Stabilising selection, exemplified by human birth weights
Stabilising selection is a type of natural selection that favours average individuals. It removes extreme traits from a population, keeping most individuals close to the middle of the range.
Real World
UK birth records consistently show that babies weighing between 3–4 kg have the lowest mortality rates; those significantly below (premature) or above (delivery complications) that range face higher risks — a pattern that has barely shifted despite modern medicine.
Exam Focus
Explain why both extremes are selected against, not just one; examiners award marks for addressing low and high birth weight separately.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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