This item is under embargo and not available online per the author's request. For access information, please visit

Date of Award

Summer 8-15-2021

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type



Background: Janzen’s physiological barrier hypothesis suggests that variation in the effectiveness of physiological barriers to dispersal underlies global patterns of speciation and biodiversity. He noted that because a species’ physiology often evolves to match the range of conditions it has experienced in evolutionary history, it is likely that lineages that occupy habitats with wider ranges of temperature variation will tend to be better at dispersing across thermal gradients and will therefore be less likely to speciate as a result of mountain barriers. Despite decades of research into different aspects of this fundamental hypothesis, its assumptions and predictions remain largely untested.

Methods: I tested the mechanistic assumptions of Janzen’s hypothesis through targeted experimental evolutions in Yeast (Saccharomyces cerevisiae) that evaluated the impact of constant and fluctuating environmental conditions on the evolution of physiological tolerance. Additionally, I tested the biogeographical predictions of this idea by assessing how global variation in the size of elevational barriers associated with recent speciation events in birds is related to local variation in temperature.

Results: Our experimental evolution tests generally support Janzen’s main assumption: yeast evolved in temporally variable conditions were able to persist under a wider range of conditions. Although such generalism typically traded off against peak performance, we note that on rare occasions, lineages evolved under fluctuating selection evolved similar or better performances than every constant selection lineage within the range of conditions in their treatments. Our biogeographic studies also supported Janzen’s expectations: elevational barriers were generally found to be higher along routes of least resistance connecting the ranges of avian sister species in environments with greater inter-annual variability in temperature.

Conclusions: Coupling biogeographical studies with experimental evolution under controlled conditions provided a powerful approach for testing the central mechanism and main predictions of Janzen’s Physiological Barrier Hypothesis. As Janzen assumed, our replicated experimental evolutions in Chapter 3 confirmed that fluctuating selection can indeed lead to wider physiological tolerances and is therefore likely to aid in dispersal across physiological gradients. Importantly, though, our observation that lineages exposed to fluctuating selection can, on rare occasions, evolve adaptations that allow them to outperform specialists across their entire physiological range demonstrates that frequently observed trade-offs between performance and flexibility are not universal. The rarity of this finding highlights the utility of having developed an experimental evolution system that enables high replication in Chapter 2. As for the predictions of Janzen’s hypothesis, Chapter 4 represents the first ever explicit demonstration that the barriers separating recent speciation events in birds are indeed higher in environments with greater inter-annual temperature variability. Overall, the work summarized in this thesis validates the assumptions and predictions of Janzen’s physiological barrier hypothesis and helps make a stronger case for this mechanism as a likely contributor to the generation of latitudinal gradients of diversity.


English (en)

Chair and Committee

Carlos A. Botero

Committee Members

Carlos A. Botero, Justin C. Fay, Michael J. Landis, Jonathan A. Myers,

Available for download on Thursday, July 06, 2023