Adaptive variation versus vicariance: what drives speciation in Eucalyptus?

Abstract

Speciation is a central process in evolutionary biology and is responsible for the diversity of life on Earth. While there has been much progress in evolutionary research over the last 150 years, understanding the many facets of speciation remains a major challenge. In this thesis, I focus on a group of Eucalyptus species called the green ashes (subgenus Eucalyptus section Eucalyptus). The green ashes comprise tall trees on fertile soils (e.g. the tallest flowering plant in the world, Eucalyptus regnans), as well as medium trees and mallees on low nutrient soils. Although Eucalyptus is Gondwanan in origin, fossil and molecular evidence suggest that many eucalypt groups (including the green ashes) diverged within the last 10 million years. Since the green ashes are highly diverse, occur across a range of habitats and are considered recently radiated, they are an appropriate group for investigating speciation mechanisms. Phylogenetic and population genetic analyses using genome-wide scans based on Diversity Arrays Technology (DArT) were used to reconstruct the evolutionary history of the species relationships. A common garden experiment was conducted to examine how seedling response to variable resource availability is associated with evolutionary events across species. Environmental modelling was used to investigate differences in the predicted range and climatic niche of species. I found that species boundaries in the green ashes were not always consistent with classifications based on morphology and there was evidence of hybridisation and ongoing gene flow between lineages. The findings suggest that the green ashes are at varying stages of speciation, with some species being highly genetically differentiated and others being at earlier stages on the speciation continuum. Inter-specific differences in seedling traits were significant, with traits such as leaf width and biomass being highly plastic across resource treatments for most species. Predicted environmental niches varied amongst species, and species with lower seedling plasticity tended to be restricted to narrower environmental ranges. Overall, this study demonstrates that an approach incorporating phylogenomics, population genomics, a common garden experiment and environmental modelling can provide insights into the speciation of a group of closely related species, where a number of speciation mechanisms (e.g. reticulate evolution, vicariance and ecological speciation) are operating in concert.