Question

Sol.(c) [Key Concept: Antibiotic Resistance — Selection of Pre-existing Variants] Appearance of antibiotic-resistant bacteria is an example of pre-existing variation in the population. When a bacterial population encounters a particular antibiotic, those bacteria that are naturally (already) sensitive die. The few that happen to have pre-existing resistance survive and reproduce. This is natural selection acting on pre-existing genetic variation — not new mutations induced by the antibiotic.