Title:

"Evolutionary pathways to trait diversity and specialisation" 

Opponent:

 Dr. Jeremy Beaulieu from the University of Arkansas in the United States

Committee:

Dr. Anthony Verboom (University of Gothenburg), Dr. Berit Gerke (University of Bergen, Norway), Dr. Gitte Petersen (DEEP, Stockholm University) and Dr. John Fitzpatrick (Reserve; Zoology, Stockholm University).

Abstract:

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.