Historical and Geographic Context for the Evolution of Climate Niche Breadth in Temperate Plants

Abstract

OF THE DISSERTATION In order to predict how species will respond to global climate change, scientists must understand the relationships between traits, fitness, environments and distributions. Niche theory provides a useful framework. Niche breadth describes the range of environmental conditions necessary for population growth. Among these conditions, climate is especially important. Climate niche breadth in turn may reflect a confluence of different forces. This dissertation presents a series of projects that assess the relative roles of historical, geographic and population processes that contribute to climate niche breadth in temperate plants. The first project evaluates the predictive power of a classical hypothesis. If gene flow slows divergent adaptation, then range fragmentation should promote niche breadth. By quantifying the relationships between environmental difference, geographic distance and genetic isolation among European plants, I show that the effect of allopatry on niche breadth depends on the role of the geographic distance among populations. The remaining projects focus in increasingly finer detail on the evolution of niche breadth in a taxonomically complex group. Dodecatheon sect. Dodecatheon grow in diverse habitats across North America. They have confounded taxonomists with polyploidy, hybridization and convergent adaptation. Currently recognized species are either widespread or rare microclimate specialists. First, with multilocus phylogenetics, I show that the difference in niche breadth among rare and widespread species is not simply due to differences in environmental tolerance. In eastern North America, geographic heterogeneity and paleoclimate history strongly contribute to taxonomic rarity. The next project focuses on this group. Both rare eastern species are considered