Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Evolution, Ecology and Population Biology


English (en)

Date of Award

January 2009

Degree Type


Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Barbara Schaal


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 glacial relicts. I test this hypothesis by combining ecophysiological and population genetic data in a new phylogeographic framework. The analysis shows that the match between traits and habitats is largely due to local gene flow and selection rather than migration and habitat sorting. Finally, through morphometrics, cytology, population genetics and greenhouse experiments, I show that dynamic polyploidy permits local movement of alleles between rare and widespread taxa. Overall, these results suggest that anthropogenic climate change may threaten biodiversity not by forcing impossible migrations, but by promoting hybridization and complicating taxonomy just as it has in the past.


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