trait reconstructions of climatic variables revealed that the common ancestor of the genus occupied relatively cool climates, with high conservatism of thermal and water availability niches across the phylogeny. This pattern suggests that Andean uplift did not require major climatic niche shifts for colonization; rather, it created novel habitats with suitable environmental conditions that matched the pre-existing cool-adapted niche of Weinmannia. These suitable habitats served as both a dispersal corridor facilitating northward migration and an ecological arena promoting diversification, allowing the genus to radiate while maintaining its ancestral climatic preferences. Together, these findings highlight the Andes as a critical driver of Weinmannia's evolutionary history, acting simultaneously as a dispersal route from southern extratropics into the tropics and as a center of in situ evolutionary radiation fueled by the generation of new, climatically suitable montane environments. Second, we investigated the role of hybridization in diversification. Using complementary phylogenomic and population genomic approaches across 10 co-occurring species, we detected widespread historical and contemporary gene flow throughout the genus, with approximately 35.3% of sampled individuals showing hybrid ancestry spanning 31 of 45 possible species pairs. A higher prevalence of backcross hybrids relative to early-generation hybrids (F1/F2), together with reticulate evolution analyses, indicated introgression occurring across both recent and deeper evolutionary timescales. Despite this pervasive genetic exchange, phylogenetic analyses showed that species boundaries persist, with lineages maintaining semi-independent evolutionary trajectories consistent with syngameon-like dynamics. Bayesian regression further revealed that the extent of hybridization was best explained by evolutionary divergence between parental species, declining as divergence increased, while geographic and elevational distances had negligible effects, suggesting that intrinsic biological barriers, rather than spatial separation, primarily constrain gene exchange. Together, these results suggest that hybridization has been a long-term evolutionary process that contributes to adaptation and diversification rather than simply eroding species integrity. Finally, we integrated taxonomy, phylogenomic, morphological, and demographic evidence to propose an evolutionary framework and generate species hypotheses in Weinmannia. Rather than validating existing taxonomic species, our approach focused on identifying independently evolving lineages as testable species hypotheses, which frequently conflicted with current classification schemes. By explicitly modelling gene flow and incomplete lineage sorting, we distinguished cases where taxonomic non-monophyly stemmed from misclassification from those reflecting genuine biological processes of reticulate divergence. The resulting process-aware species-level framework recognizes both well-supported, taxonomically and phylogenetically cohesive lineages and more complex species complexes, accommodating the continuum of divergence that characterizes this rapidly radiating group. Critically, our analyses demonstrate that Weinmannia species are not discrete endpoints of a bifurcating tree but nodes embedded within an evolutionary network, where hybridization and shared ancestral polymorphism are fundamental drivers rather than exceptions to diversification. This framework moves beyond static, name-based taxonomy to provide an evolutionarily informative foundation, grounded in explicitly testable hypotheses, that not only revises species boundaries within the group but also offers valuable insights for systematists working on other plant lineages where reticulate evolution obscures the relationship between phenotypic variation and evolutionary independence. Collectively, this dissertation demonstrates that Weinmannia is an informative model system for investigating the evolutionary processes that have shaped the Andean flora. The results show that Andean biodiversity has emerged through the interaction of historical dispersal, ecological opportunity, niche conservatism, geographic isolation, and recurrent gene flow. By integrating evolutionary history across multiple temporal and biological scales, this work contributes to a broader understanding of the mechanisms responsible for the exceptional diversity of the tropical Andes.

Committee Chair

J. Sebastian Tello

Committee Members

Christine Edwards, Iván Jiménez; Jonathan Myers; Michael Landis

Degree

Doctor of Philosophy (PhD)

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-13-2026

Language

English (en)

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Available for download on Friday, February 12, 2027

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