In microorganisms, specifically free-living bacteria and archaea, the equivalent of the biological species concept does not exist, creating several barriers to the study of the fine-scale genetic structure of microbial populations and thus, the processes contributing to microbial diversification. More often than not, studies attempting to delineate microbial populations overestimate traditionally-defined eukaryotic populations, or groups of individuals with the potential for contemporary interactions and exchange of genetic material. Here, we investigate the abundant leaf litter bacterium, Curtobacterium, and asked whether population structure in a free-living soil bacterium was consistent with patterns of allopatric or sympatric speciation. By investigating the fine-scale genetic structure within a single bacterial ecotype, we concluded that the distributional patterns of the populations suggest that both isolation-by-distance and isolation-by-environment contribute to Curtobacterium population structure. FIG 2. Recombination network across all pairwise strains. Thicker edges represent increased recombination between strains. Nodes are colored by population designation and node size indicates number of clonal clusters (strains too closely-related to differentiate recombination). D = Desert, Sc = Scrubland, G/MMLR = Grassland, SS = Salton Sea, MCBA = Boston