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Flip to reveal answersIn natural selection, where does the variation come from — before or after the selection pressure?
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All 7 Flashcards — Natural selection in action
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Question
In natural selection, where does the variation come from — before or after the selection pressure?
Answer
**Before.** Variation (often from a **random mutation**) already exists; the pressure only **selects** which variants survive — it never **creates** the trait.
Question
Outline how bacteria become resistant to an antibiotic.
Answer
A **random mutation** makes a resistant variant; the antibiotic **kills the susceptible** bacteria; the **resistant survive and reproduce** (incl. via **plasmids**); so the **resistance allele becomes common**.
Question
Why do resistant weeds increase as herbicide use rises?
Answer
The herbicide is a **selection pressure** — it kills non-resistant weeds, so the rare **resistant** variant survives and reproduces, and its frequency **rises as spraying continues**.
Question
What is heterozygote advantage?
Answer
When the **heterozygote** has **higher fitness** than either homozygote, so **both alleles are kept** in the population (balancing selection).
Question
Explain why the sickle-cell allele persists in malaria regions.
Answer
**Carriers** (heterozygotes) are **more resistant to malaria** and avoid severe anaemia, so they are the **fittest** and reproduce most — keeping the sickle allele at **moderate frequency**.
Question
What did Endler's guppy experiments demonstrate?
Answer
Natural selection in **real time**: **predation** favours dull, camouflaged males while **sexual selection** favours bright males, so colouration **shifts in a few generations**.
Question
State the four steps common to every case of natural selection in action.
Answer
**Variation** → a **selection pressure** removes some variants → favoured variants **survive and reproduce** → the helpful **allele becomes more common** over generations.
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Natural selection
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