Publication Date
12-2025
Date of Final Oral Examination (Defense)
10-1-2025
Type of Culminating Activity
Dissertation
Degree Title
Doctor of Philosophy in Ecology, Evolution, and Behavior
Department
Biological Sciences
Supervisory Committee Chair
Sven Buerki, Ph.D.
Supervisory Committee Member
Martin W. Callmander, Ph.D.
Supervisory Committee Member
Stephen Novak, Ph.D.
Supervisory Committee Member
James Smith, Ph.D.
Abstract
Hybridization—the interbreeding of genetically distinct populations or species—is hypothesized as a major force in plant evolution, driving speciation and enabling the spread of beneficial traits through adaptive introgression. Advances in genomics, phylogenomics, and bioinformatics have transformed our ability to study these processes. Recent research showed that hybridization is even more widespread among flowering plants that predicted and may have shaped their early diversification. This thesis examines how hybridization leads to adaptive introgression in tropical flowering plants. Pandanaceae is used as a model system due to its relatively old origin, ecological success, and the presence of morphological features that could have evolved as a response to past climate change conditions.
In Chapter 1, I reconstruct a chloroplast-based phylogeny for Pandanus, the largest genus, to determine links between morphological traits and climate, and infer spatio-temporal patterns. The evolution of a specialized water-storage tissue, associated with drought tolerance, emerged as key morphological feature. The emergence of this trait mirrors both phylogenetic clustering and climatic niches. Divergence timing suggested Miocene environmental change as a major driver of speciation.
In Chapter 2, I examine hybridization and adaptive introgression in Pandanus by comparing nuclear and chloroplast phylogenies and reconstructing the evolution of morphological features through time. Genomic discordance was observed, which was supported by a Miocene hybridization event that introduced a key drought-adaptive trait (i.e., the specialized water-storage tissue) into the hybrid clade. The emergence of this trait was associated with increased ecological niche and speciation rate. This result highlights hybridization and adaptive introgression major drivers of evolution in Pandanus.
In Chapter 3, my focus expands to the entire family, exploring how shifts in life form—i.e. transitions between tree to liana—shaped evolution of these plants in response to past climate change. Results showed that lianas evolved from palm-like trees following the Cretaceous–Paleogene extinction event, likely driven by the rise of closed-canopy forests (due to dinosaurs’ extinction). This shift coincided with the emergence of the adaptation for water-use efficiency (not only in Pandanus, but also in its sister genus, Benstonea) and reproductive changes linked to a shift to vertebrate pollination (from beetle pollination). During the Paleocene–Eocene transition, a novel tree form evolved from lianas, possibly due to forest contraction caused by climate change. Finally, in the Oligocene, further changes in male reproductive structures suggest a shift of pollination syndrome from vertebrates to beetles, coinciding with a mass turnover of vertebrate species.
Overall, my research illuminates how past climatic fluctuations, morphological innovation, hybridization, and ecological trade-offs have shaped Pandanaceae evolution. This work uncovers the genetic and functional bases of key morphological adaptations and underscores the evolutionary significance of hybridization and life form transitions in tropical plants.
Recommended Citation
Wojahn, John Michael Adrian, "Understanding Tropical Plant Evolution Through Hybridization and Adaptive Introgression: A Case Study of Pandanaceae" (2025). Boise State University Theses and Dissertations. 2473.
https://scholarworks.boisestate.edu/td/2473
Comments
Degree emphasis is in global change biology.