Stressful environments can commonly be found at the edge of a species range and may be a driver for adaption in suboptimal environments. Furthermore, the edge of a species' range can expand and contract over time, resulting in multiple independent invasions of the same stressful habitat. Elucidating population genetic structure and demographic history can aid in determining the which geologic factors impact range distributions and when climatic changes occurred driving genetic patterns observed in contemporary populations. Moreover, populations at the edge of the species range may adapt to the stressful environments that occur at the range edge and exhibit genetic traits divergent from populations in the core of the species range. In this dissertation, I first examined how a stressor (salinity) has impacted genetic structure and demographic history in a wide-ranging, large semi-aquatic species, the American alligator (Alligator mississippiensis; Chapter 2). I estimated the splitting of genetic clusters and matched them with geologic events of past sea level rise. Then, I tested if coastal populations respond differently to changes in salinity compared to alligators from inland populations (Chapter 3). To do this I randomly placed juvenile alligators from coastal and inland populations in one of three salinities (0, 10, or 20 ppt) for two weeks. I collected behavioral, physiological, and histological datasets and found a habitat by salinity interaction with coastal alligators exhibiting a pattern of increased plasticity relative to inland alligators. In Chapter 4, I hypothesized that coastal and inland alligators would exhibit differentially expressed genes in osmoregulatory organs in response to salt stress. My data supported this hypothesis, and I found that the most differentially expressed genes functioned in signal transduction, metabolic pathways, and secretion. In addition, I found that at high salinities, coastal alligators upregulated genes coding for solute carriers compared to inland alligators. Overall, my dissertation contributed to the study of adaptive evolution by demonstrating that salinity has been a past and current stressor for American alligators. High salinity levels continue to limit the alligator's species range and lead to genetic differentiation among historically isolated regions. Yet, at the same time, I found evidence that coastal populations exhibit incipient adaptation to high salt environments. The patterns I found here are similar to other species that inhabit both freshwater and saltwater environments. As there appears to be evidence of convergent evolution for mechanisms to excrete salt in fully marine reptiles, my dissertation is starting to provide evidence for patterns of convergent evolution among reptiles that similarly use both freshwater and brackish water environments.


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Graduation Date





Hoffman, Eric


Doctor of Philosophy (Ph.D.)


College of Sciences



Degree Program

Conservation Biology; Integrative Biology


CFE0009887; DP0028420





Release Date

February 2024

Length of Campus-only Access


Access Status

Doctoral Dissertation (Open Access)