Selection on Quantitative Traits and Predicting the Response · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Predicting a response to selection

Science · Genetics & Evolution · ages 19-20
Name ______________________   Date ____________

  1. Use R = h²S. The selection differential is 10 units and narrow-sense heritability is 0.5. Enter the predicted response in units.

    Answer: ______________

  2. Use R = h²S, then add R to the starting mean. The starting mean is 20 cm, S = 4 cm and h² = 0.5. What next-generation mean is predicted?

    • 24 cm
    • 22 cm
    • 2 cm

  3. Selected parents are taller on average than the starting population. Narrow-sense heritability is positive and the prediction assumptions hold. Which prediction is supported?

    • The mean should rise; the spread may change
    • The mean should stay fixed; the spread must shrink
    • The mean should fall; the spread must stay fixed

  4. To calculate the selection differential S, which mean do you subtract from the selected parents' mean?

    • The selected parents' mean after breeding
    • The offspring mean in the next generation
    • The population mean before selection

  5. In a symmetric trait distribution, equal selection against both extremes favours intermediate values and tends to reduce the spread.

    Circle one:   True   False

  6. Use R = h²S. A trait has S = 20 units and h² = 0.01. What response is predicted?

    • 0.2 units of change
    • 20.01 units of change
    • 2000 units of change

  7. Short and tall plants each leave an average of 6 offspring. Intermediate plants leave an average of 2. Which selection mode does this pattern show?

    • Directional selection favouring only the taller extreme
    • Stabilising selection favouring intermediate trait values
    • Disruptive selection favouring both trait extremes

  8. A population's mean stops rising although taller parents are still chosen. No variance or environmental measurements are available. Which conclusion is justified?

    • All genetic variation has disappeared from the population
    • Several causes are possible; more evidence is needed
    • Environmental conditions alone must explain the entire plateau
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Answer key

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

Predicting a response to selection W1-mt_M-czTLDmfl-s1

  1. 5 · R = 0.5 × 10 = 5 units. This is the predicted change in the mean.
  2. 22 cm · R = 0.5 × 4 = 2 cm. Add this response to the starting mean: 20 + 2 = 22 cm.
  3. The mean should rise; the spread may change · The higher selected-parent mean gives positive S. With positive h², R = h²S is positive. The equation predicts a rise in mean but does not determine the spread.
  4. The population mean before selection · S = selected-parent mean minus population mean before selection.
  5. True · This is stabilising selection. Intermediate values have greater reproductive success, so the distribution tends to narrow.
  6. 0.2 units of change · R = 0.01 × 20 = 0.2 units. Calculate the product to find the response.
  7. Disruptive selection favouring both trait extremes · Both extremes have greater reproductive success than intermediate plants. That defines disruptive selection.
  8. Several causes are possible; more evidence is needed · Reduced additive variance, opposing selection and changed conditions are possible explanations. The plateau alone does not distinguish them.
Worksheet · LightMySky