PLASTICITY-LED EVOLUTION IN NATURE

An enduring problem in evolutionary biology is to explain how novel, complex traits originate. A controversial solution to this problem is that environmentally induced phenotypic change––phenotypic plasticity––can play a leading role in the origins of novelty. This process of plasticity-led evolution (PLE) posits that a change in the environment triggers a change in development via plasticity, and this pre-existing plasticity is subsequently molded by selection into an adaptive trait. The significance of PLE is controversial because few tests have been conducted in natural populations. Here, I address that problem. Conceptually, I provide an annotated bibliography of significant works relating to phenotypic plasticity’s role in evolution. I then define the PLE hypothesis, identify its key criteria, produce a framework for evaluating the criteria in natural populations, and describe study systems that have begun to test the hypothesis. Next, I focus on studies of PLE in amphibians and identify mechanisms that are important to PLE. Together, these chapters (2-4) provide a foundation for assessing the broad relevance of PLE and enable tests of PLE in diverse systems. Empirically, I use my analytical framework mentioned above to test the key criteria of PLE in spadefoot toad (genera Spea and Scaphiopus) tadpoles (chapters 5-7). Tadpoles in the genus Spea have evolved a novel, complex feeding strategy (a carnivore/cannibal morph). Using a combination of morphological and molecular approaches in diverse lineages, I provide evidence in support of PLE’s key criteria. The carnivore morph appears to have begun as an environmentally-induced developmental variant that was subsequently refined into a coordinated plastic response. Some populations have secondarily lost this plastic response and are becoming exclusively well-adapted carnivores. Looking forward, chapter 8 provides a number of additional hypotheses and research directions for the PLE field. It describes predictions with respect to particular developmental features, identifies possible taxa and traits that could be used to test these predictions, notes gaps between theory and experimentation, and concludes with a list of specific future directions. In total, this dissertation not only provides empirical tests of a significant evolutionary question, but also provides a means for this question to be addressed in the field more broadly.Doctor of Philosoph