Bacterial genomes differ dramatically in size: from 140Kb to 13Mb (those numbers might be off now...please let me know if something has broken the record. Yes, I know the lower estimate can change based on semantics, but there are a bunch in that range). Although we have some clues as to how selection acts (or fails to act) on genome size, outside of intracellular parasites it's a bit of a mystery how selection shapes total genomic content. Perhaps the most interesting case out there involves genome streamlining in marine bacteria, which has been attributed to selection but which remains a just so story to this point.
My Chrobial Romance blog
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