Life history evolution in seasonal environments: phenological and environmental determinants of thermal adaptation in Wyeomyia smithii Public Deposited

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  • March 21, 2019
  • Ragland, Gregory J.
    • Affiliation: College of Arts and Sciences, Department of Biology
  • Environmental variation poses a major evolutionary challenge to organisms. This is particularly true for seasonal environments where environmental factors fluctuate radically but predictably on an annual basis. Dormant life history stages often evolve to mitigate exposure to harsh seasonal environments (e.g., winter). In addition, norms of reaction, or the relationship between phenotype and environment, often evolve as a response to local environmental heterogeneity. My thesis explores how the seasonal timing of dormancy affects selection on reaction norms of active, non-dormant life history stages in temperate insects. Changing the dates of initiation and termination of winter dormancy changes the thermal habitat experienced during active growth and reproduction. Thus, geographic variation in the timing of dormancy complicates geographic patterns of thermal selection on active life history stages. Using available inter- and intraspecific life history data in conjunction with long-term weather data, I show that geographic clines in dormancy timing cause populations along the cline to experience similar exposure to cold temperatures during active growth. As a result, strong latitudinal trends in the timing of dormancy predict weaker latitudinal trends in thermal adaptation of active stages. I further illustrate this concept by examining geographic variation in the timing of winter dormancy, thermal sensitivity of development, and tolerance to thermal stress in the pitcher plant mosquito, Wyeomyia smithii. The results from W. smithii suggest that selection applied specifically by the thermal environment of the growing season best explains geographic variation in the thermal sensitivity of development time. In contrast, geographic variation in the thermal environment of the entire year best explains geographic trends in thermal stress tolerance of active life history stages. These results suggest two major conclusions. First, thermal sensitivity and thermal tolerance can exhibit local adaptation in populations that also demonstrate local adaptation in diapause timing. Thus the evolution of one type of adaptation does not preclude the other. Second, dormancy timing unquestionably influences direct selection on active life history stages, but correlated selection on overwinter survival may strongly influence temperature tolerance of active life history stages.
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  • Kingsolver, Joel
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