Answer:
Mutation.
Explanation:
Mutation may be defined as the sudden, heritable change in the individual that might affect the genotype and phenotype of the organisms. The mutation can be somatic mutation or germ line mutation.
The two population of bacteria are same but after sometime the mutation might occur in the bacteria that makes their genetic material different. These mutations can make the bacteria strain resistant against the particular chemical and increase their chance of survival in the harsh environment.
Thus, the correct answer is mutation.
