Phylogeny, biogeography, and diversification of the tea family (Theaceae)
Research output: Book/Report › Ph.D. thesis › Research
The distribution of life on earth is uneven between regions and taxonomic clades. It is well acknowledged that much of the diversity patterns we see today are linked to the past environment and reflect the variation in the processes of speciation, movement, and extinction in deep times. For example, variations between different regions may reflect differences in the movement of lineages between regions, and variations in morphologicial diversity may be shaped by the different diversification rates of different traits. Those processes are all imprinted in species’ genome. However, our understanding of the relative importance of these processes remains inadequate. In this thesis, I focused on the tea family (Theaceae) and explored factors that contribute to its macroevolutionary patterns. This woody plant familyhas an unusual amphi-Pacific distribution. In essence, the center of diversity is restricted to the subtropical and tropical evergreen broadleaf forests in East Asia, while a handful of species occur exclusively in the subtropical and tropical Americas. Within the family, the largest genus Camellia has ca. 200 species, while the rest 6-8 genera have only 10-40 species each. It comprises predominantly evergreen species with lifeforms from shrubs to tall trees and a few temperate deciduous outliers. Researchers have been long curious about the cause of disjunctive distributions and the unbalance in species number among different groups. However, many of the previous attempts in solving the problems have been hampered by the controversy within the taxonomy classification and the lack of a robust and time-calibrated phylogeny with a complete taxon coverage. This dissertation aims to elucidate the origin of the heterogeneity in large-scale evolutionary trends based on newly generated molecular data. I attempted to approach the problem from three aspects, namely, the biogeographical history, the evolutionary history of different genomic compartments, and the trait evolution. In the first chapter, I aimed at understanding the unusual disjunctive distribution patterns of Theaceae started from a newly generated robust timecalibrated phylogeny of the family (c.140 species). By incorporating evidence from fossil specimens, while performing a biogeographic analysis of the spatiotemporal history of the group, I revealed a circumboreal distribution of the family from the Eocene to the Miocene. I inferred repeated expansions and retractions of the modelled distribution in the Northern Hemisphere, supporting the boreotropical forest hypothesis. In the second chapter, I explored the reason behind the incongruence of the phylogenetic relationships of this family. By generating a dataset for ~350 nuclear genes for c. 70 species, and comparing the results of different phylogenetic method and different genes, I suggested that incomplete lineage sorting events, which possibly resulted from rapid radiations, are main reasons causing the discordances between different phylogenetic trees. In the third chapter, I tested whether the explosive diversification events in the family associate with a loss in dispersal ability. Multiple transitions from winged to wingless seeds in the evolution of the family, occurring shortly after a significant acceleration of diversification rate in the late Oligocene/ early Miocene, were observed. Evidence supports that lineages with lower dispersal abilities have a higher diversification rate than lineages with higher dispersal abilities. Together, my results provide evidence to link the boreotropical forest to present-day subtropical forest, and indicate complex processes underlying the disjunctive distribution and the explosive diversification driven by changing climate and evolving biota. I demonstrate that the variation in the rates at which species diversity accumulates can be linked to differences in ecology among species. Such differences may implicate different adaptation strategies to the change in the environment. The study provides a concrete basis for continued research into more detailed explorations on the relationships between environmental change, trait evolution, and species diversification of the focal group.
|Publisher||Center for Macroecology, Evolution and Climate, University of Copenhagen|
|Number of pages||210|
|Publication status||Published - 2020|