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Adaptive diversification in experimental populations of Escherichia coli Tyerman, Jabus

Abstract

What processes contribute to the origin and maintenance of biological diversity? When populations occupy different environments, that have divergent ecological characteristics, natural selection can cause each population to adapt to its environment, resulting in phenotypic divergence between populations. But can natural selection cause a single population to diverge? Adaptive dynamics theory predicts that ecological interactions between individuals in a population can result in negative frequency-dependent selection, and the branching of the population into phenotypically distinct subpopulations. Here, I tested predictions from adaptive dynamics theory. To do this, I experimentally evolved populations of the clonal bacterium Escherichia coli in the lab. First, using replicate populations of E. coli that had diversified in parallel, I tested whether convergent ecotypes among replicate populations competed in a likewise manner. I found they do not, suggesting that the genetic underpinnings of the convergent ecotypes among populations were different. The ecological interaction that has received the most attention from adaptive dynamics theory is resource competition, yet there are few direct tests that competition for resources does cause phenotypic distributions to evolve. I used diversified populations of bacteria, and experimentally demonstrated that competition can cause ecological character displacement. Subsequently, I explored whether mutational bias for novelty can regulate adaptive radiation when ecological opportunities and selection were similar. I found an asymmetry in the extent and range of diversification among replicate populations initiated from different genotypes, suggesting that mutational bias can regulate adaptive diversification. Finally, I tested for resource specialization trade-offs by measuring the extent of variation in populations evolved in environments that range in complexity from one to three resources and found that diversity was not related to environmental complexity. Additionally, I explored the role of growth rate vs. yield trade-offs within and among populations that have diversified, and found little support for the hypothesis that these trade-offs underlie adaptive diversification in our bacterial populations. Together, these chapters test the predictions of adaptive diversification, so that we may better understand adaptive diversification in nature.

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