Appearance of antibiotic-resistant bacteria is an example of pre-existing variation in the population. When a bacterial population encounters a particular antibiotic, those sensitive to it die. But some bacteria having mutations become resistant to the antibiotic. Soon, the resistance providing genes become widespread and the entire population becomes resistant. It is not due to adaptive radiation because adaptive radiation is development of different functional structures from a common ancestral form, which is also known as divergent evolution. Transduction is a process whereby, foreign DNA is introduced into another cell via a viral vector.