Bacterial Resistance via Pre-Existing Mutation

Steps:

• Population exposed to penicillin kills susceptible bacteria, leaving resistant survivors.
• Resistant allele must pre-exist in population for natural selection to act.
• New alleles arise randomly via mutation before antibiotic exposure.
• Other mechanisms like directed evolution or post-exposure acquisition don't explain initial presence.

Why C is correct:

• Mutation (1 only) introduces novel alleles randomly, per evolutionary theory, allowing variation for selection without anticipating the antibiotic.

Why the others are wrong:

• A includes 2 and 3, which imply non-random or post-exposure changes, violating Darwinian evolution.
• B includes 3, likely a Lamarckian or environmental induction not supported by genetics.
• D excludes 1, ignoring mutation as the source of genetic variation.

Final answer: C