Scientists grow bacteria on a plate without antibiotic. They transfer samples from each colony to the same positions on two plates containing antibiotic X. Colonies grow at the same few positions on both plates. Each surviving colony remains resistant after several generations without antibiotic.
Which explanation best accounts for both the matching positions and persistent resistance?
Antibiotic X caused matching mutations after transfer; resistance then persisted because all mutations are advantageous.
Resistant variants were present before transfer; their descendants inherited resistance even when antibiotic X was absent.
Antibiotic X selected variants after transfer; resistance then persisted because exposure was needed in every generation.
Resistant variants appeared independently after transfer; their matching positions show that mutation followed bacterial need.
40 exam-style questions on AQA GCSE Biology 6.3 The development of understanding of genetics and evolution, covering 6.3.1 Theory of evolution, 6.3.2 Speciation, 6.3.3 The understanding of genetics, 6.3.4 Evidence for evolution, 6.3.5 Fossils, 6.3.6 Extinction, and 6.3.7 Resistant bacteria. Each one has a worked solution and a mark scheme showing where the marks go.