Abstract
Angiosperms (the flowering plants) split from relatives ~310-370 million years ago (Mya) and began diversifying into modern lineages ~140-270 Mya. They now constitute the largest plant lineage on Earth, with ~290,000 extant species distributed in ~435 families, and dominate terrestrial ecosystems through staggeringly diverse ecological and life-history adaptations. The explosive diversification of angiosperms was one of the most important events in the history of Life, altering evolutionary trajectories of animals, fungi and other plants. Characterising the rise of angiosperms has been a major focus of evolutionary biologists for over 150 years, but our understanding of the forces shaping diversity remain incomplete. In order to fully understand angiosperm diversity, we need to investigate some of the most fundamental trends and processes shaping their evolution. In this thesis, I use comparative methods to analyse why and when angiosperms diversified, with focussed analyses on some of the richest and most iconic lineages, including analyses of angiosperms as a whole.I demonstrate a previously unknown link between Cenozoic global cooling and diversification rates in the largest terrestrial Orchid lineage. While the impact of temperature is known to have impacts on diversification throughout the Tree of Life, it has been largely neglected in angiosperm research. Diversification rates of Orchidoideae, as well as all tribes, are negatively correlated with paleoclimatic reconstructions of temperature. Unusually, this pattern is remarkably homogeneous across Earth, contrasting other climatic factors with locally-varying impacts. Models suggest that terrestrial Orchids are one of the best-supported examples of climate-dependent diversification yet recorded
I investigate the rise of the seven rich succulent lineages in arid floras around the world. The succulent syndrome is an iconic suite of traits which has evolved convergently dozens of times in 3-5% of angiosperms, allowing survival in inhospitable water-scarce environments. Although succulence is touted as one of the best cases of climate-driven convergence, the tempo and drivers of the richest radiations have been underexplored. These analyses reveal the complexity underlying succulent macroevolution and the rise of arid floras. Aridity-dependent diversification is supported, but the impact of specific aridities varies by lineage, and the impact of atmospheric CO2 is supported only by two lineages. Succulence tends to elevate diversification rates, but not in every lineage, despite its highly adaptive benefits. Mixed levels of synchronicity in transitions to succulence and radiations are found, with the majority occurring in recent geological time, consistent with a global aridification driver. But many were reconstructed much earlier, indicating complex lineage and region-specific patterns.
I demonstrate the complexity underlying the forces shaping diversification, with Cactaceae as a case study. The complexity of macroevolution has long been recognised, but it has been difficult to quantify this, and identify key drivers of diversification in the network of interdependencies. Cacti represent an ideal case for this investigation, given their remarkable diversity and distribution across heterogeneous environments. Machine Learning models provided support for complexity and identified eleven drivers, the power of which were all confirmed with phylogenetic models. Some of these were confirmations of previous hypotheses, whereas some were novel, reinforcing the need to sample potential drivers widely.
I provide phylogenetic support for macroevolutionary resilience of angiosperms to the Cretaceous-Paleogene (K-Pg) mass extinction event. The fossil record reveals the instantaneous loss of 75% of diversity in many ecosystems, including non-avian dinosaurs, and a significant fraction of squamates and birds. The angiosperm fossil record is exceptionally poor and biassed, but initial research has suggested a puzzling pattern of minimal impacts of K-Pg on angiosperm extinction. Analyses of two of the largest angiosperm phylogenies extend support for a lack of mass extinctions to the majority of branches which did not fossilise.
These analyses make use of the wealth of publicly available data, and implement modern statistical methods to reveal forces shaping the evolution of major angiosperm lineages. It is clear that the rise of angiosperms was a very complex process, and that understanding their evolution requires both focussed and broad-scale investigation.
| Date of Award | 28 Jun 2023 |
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| Original language | English |
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| Supervisor | Tiffany Taylor (Supervisor) & Nicholas Priest (Supervisor) |