Question

appearance of antibiotic resistant Bacteria is an example of

Answer 1

could be due to the introduction of an antibiotic resistance plasmid

Answer 2

The correct answer is Pre-existing variation

It can be defined as a number of bacterial colonies are exposed to a particular antibiotics, those which are sensitive to the antibiotic will die whereas those with mutation developed in them survive the antibiotics. And eventually this resistance provided gene spreads all over and the entire population becomes resistant.