Warm, dry environments create living conditions that challenge plant growth, reproduction and survival. Plants in these environments have evolved adaptive strategies to enhance water-use efficiency and ensure reproductive success, two of which are annuality and C₄ photosynthesis. However, life history variation is rarely included in large-scale studies of plant diversity, and the extent to which these traits coevolve and are jointly selected for is not known. To address this, we used Pagel’s models of independent and correlated evolution for over 4000 species of grasses (Poaceae), while accounting for evolutionary rate heterogeneity and potential type I statistical errors. We found that there are more C₄ than C₃ annuals and that C₄ origins predate evolution of annuality, but no support for correlated evolution between the two traits. Our results indicate that any habitat or trait similarities (e.g. small seeds, fast growth) between annuals and C₄ species reflect independent adaptations to similar environmental conditions or are contingent on the two traits themselves, rather than the result of evolutionary or functional links between them. Our results further highlight the importance of appropriate null model specification for testing evolutionary hypotheses across large, old clades.

Accumulating evidence is suggesting more frequent tropical-to-temperate transitions than previously thought. This raises the possibility that biome transitions could be facilitated by precursor traits. A wealth of ecological, genetic, and physiological evidence suggests overlap between drought and frost stress responses, but the origin of this overlap, i.e., the evolution of these responses relative to each other, is poorly known. Here, we test whether adaptation to frost and/or severe winters in grasses (Poaceae) was facilitated by ancestral adaptation to drought. We used occurrence patterns across Köppen-Geiger climate zones to classify species as drought, frost, and/or winter tolerant, followed by comparative analyses. Ancestral state reconstructions revealed different evolutionary trajectories in different clades, suggesting both drought-first and frost-first scenarios. Explicit simultaneous modelling of drought and frost/winter tolerance provided some support for correlated evolution, but suggested higher rates of gain of frost/winter tolerance in drought-sensitive rather than drought-tolerant lineages. Overall, there is limited support across grasses as a whole that drought tolerance acted as an evolutionary precursor to frost or severe winter tolerance. Different scenarios in different clades is consistent with present-day grasses being either cold or drought specialists, possibly as a consequence of trade-offs between different stress tolerance responses.

Plant traits shape the distribution, environmental tolerances and evolutionary histories of species. In this thesis, I focus on the evolutionary history and diversity of key traits in the grass family (Poaceae), one of the most globally distributed and ecologically diverse lineages of flowering plants. This thesis addresses fundamental questions related to the evolutionary history and diversity of traits in grasses, including the rates and patterns of their evolution, relationships among them and methodological considerations for inferring the above. Using comparative phylogenetic approaches, I address four main aims: (1) determine if drought tolerance acted as a precursor to frost and/or severe winter tolerance; (2) test whether variation in germination rates among temperate grasses is shaped by life history strategy, growing season length and seed size; (3) assess how trait lability can be quantified at macroevolutionary scales and investigate the lability of life history; and (4) test whether evolutionary transitions in life history strategy are correlated with changes in photosynthetic pathway.

Trait data were compiled from large-scale databases such as GrassBase, while abiotic stress tolerances were inferred by linking occurrence records (GBIF) to the climates a species occupies using Köppen-Geiger climate zones. Analyses were performed using multiple phylogenetic trees representing different data sources and taxon sampling, together with a range of evolutionary models, including Pagel’s correlation, hidden Markov models and Ornstein-Uhlenbeck models.

This thesis challenges several assumptions about trait evolution. Drought tolerance has long been thought to have facilitated evolution of frost and severe winter tolerance, but chapter I only finds limited evidence for this. Instead, many lineages are specialised in either drought or cold, suggesting possible constraints to adapting to both traits. Chapter II found that germination rate evolves towards consistently faster optima in annuals. Chapter III revealed frequent transitions from perennial to annual, but the classification of clades as “labile” or “stable” depends strongly on the analytical approach used. Finally, Chapter IV showed that while annuality and C₄ photosynthesis are both prevalent in hot, dry environments, these traits evolve independently rather than correlated.

Overall, this thesis advances our understanding of the evolutionary dynamics of key traits in grasses and demonstrates the value of thoughtful methodological choices to accurately infer macroevolutionary patterns and processes.

Frost tolerance has evolved many times independently across flowering plants. However, conservation of several frost tolerance mechanisms among distant relatives suggests that apparently independent entries into freezing climates may have been facilitated by repeated modification of existing traits (‘precursor traits’). One possible precursor trait for freezing tolerance is drought tolerance, because palaeoclimatic data suggest plants were exposed to drought before frost and several studies have demonstrated shared physiological and genetic responses to drought and frost stress. Here, we combine ecophysiological experiments and comparative analyses to test the hypothesis that drought tolerance acted as a precursor to frost tolerance in cool-season grasses (Pooideae). Contrary to our predictions, we measured the highest levels of frost tolerance in species with the lowest ancestral drought tolerance, indicating that the two stress responses evolved independently in different lineages. We further show that drought tolerance is more evolutionarily labile than frost tolerance. This could limit our ability to reconstruct the order in which drought and frost responses evolved relative to each other. Further research is needed to determine whether our results are unique to Pooideae or general for flowering plants.

Low temperature constitutes one of the main barriers to plant distributions, confining many clades to their ancestrally tropical biome. However, recent evidence suggests that transitions from tropical to temperate biomes may be more frequent than previously thought. Here, we study the evolution of cold and frost tolerance in the globally distributed and highly stress-tolerant Salicornieae (Salicornioideae, Amaranthaceae s.l.). We first generate a phylogenetic tree comprising almost all known species (85-90%), using newly generated (n = 106) and published nuclear-ribosomal and plastid sequences. Next, we use geographical occurrence data to document in which clades and geographical regions cold-tolerant species occur and reconstruct how cold tolerance evolved. Finally, we test for correlated evolution between frost tolerance and the annual life form. We find that frost tolerance has evolved independently in up to four Northern Hemisphere lineages but that annuals are no more likely to evolve frost tolerance than perennials, indicating the presence of different strategies for adapting to cold environments. Our findings add to mounting evidence for multiple independent out-of-the-tropics transitions among close relatives of flowering plants and raise new questions about the ecological and physiological mechanism(s) of adaptation to low temperatures in Salicornieae.

Background

Variation in floral shapes has long fascinated biologists and its modelling enables testing of evolutionary hypotheses. Recent comparative studies that explore floral shape have largely ignored 3D floral shape. We propose quantifying floral shape by using geometric morphometrics on a virtual3D model reconstructed from 2D photographical data and demonstrate its performance in capturing shape variation.

Methods

This approach offers unique benefits to complement established imaging techniques (i) by enabling adequate coverage of the potential morphospace of large and diverse flowering-plant clades; (ii) by circumventing asynchronicity in anthesis of different floral parts; and (iii) by incorporating variation in copy number of floral organs within structures. We demonstrate our approach by analysing 90 florally-diverse species of the Southern African genus Pelargonium (Geraniaceae). We quantify Pelargonium floral shapes using 117 landmarks and show similarities in reconstructed morphospaces for nectar tube, corolla (2D datasets), and a combined virtual3D dataset.

Results

Our results indicate that Pelargonium species differ in floral shape, which can also vary extensively within a species. PCA results of the reconstructed virtual3D floral models are highly congruent with the separate 2D morphospaces, indicating it is an accurate, virtual, representation of floral shape. Through our approach, we find that adding the third dimension to the data is crucial to accurately interpret the manner of, as well as levels of, shape variation in flowers.