Horizontal gene transfer (HGT) confers the rapid acquisition of novel traits and is pervasive throughout microbial evolution. Despite the central role of HGT, the evolutionary forces that drive the dynamics of HGT alleles in evolving populations are poorly understood. Here, we show that HGT alters the evolutionary dynamics of genetic variation, so that deleterious genetic variants, including antibiotic resistance genes, can establish in populations without selection. We evolve antibiotic-sensitive populations of the human pathogen Helicobacter pylori in an environment without antibiotic but with HGT from an antibiotic-resistant isolate of H. pylori. We find that HGT increases the rate of adaptation, with most horizontally transferred genetic variants establishing at a low frequency in the population. When challenged with antibiotic, this low-level variation potentiates adaptation, with HGT populations flourishing in conditions where nonpotentiated populations go extinct. By extending previous models of evolution under HGT, we evaluated the conditions for the establishment and spread of HGT-acquired alleles into recipient populations. We then used our model to estimate parameters of HGT and selection from our experimental evolution data. Together, our findings show how HGT can act as an evolutionary force that facilitates the spread of nonselected genetic variation and expands the adaptive potential of microbial populations.

水平基因转移（HGT）赋予了新特性的快速获取，并且在整个微生物进化过程中无处不在。尽管HGT发挥着核心作用，但对于进化种群中驱动HGT等位基因动力学的进化力知之甚少。在这里，我们表明HGT改变了遗传变异的进化动力学，因此有害的遗传变异（包括抗生素抗性基因）可以在未经选择的人群中建立。我们在没有抗生素但从幽门螺杆菌的抗药性菌株中分离出具有HGT的环境中进化人类致病菌幽门螺杆菌的抗生素敏感性种群。我们发现，HGT增加了适应率，大多数水平转移的遗传变异在人群中建立的频率较低。当受到抗生素攻击时，这种低水平的变异增强了适应能力，在未强化种群灭绝的情况下，HGT种群繁盛。通过扩展以前在HGT下的进化模型，我们评估了HGT获得的等位基因的建立和传播到受体群体中的条件。然后，我们使用我们的模型来估算HGT的参数，并从我们的实验进化数据中进行选择。在一起，我们的研究结果表明HGT如何作为一种进化力量，促进非选择遗传变异的传播并扩大微生物种群的适应潜力。