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WAHuman BiologyQuick questions
Unit 4: Human Variation and Evolution
Quick questions on Natural selection and types of selection: WACE Year 12 Human Biology Unit 4
4short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the conditions for natural selection?Show answer
Natural selection follows from four observations:
What is directional selection?Show answer
Directional selection favours one extreme of a trait, so the population mean shifts in that direction over generations. It happens when the environment changes or a new selection pressure appears, making one extreme advantageous. The classic example is antibiotic resistance: when an antibiotic is used, the resistant extreme is favoured and resistance increases. On a graph, the whole distribution moves toward the favoured extreme.
What is stabilising selection?Show answer
Stabilising selection favours the average phenotype and selects against both extremes, so variation is reduced and the population becomes more uniform around the mean. Human birth weight is the standard example: very small and very large babies historically had lower survival, so intermediate birth weights were favoured. On a graph, the distribution becomes taller and narrower around the same mean.
What is disruptive selection?Show answer
Disruptive selection favours both extremes of a trait and selects against the intermediate, so the population may split into two groups. It can occur when two different niches favour different extremes. Over time, if the two groups stop interbreeding, disruptive selection can contribute to speciation. On a graph, the distribution develops two peaks with a dip in the middle.
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