Studies of phylogenetic relationships and evolution of functional traits in diatoms

The research presented here deals with inferring phylogenetic trees and their use to study the evolution of functional traits in diatoms (Heterokontophyta: Bacillariophyceae). Two chapters are concerned with the phylogeny of a mainly freshwater group, the Cymbellales, with a convoluted taxonomic history and classification. I generated a multi-gene dataset to test the monophyly of the Cymbellales and reconstruct the relationships within the order. The molecular data were equivocal with respect to the monophyly of the Cymbellales, especially when taking into account some problematic taxa like Cocconeis and Rhoicosphenia. Aside from the problem with their monophyly, my work shows that the current genus- and family-level classification of the Cymbellales is unnatural, arguing for the need of nearly wholesale re-classification of the group. The two following chapters make use of phylogenetic trees to model the evolution of functional traits. I explored the evolution of cell size across the salinity gradient finding that the opposing selective forces exerted by marine and fresh waters select for different optimal cell sizes -- larger in the oceans and smaller in lakes and rivers. Thereafter, I modelled the evolutionary histories of habitat preference (planktonic-benthic) and growth form (solitary-colonial) across the diatoms. These traits exhibit markedly different evolutionary histories. Habitat preference evolves slowly, is conserved at the level of large clades, and its evolution is generally uniform across the tree. Growth form, on the other hand, has a more dynamic evolutionary history with frequent transitions between the solitary and colonial growth forms and rates of evolution that vary through time. I hope that these empirical studies represent an incremental advancement to the understanding of the evolution diatom species and functional diversity.