Ecological and Evolutionary Consequences of Two Distinct Modes of Speciation in Plants

Abstract

Speciation—the evolutionary process by which new species arise—can occur through genetic divergence maintaining the ploidy—the number of complete sets of chromosomes in the cells—of the descendant lineages relatively unchanged (i.e., homoploid speciation) or through drastic changes in ploidy associated with whole-genome duplications (i.e., polyploid speciation). In this thesis, I use the Arabidopsis arenosa species complex and the Angiosperm family Brassicaceae to explore different hypotheses about the ecological and evolutionary consequences of these two distinct modes of speciation in plants. The A. arenosa complex includes at least three highly divergent diploid lineages and one recently formed polyploid lineage. In chapter 1, I explore the potential consequences of human-induced global warming on one instance of homoploid speciation in A. arenosa. Here, I offer empirical evidence supporting the hypothesis that climate-induced range shifts may increase sympatry between two parapatric lineages and demonstrate that rising temperatures will undermine reproductive isolation by increasing the probability of introgressive hybridization. Next, in chapter 2, I evaluate the ecological consequences of the recent case of polyploid speciation in A. arenosa. Here, I find no evidence supporting the hypothesis that successful establishment of neopolyploids is contingent on climatic niche shifts from their diploid progenitors, but I show how this neopolyploid lineage has significantly expanded its niche instead. Finally, in chapter 3, I study the long-term evolutionary consequences of polyploidy in the family Brassicaceae which exhibits an extensive inter-specific variation in chromosome numbers and a high incidence of polyploid species. Here, I show macroevolutionary evidence supporting the hypothesis that polyploidy has significant effects on the long-term diversification of lineages, contradicting the traditional model that considers polyploidy as an evolutionary dead-end. Altogether, this dissertation contributes to our understanding of the short- and long-term ecological and evolutionary consequences of homoploid vs. polyploid modes of speciation.