In a polluted aquatic environment, bacteria are chronically exposed to low concentrations of multiple antibiotics released from pharmaceutical effluents. Over time, resistant colonies appear even when the antibiotic concentration remains constant. To determine whether resistance arises through induced mutation (a physiological response to antibiotics) or random mutation followed by selection, an ecologist performs a fluctuation test similar to that of Luria and Delbrück (1943) using replicate bacterial cultures.
Which one of the following outcomes would best support the conclusion that antibiotic resistance arises by random mutation followed by selection, rather than by induction?

Correct option is B

Correct Answer: (b)
Explanation:
The Luria–Delbrück fluctuation test showed that if mutations arise randomly before selection, then replicate cultures will show large variation (fluctuation) in the number of resistant colonies. Some cultures acquire mutations early and produce many resistant descendants, while others acquire them late or not at all.
Information Booster :
· Random mutation leads to high variance in mutant numbers across identical cultures.
· Induced mutation would produce similar numbers of resistant colonies in each culture.
· Antibiotics act as selective agents, not mutagens.
· The fluctuation test was key evidence for the Darwinian model of mutation and selection.
Additional Information (Incorrect Options):
(a): Suggests induced mutation, not random mutation.
(c): Implies resistance is caused by antibiotic exposure itself.
(d): Predictable increases are inconsistent with random, pre-existing mutations.