Effects of the Dynamic Pleistocene Climate on Plant Evolution: Integrative Tests Across Multiple Spatial and Temporal Scales.

Abstract

New analytical techniques and abundant genomic data provide an unparalleled opportunity to illuminate evolutionary questions. This dissertation utilizes these resources to investigate plant evolution in ecosystems that were highly impacted by Pleistocene climatic oscillations. At the broadest spatial and temporal scales, I resolve relationships among species in Carex section Racemosae, which are primarily distributed at high elevations and high latitudes. Analyses utilizing the resulting phylogeny suggest that much of the extant diversity within sect. Racemosae was generated during the Pleistocene, and that montane clades diversified regionally, but failed to disperse between continents. This latter result, in light of floristic similarities of Asian and North American mountains, suggests that shared evolutionary lineages were generated when widespread species adapted to high latitudes became isolated in mountains at southern latitudes during glacial periods. Refocusing on a regional scale, I investigate whether genomic patterns of variation in two closely related and co-distributed species differ in a manner consistent with expectations of how species’ respective microhabitats were influenced by Pleistocene glaciations. Results support plant populations of wet-adapted species being more isolated through time compared to populations of a dry-adapted species. To test whether interactions with glaciers caused these genomic patterns, I use demographic modeling that incorporates spatial and temporal heterogeneity. Coalescent simulations generate expectations for geographic patterns of molecular variation under different combinations of parameters for two models, and I subsequently use approximate Bayesian computation to select the most likely model for each species. Glaciers impacted wet and dry microhabitats differently, forcing wet-adapted species to establish isolated populations at lower elevations around the margins of glaciers. Alternatively, populations of dry-adapted species remained more connected by persisting within glaciated regions as well as establishing populations at lower elevations. These results suggest that deterministic processes are important to consider in comparative phylogeographic studies, and may partially explain discordant patterns commonly resolved among species. Many factors influence the evolution of organisms adapted to ecosystems highly impacted by Pleistocene glacial cycles, and here I demonstrate how genomic data and new analytical techniques can be utilized to shed light on processes across spatial and temporal scales.