Genetic Drift and Effective Population Size · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Sampling error that evolves

Science · Genetics & Evolution · ages 18-19
Name ______________________   Date ____________

  1. Drift pushes allele frequencies steadily toward higher fitness.

    Circle one:   True   False

  2. What causes genetic drift?

    • A weather event selects only the strongest individuals
    • Random sampling luck
    • A mutation aims the population toward fitness

  3. A neutral allele in a small group over many generations may do what?

    • Stay at exactly one half in every generation
    • Become harmful immediately
    • Fix or vanish by chance

  4. A random crash leaves five survivors whose alleles shape the future. What is this?

    • Natural selection always changes the most fit survivors
    • A bottleneck effect
    • Gene flow brings migrants between populations

  5. A herd of 1000 has one male fathering most offspring. What follows?

    • Drift stops entirely because the herd has many individuals
    • Effective size is much smaller than 1000
    • Selection becomes twice as strong

  6. How does drift differ from selection?

    • Drift acts faster in large populations with more individuals
    • Selection changes frequencies by pure random chance
    • Drift is directionless; selection tracks fitness

  7. A population has 200 females and 10 males. Using Ne = 4 × males × females ÷ (males + females), about what is its effective size?

    • About 38
    • About 105
    • About 210

  8. Two groups each count 500. One has even family sizes, and the other has wildly uneven ones. Which loses variation faster?

    • The group with even breeding, because equal families create more drift
    • Both groups lose variation at exactly the same rate
    • The group with wild family-size swings
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Sampling error that evolves W1-mt_lbB1GbKv_t-s1

  1. False · Drift has no preferred direction; that push belongs to selection.
  2. Random sampling luck · Drift is the wobble of random sampling, not adaptation.
  3. Fix or vanish by chance · With no selection, luck alone can fix or erase a neutral allele.
  4. A bottleneck effect · A crash that resamples the gene pool is a bottleneck.
  5. Effective size is much smaller than 1000 · Few gene contributors shrink the genetic size below the headcount.
  6. Drift is directionless; selection tracks fitness · Drift comes from random sampling, while selection steadily favors higher fitness.
  7. About 38 · Ne = 4 × 10 × 200 ÷ (10 + 200), which is 8000 ÷ 210, or about 38. That is far below the census of 210, so drift acts as it would in a much smaller population.
  8. The group with wild family-size swings · Wild family-size swings make genetic contributions uneven, which lowers effective size and speeds drift.
Worksheet · LightMySky