Imagine a bacterium whose only mode of reproduction is binary fission (it splits itself into two). And then the process goes on and on. All the offspring are clones. You’d expect zero variation in their DNA, and hence in their appearance and function.
Pretty dull huh? Not only that, but this population would be doomed. With only one allele of every gene, ALL of them will either be resistant to certain antibiotics, or NONE will. So, antibiotic comes around, and there is a 50% probability the population will survive. Throw in 100 antibiotics and 10 different environmental pressures, and the population is good as dead.
Good thing for them that that’s not the case, that is, not all individual bacteria have identical DNA. How is that possible, since they reproduce by binary fission?? In fact, it is mind-opening just how many different ways they have of achieving just that: variation. Some are:
1. Conjugation. One bacterium produces a mating bridge through which a plasmid (circular piece of DNA) can pass to another bacterium.
2. Transformation. Bacteria can take up DNA from dead bacteria and use it as their own.
3. Transduction. Viruses which enter bacteria may pick up some of their DNA and pass it to the next bacteria they infect.
But none of these actually explain the CAUSE of DNA variation itself. By far the biggest cause of DNA variation in bacteria is, of course, mutation. One might say mutation is the inevitable effect of the very way DNA replicates, perhaps not a mistake, as it is often perceived, but as crucial a part of the overall process as the multitude of enzymes which take part. Just because it defies our (designer?) expectations of rigid rules that are never broken, doesn’t mean it is not integrated in nature as obviously as the pairing of DNA nucleotides is.