MOVING TOWARD AN OPTIMUM: THE ADAPTATION GENETICS OF ARABIDOPSIS THALIANA.
Stearns, Frank Warren
Fenster, Charles B
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Adaptation accounts for many of the interesting characteristics of biodiversity. Despite this, the genetic mechanisms underlying the process of adaptation in nature are largely unknown. While general principles are emerging, important questions remain. Although experimental evidence has corroborated theoretical predictions, very few studies have tested macroorganisms in nature, where adaptation is most relevant. My dissertation addresses several important questions in adaptation genetics in the context of fitness landscapes, primarily using the model plant Arabidopsis thaliana. Fitness landscapes are used to visualize the relationship between genetics and fitness (evolutionary success of an individual). Although fitness landscapes have been considered metaphorical, recent work (and this dissertation) suggests they may approximate reality, providing testable predictions. I first assess pleiotropy (when one gene has multiple effects), an important component of fitness landscape models. I examine this concept in historical context and suggest future directions for research. Next I evaluate how well genetic relatedness corresponds to climate adaptation across the native range of A. thaliana and find support for parallel evolution (identical but independent genetic changes), suggesting that fitness landscapes are complex. In my next chapter, using a combination of natural and artificial conditions, I examine how induced mutations impact traits that are fitness indicators as compared to general traits. I find that new mutations tend to reduce fitness, whereas their effect on general traits is bidirectional. This result is more pronounced under stressful field conditions. Finally, I evaluate a mathematical model of adaptation in the field using induced mutations in A. thaliana. I find support for a previous result from laboratory studies - that lineages that are less well adapted to an environment are more likely to benefit from new mutations whereas lineages that are well adapted are more likely to be disrupted by new mutations - and extend that to the wild. Throughout I explore the importance of contingency in evolution, sometimes underscoring how it leads to unpredictable adaptation (chapters one and two), yet also demonstrating that the actions of mutations can be fit to simplifying assumptions (chapters three and four). These studies therefore significantly contribute to the emerging scholarship on adaptation genetics.