The effects of stabilising, directional and disruptive selection
Natural selection can change a population's traits in three ways. It can keep traits the same, shift them toward one extreme, or split the population into two distinct groups.
Real World
African black-bellied seedcrackers show disruptive selection: birds with very large or very small beaks crack hard or soft seeds efficiently, while medium-beaked birds struggle with both, so the population splits into two beak-size peaks.
Exam Focus
Sketch and label the before-and-after distribution curves for each type; diagrams earn marks that words alone may not.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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