Title: Understanding Tropical Plant Evolution Through Hybridization and Adaptive Introgression: A Case Study of Pandanaceae

Program: Ecology, Evolution, and Behavior PhD

Committee Chair: Sven Buerki

Committee: Sven Buerki, Martin Callmander, James Smith, Stephen Novak

Abstract: Hybridization—the interbreeding of genetically distinct populations or species—is a major force in plant evolution, driving speciation and spreading beneficial traits through adaptive introgression. Advances in genomics, phylogenomics, and bioinformatics have transformed how we study these processes. Recent research shows hybridization is more widespread among flowering plants than previously thought and may have shaped their early diversification. This thesis examines how hybridization leads to adaptive introgression in tropical flowering plants. Pandanaceae serves as a model system due to its ancient origin, ecological success, and morphological traits potentially evolved in response to past climate changes.
Chapter 1 uses a chloroplast-based phylogeny for Pandanus, the largest genus, to link morphological traits with climate and infer spatio-temporal patterns. The evolution of specialized water-storage tissue, linked to drought tolerance, emerged as a key feature. Its emergence mirrors phylogenetic clustering and climatic niches, with divergence timing suggesting Miocene environmental change as a major speciation driver.
Chapter 2 explores hybridization and adaptive introgression in Pandanus by comparing nuclear and chloroplast phylogenies and tracing morphological evolution. Genomic discordance supports a Miocene hybridization event introducing the drought-adaptive water-storage trait into the hybrid clade. This trait’s emergence coincides with increased ecological niche and speciation rates, highlighting hybridization and adaptive introgression as major evolutionary drivers in Pandanus.
Chapter 3 expands to the entire family, exploring how life form shifts—such as tree-to-liana transitions—shaped evolution in response to past climate change. Results show lianas evolved from palm-like trees after the Cretaceous–Paleogene extinction, likely driven by the rise of closed-canopy forests. This shift coincided with adaptations for water-use efficiency (in Pandanus and its sister genus Benstonea) and reproductive changes linked to a transition from beetle to vertebrate pollination. During the Paleocene–Eocene transition, a novel pseudo-tree form evolved from lianas, possibly due to forest contraction from climate change. In the Oligocene, changes in male reproductive structures suggest a shift back from vertebrate to beetle pollination, coinciding with vertebrate species turnover.
Overall, my research reveals how past climatic fluctuations, morphological innovations, hybridization, and ecological trade-offs shaped Pandanaceae evolution. It uncovers the genetic and functional bases of key adaptations and underscores the evolutionary importance of hybridization and life form transitions in tropical plants.