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

Population evolution is the process by which gene frequencies change within a gene pool. Evolution is the theory that explains how populations of organisms change over time.  Evolution is a change in the inherited traits of a population over successive generations. It is not directed towards a goal, nor is it solely dependent on natural selection.  The theory of evolution explains the similarities and differences within life's organisms and processes. According to this theory, every organism shares a common ancestor.  The two major mechanisms that drive evolution are natural selection and... Show more

Population evolution is the process by which gene frequencies change within a gene pool. Evolution is the theory that explains how populations of organisms change over time. 

Evolution is a change in the inherited traits of a population over successive generations. It is not directed towards a goal, nor is it solely dependent on natural selection. 
The theory of evolution explains the similarities and differences within life's organisms and processes. According to this theory, every organism shares a common ancestor. 
The two major mechanisms that drive evolution are natural selection and genetic drift. 

Other mechanisms include:
Non-random mating
Allele (gene) flow
Mutation
Migration
Competition
Speciation 

Natural selection can only take place if there is variation, or differences, among individuals in a population. These differences must have some genetic basis. 
Genetic drift is the evolutionary mechanism that causes random fluctuation in allele frequencies across generations. 

It can be caused by:
Population bottlenecks: A sharp reduction in the size of a population
The founder effect: When a small portion of individuals leave a population 

Related Test: Basic Biology Practice Test: Darwin's Theories

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