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Summary
Most people today recognize bacterial names like Escherichia coli and Neisseria meningitidis. Yet, from an evolutionary viewpoint, the clarity of species labels for bacteria is blurred by rampant horizontal gene transfer between bacteria (1). The forces driving speciation in bacteria include niche adaptation, selective sweeps, genetic drift, recombination of genetic material, and geographic isolation. How do those forces maintain species homogeneity or bring about lineages, when gene swapping is apparently so rife?