The shift shown from the dashed (before) to the solid (after) curve is most consistent with which mode of selection?

Answer: Directional selection The whole distribution shifted to one extreme (right) without splitting or narrowing, which is the signature of directional selection favoring one tail of the trait distribution.

The graph shows reduction of variance with the mean unchanged. Which selection mode is operating?

Answer: Stabilizing selection When extremes are selected against and the mean is preserved, variance decreases, producing a narrower curve centered on the same mean - the hallmark of stabilizing selection.

What mode of selection produces the bimodal post-selection (solid) curve from a unimodal pre-selection (dashed) curve?

Answer: Disruptive selection Disruptive selection favors both extremes and disfavors intermediates, splitting a single peak into two and potentially driving sympatric speciation.

AP Biology · Topic 7.12

Variations in Populations Practice

Part of Natural Selection.(EVO-1.L)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$The graph shows the distribution of a continuous trait in a population before (dashed) and after (solid) several generations of selection.$
$The shift shown from the dashed (before) to the solid (after) curve is most consistent with which mode of selection?$
- A
  $Directional selection$
  check_circle
- B
  $Disruptive selection$
- C
  $Stabilizing selection$
- D
  $Frequency-dependent selection$
Why
$The whole distribution shifted to one extreme (right) without splitting or narrowing, which is the signature of directional selection favoring one tail of the trait distribution.$
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Sample 2difficulty 1/5
$The phenotype distribution narrows around the same mean across many generations.$
$The graph shows reduction of variance with the mean unchanged. Which selection mode is operating?$
- A
  $Sexual selection$
- B
  $Directional selection$
- C
  $Stabilizing selection$
  check_circle
- D
  $Disruptive selection$
Why
$When extremes are selected against and the mean is preserved, variance decreases, producing a narrower curve centered on the same mean - the hallmark of stabilizing selection.$
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Sample 3difficulty 1/5
$Beak-size variation in a population shifts from a single peak (dashed) to two peaks (solid) after selection.$
$What mode of selection produces the bimodal post-selection (solid) curve from a unimodal pre-selection (dashed) curve?$
- A
  $Directional selection$
- B
  $Disruptive selection$
  check_circle
- C
  $Genetic drift$
- D
  $Stabilizing selection$
Why
$Disruptive selection favors both extremes and disfavors intermediates, splitting a single peak into two and potentially driving sympatric speciation.$
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Sample 4difficulty 2/5
$A population stabilizes p near 0.5 over time, as in heterozygote-advantage scenarios (e.g., sickle-cell HbS in malaria zones). This pattern is$
- A
  $Directional selection toward fixation$
- B
  $Bottleneck effect$
- C
  $Balancing selection maintaining polymorphism$
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- D
  $Genetic drift to fixation$
Why
$Balancing selection (such as heterozygote advantage) maintains both alleles at intermediate frequencies because heterozygotes have higher fitness, preventing fixation of either allele.$
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Sample 5difficulty 2/5
$Frequency of the dark (carbonaria) allele in peppered moths in Britain over time.$
$What evolutionary explanation fits this pattern?$
- A
  $Random drift caused the rise and fall by coincidence$
- B
  $Mutation rate changed in response to industrial pollution$
- C
  $Pollution darkened tree bark, favoring dark moths via directional selection; cleaner air reversed the selection$
  check_circle
- D
  $Hardy-Weinberg equilibrium was maintained throughout$
Why
$Soot-darkened bark reduced predation on dark moths, increasing dark allele frequency. The Clean Air Act lightened bark, reversing the selective advantage and lowering dark allele frequency.$
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