Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Zachary A. Cheviron

Commitee Members

Creagh Breuner, Douglas J. Emlen, Jeffrey M. Good, Andrew Whitehead


University of Montana

Subject Categories



Intraspecific patterns of geographic variation reflect a dynamic history of colonization and divergence in response to spatially varying selective pressures. Analysis of this phenotypic variation has long stimulated biological thought, yet many outstanding questions remain regarding the ecological and evolutionary mechanisms driving patterns of geographic variation. In my dissertation, I collected data on demographic history, physiological traits contributing to salinity tolerance, and acclimation responses to different salinities to elucidate the mechanisms shaping colonization and adaptive divergence in tidal marsh populations of Savannah Sparrows (Passerculus sandwichensis). Specifically, I addressed two main questions related to adaptive divergence in this species: (1) how do demographic and ecological forces interact to shape spatial patterns of local adaptation? And (2) what role does ancestral plasticity play in adaptation to new environments?

In Chapter 1, phylogeographic analyses of a population genomic dataset revealed that tidal marshes have been colonized twice by Savannah Sparrows and include a younger, less isolated tidal marsh lineage from the California coast, and an older, more isolated lineage from northwest Mexico. In the second chapter, I assessed how variation in demographic history and environmental factors interacted to shape patterns of divergence in physiological traits associated with salinity tolerance across multiple tidal marsh populations. Finally, in Chapter 3, I compared acclimation responses to salinity in freshwater-adapted and tidal marsh Savannah Sparrows to test whether ancestral plasticity may have contributed to tidal marsh adaptation in this species. The outcomes of adaptation to spatially varying selective pressures will be contingent on many interacting ecological, demographic, and evolutionary processes. The results of my dissertation show that demographic history, variation in selective pressures, and plasticity all contributed to patterns of adaptive divergence within tidal marsh Savannah Sparrows. This underscores the necessity of considering multiple interacting processes to thoroughly understand the evolution of geographic variation.

Included in

Biology Commons



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