The restriction enzymes of bacteria protect the bacteria from successful attack by bacteriophages, whose genomes can be degraded by the restriction enzymes. The bacterial genomes are not vulnerable to these restriction enzymes because bacterial DNA is methylated. This situation selects for bacteriophages whose genomes are also methylated. As new strains of resistant bacteriophages become more prevalent, this in turn selects for bacteria whose genomes are not methylated and whose restriction enzymes instead degrade methylated DNA. The outcome of the conflict between bacteria and bacteriophage at any point in time results from

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The conflict between bacteria and bacteriophages is an example of frequency-dependent selection. The prevalence of one strategy (methylated or unmethylated DNA) is dependent on the frequency of the other strategy in the population. In this case, as more bacteriophages with methylated DNA become prevalent, bacteria with unmethylated DNA are selectively favored. This creates a cyclical pattern of selection.

The Restriction Enzymes of Bacteria Protect the Bacteria from Successful