Evidence for microbial local adaptation in nature.

Local adaptation is an outcome of divergent selection on microbial populations and has been linked to the immense genetic diversity of microbes observed in nature. Because it is difficult to study microbes in their natural habitats, most tests of microbial local adaptation have been performed in model laboratory systems; as a result, microbiologists have limited understanding of local adaptation among natural microbial populations. In this review, we summarize the evidence for microbial local adaptation in nature. Local adaptation has been most frequently studied, and most frequently found, in host-pathogen systems. We argue that more research is needed to understand the prevalence of local adaptation in free-living microbial populations. However, researchers will need to overcome a variety of logistical and conceptual challenges when studying natural microbial local adaptation, including a lack of solid understanding of many microbes' natural histories. We propose strategies to facilitate future natural history research on divergent selection. We also summarize laboratory experimental work investigating the ecological and evolutionary processes leading to local adaptation. Microbiologists' ongoing challenge is to integrate the valuable knowledge gained from laboratory experiments into well-designed field experiments. Such integration will help us understand the prevalence of, and circumstances leading to, local adaptation among microorganisms.