AP Biology · Free-response skill

Interpreting data: leaf hairs and a new beetle

The move. Read trait and genotype data, compute a change and an allele frequency, compare observed numbers with a no-evolution baseline, and explain the change by natural selection.

How to do it

  1. Read the passage first and note what changed in the environment, what was measured, and which facts link the trait to survival.
  2. To describe a change, subtract and give the start and end values with units.
  3. To test for evolution at a gene between two samples, compare its allele frequencies (or genotype counts): if they are the same, the gene shows no sign of change. Hardy-Weinberg numbers (p squared, 2pq and q squared, times the total) give the counts expected with no evolution, for comparison.
  4. To explain by natural selection, give each link: which variant survives or reproduces better in this environment and why, and that it passes the trait on, so the trait grows more common over generations.

Practice prompt Try it

In a fictional meadow, a wildflower varies in how many fine hairs cover its leaves, and hair density is strongly heritable. A leaf-eating beetle that had never lived there arrived and spread. The beetles fed far more on leaves with few hairs, and many sparsely haired plants were stripped bare and died before setting seed. Researchers measured a sample of 500 adult plants just before the beetle arrived and 500 adult plants grown from seed set after ten years of beetle feeding. They also counted genotypes at a separate gene, for flower color, that has no known effect on the beetles or on survival.

Average leaf hair density and flower-color genotypes in samples of 500 adult wildflowers

SampleAverage hairs per square millimeter of leafFlower gene: AAFlower gene: AaFlower gene: aa
Before the beetle arrived42590405
Grown from seed set after ten years of beetle feeding51590405

Use the passage and the table to respond to each part.

  1. Work out how much the average leaf hair density changed from the first sample to the second. Give the size and the direction of the change, with units.
  2. Using the flower-gene counts in the first sample, work out the frequency of allele A, showing each step.
  3. Using your answer to part b, work out how many Aa plants Hardy-Weinberg predicts in a sample of 500, showing each step. Then say whether the flower gene shows any sign of evolution between the two samples, and use the counts in the table to back up your answer.
  4. Using natural selection, say why the average leaf hair density changed.

Write each part, then check yourself against the checklist. Nothing you type here is saved or graded.

Check yourself and see a model answer

Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 6 points

Model answer, annotated

(a) The average hair density increased by 51 - 42 = 9 hairs per square millimeter, from 42 in the first sample to 51 in the second.

  • Point 1 Gives the direction (increase), the size (9 hairs per square millimeter) and the subtraction behind it.

(b) Each plant has 2 copies of the gene, so 500 plants have 1,000 copies. A copies = 2 x 5 (from AA) + 90 (from Aa) = 10 + 90 = 100. Frequency of A = 100 / 1,000 = 0.10.

  • Point 2 Counts gene copies, not plants, and writes out 100 divided by 1,000 to reach 0.10.

(c) With p = 0.1 for A and q = 1 - 0.1 = 0.9 for a, the expected share of Aa plants is 2pq = 2 x 0.1 x 0.9 = 0.18, and 0.18 x 500 = 90 Aa plants. The flower gene shows no sign of evolution: its counts are 5, 90 and 405 in both samples, so its allele frequencies did not change between them. The counts also match what Hardy-Weinberg predicts, 90 Aa plants.

  • Point 3 Shows 2pq and multiplies by 500 to get 90.
  • Point 4 Concludes no sign of evolution and backs it with the identical counts, and so unchanged allele frequencies, in the two samples.

(d) The beetles fed far more on leaves with few hairs, so many sparsely haired plants were stripped and died before setting seed, while hairier plants survived and set seed more often. Hair density is strongly heritable, so the hairy survivors passed dense hairs to their offspring, and the plants grown from their seed had a higher average hair density.

  • Point 5 Ties more leaf hairs to less beetle feeding and to better survival and seed set.
  • Point 6 Finishes the chain: the trait is passed on, so the next generation’s average is larger.