Directional selection, exemplified by antibiotic resistance in bacteria
Directional selection happens when one extreme version of a trait gives organisms a survival advantage. Over generations, that extreme trait becomes more common in the population.
Real World
MRSA (methicillin-resistant Staphylococcus aureus) emerged in hospitals after widespread antibiotic use killed susceptible bacteria, leaving only resistant mutants to reproduce — by the 2000s it was responsible for thousands of UK deaths annually.
Exam Focus
Always state that resistant bacteria survived because of a pre-existing mutation, not because they 'adapted' or 'became resistant' in response to the antibiotic.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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