History Leaves Its Mark on Soil Bacterial Diversity

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History Leaves Its Mark on Soil Bacterial Diversity Jennifer B. H. Martiny Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, California, USA ABSTRACT Dispersal is closely tied to the origin and maintenance of microbial diversity. With its focus on a narrow group of soil bacteria, recent work by Andam and colleagues on Streptomyces has provided perhaps the strongest support so far that some bacterial diversity in soils can be attributed to regional endemism (C. P. Andam et al., mBio 7:e02200-15, 2016, ). This means that dispersal is limited enough to allow for evolutionary diversi- fication. Further analyses suggest that signatures of climate conditions more than 10,000 years ago can be detected in contempo- rary populations of this genus. These legacies have implications for how future climate change might alter soil microbial diversity. A long-held tenet in microbiology is that “everything is every- where” (1). Indeed, many microbial taxa, often defined by 97% similarity of rRNA sequences, are widely distributed across the planet. The same taxa can be found on different continents, in separate ocean basins, and at both poles. These examples highlight the large geographic ranges of many microbial taxa and have been used as support for the idea that microbes are not dispersal lim- ited. One implication of unlimited dispersal, or the complete mix- ing of microbial populations, is that any influence of past condi- tions on microbial diversity would be immediately erased by contemporary conditions. Dispersal limitation is key to understanding the origin and maintenance of biodiversity—microbial and otherwise. In gen- eral, restricted dispersal between populations contributes to evo- lutionary diversification. If gene flow is restricted, then chance events within populations can contribute to population diver- gence. These events include unique mutations, the random order in which mutations arise, and genetic drift (2). Even the outcome of local adaptation to similar environments might differ between populations, because selection acts on these chance events. In this
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