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Science 7–10 · Year 10

Sooty selection: camouflage, predation and allele frequencies over generations

Science understanding: Biological sciences

Practical, model not builtLow risk

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The idea

Predators remove the individuals they can see, so the colour that matches the background rises in frequency generation by generation, and the direction reverses when the background changes.

What you need

  • A bag of wild rice with light and dark grains (the Arizona State University Sooty Selection activity), or light and dark rice of the same size
  • Sheets of crumpled light and dark construction paper as two backgrounds
  • Forceps for the predator, a separate bag for captured grains (captured grains are not reused), a stopwatch, tally sheet

How to do it

  1. Spread 15 light and 15 dark grains over the light background.
  2. The predator, using forceps, removes one grain per peck for 10 seconds and puts each captured grain in the spare bag.
  3. Count survivors of each colour. Each survivor reproduces: count out the same number of each colour from the full supply bag, not from the captured grains, and add them so the population doubles; record the new percentages.
  4. Repeat the predation and reproduction for three generations.
  5. Repeat the whole run on the dark background.
  6. Plot the percentage of dark grains against generation for both backgrounds.

What you should see

On the light background the light grains rise in percentage over three generations and dark grains fall; on the dark background the reverse. The Arizona State activity has learners calculate exactly these percentage changes. The learner knows it worked when the two plots trend in opposite directions and when the predator's catch in generation 1 was biased toward the visible colour.

What changes

What you change
Background colour
What you measure
Percentage of dark grains after each generation
What you keep the same
  • Starting numbers (15 and 15)
  • Predation time (10 s)
  • One grain per peck
  • Reproduction rule (double each survivor count)

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • The grains change colour to match the background.
  • Once a colour is rare it must be harmful.
  • A recessive allele can be removed completely by selection; it lingers in heterozygotes, which is why the recessive curve flattens.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

  • Grains are not eaten; swept up at the end

Note

No hazards.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fa16e6f2bd
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fa9f532d80
  3. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  4. askabiologist.asu.edu/experiments/sooty-selection
  5. www.biointeractive.org/classroom-resources/making-fittest-natural-selection-and-adaptation
  6. www.biointeractive.org/classroom-resources/allele-and-phenotype-frequencies-rock-pocket-mouse-populations

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