Population size and the limits of natural selection

进化生物学家主要关注理解和预测种群如何随着时间的推移而变化。广泛的种群特征和过程可以影响进化反应，包括遗传学、环境变异和迁徙。根据这一建议，我们将研究种群规模对进化速度和方向的影响：种群规模对于决定变异(即突变)对种群的输入以及突变出现后的命运都是至关重要的。*我们将检验关于种群规模对进化速度和模式的影响的重要预测。理论预测，小种群的进化应该比大种群的进化带来更多不同的结果。此外，在大种群中，个体之间的基因交流预计会增加适应的速度。我们将在实验室通过实验进化大肠杆菌种群来验证这些预测。使用细菌进行这些实验将使我们能够直接控制种群规模，并跟踪许多复制种群的进化。我们将能够通过测量进化种群的适合度来跟踪进化，并通过对它们的基因组进行测序来确定进化的遗传基础。对已测序的细菌基因组的计算分析将使我们能够测试额外的理论预测。基因交换水平低的基因组区域预计比交换水平高的区域包含的遗传变异更少。这一预测之前曾在动物身上进行过测试，但没有在细菌中进行过测试。*我们拟议的工作将解决我们在理解种群规模和进化过程之间的关系方面的关键差距。除了与进化理论的相关性外，我们的工作还可以为其他被认为是重要的人口过程的进化情景提供信息，例如病原体进化、疾病传播和范围边缘的进化。

Evolutionary biologists are centrally concerned with understanding and predicting how populations change over time. A wide range of population features and processes can influence evolutionary responses, including genetics, environmental variation, and migration. Under this proposal, we will investigate the influence of population size on the speed and direction of evolution: population size is crucial in determining both the input of variation (that is, mutations) into a population, and in the fate of mutations once they have arisen.******We will test important predictions concerning the influence of population size on rates and patterns of evolution. Theory predicts that evolution in small populations should lead to more varied outcomes than in large populations. Moreover, in large populations, genetic exchange between individuals is expected to increase the rate of adaptation. We will test these predictions by experimentally evolving populations of the bacterium Escherichia coli in the laboratory. Use of bacteria for these experiments will allow us to directly control population size and to follow evolution in many replicate populations. We will be able to track evolution both by measuring fitness of the evolved populations, and by sequencing their genomes so as to identify the genetic basis of evolution. Computational analyses of sequenced bacterial genomes will allow us to test additional theoretical predictions. Regions of the genome with low levels of genetic exchange are expected to harbor less genetic variation than regions with high levels of exchange. This prediction has previously been tested in animals, but not in bacteria. ******Our proposed work will address key gaps in our understanding of the relationship between population size and the evolutionary process. Beyond its relevance to evolutionary theory, our work can inform other evolutionary scenarios where demographic processes are thought to be important, such as pathogen evolution, disease transmission and evolution at range margins.