This set covers the whole of variation and selection: continuous and discontinuous variation, sources of genetic variation, natural selection, the difference between selection and choosing to adapt, and careful reading of distributions. The questions run from easy to harder. All organisms and data are invented.
Work on paper, with a pencil and a ruler ready. Open the answer only after you have written your own. Use the inheritance model board and the probability tree and counting board to test simple models, and the mistake log to record errors you repeat.
Questions
Q1 (easy). Classify each feature as continuous or discontinuous variation: (a) shell length of fictional snails in millimetres; (b) number of stripes on a fictional fish, which is 0, 1, 2 or 3; (c) fur colour, either black or white.
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(a) Continuous: length is measured and can take any value in a range. (b) Discontinuous: stripes are counted in whole numbers with nothing in between. (c) Discontinuous: two separate categories.
Q2 (easy). State which type of graph suits (a) and (c) in Q1, and why.
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(a) A histogram or line graph, because the data are continuous and the classes join on to each other. (c) A bar chart with gaps, because the categories are separate.
Q3 (easy). A sample of 40 fictional seedlings is measured.
| Height (cm) | Frequency |
|---|---|
| 10 to under 15 | 5 |
| 15 to under 20 | 11 |
| 20 to under 25 | 14 |
| 25 to under 30 | 8 |
| 30 to under 35 | 2 |
Check the total, give the modal class, and find the percentage of seedlings from 15 to under 25 cm.
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Total: 5 + 11 + 14 + 8 + 2 = 40, so the table is complete. The modal class is the largest frequency, 20 to under 25 cm (14 seedlings). Seedlings from 15 to under 25 cm: 11 + 14 = 25, and 25/40 = 0.625, so 62.5%.
Q4 (medium). Two identical cuttings from the same fictional plant are grown, one on rich soil and one on poor soil. After a month the first is taller. Explain why this is not genetic variation.
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Cuttings come from one parent by asexual reproduction, so they have the same alleles. The difference must come from the environment, here the nutrients in the soil.
Q5 (medium). A fictional organism has 5 pairs of chromosomes. Using the simple model of independent chromosome pairs, how many different gametes can one parent produce? Name the source of variation this illustrates.
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Each pair gives 2 choices, so 2⁵ = 2 × 2 × 2 × 2 × 2 = 32 gametes. This illustrates meiosis, which makes genetically different gametes. Check: 2⁴ = 16 and 16 × 2 = 32.
Q6 (medium). Explain the difference between mutation and random fertilisation as sources of genetic variation.
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Mutation is a random change in genetic material that can create a new allele. Random fertilisation is chance pairing of gametes, which makes new combinations of existing alleles. Only mutation produces new alleles.
Q7 (medium). A lake holds 500 fictional fish, 300 silver and 200 gold. A bird hunts by sight, and gold fish are easier to see. The survival rate is 40% for silver fish and 10% for gold fish. Find the proportion of survivors that are gold, to 1 decimal place, and compare it with the proportion before.
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Silver survivors: 300 × 0.40 = 120. Gold survivors: 200 × 0.10 = 20. Total survivors: 140. Gold share after: 20/140 = 0.142857, so 14.3%. Before, it was 200/500 = 40%. Selection reduced the proportion of gold fish. Check: 14.3% of 140 is about 20.
Q8 (harder). A culture of 10 000 bacteria contains 4 that carry a resistance allele. An antibiotic kills every non-resistant cell. Explain, in steps, why most bacteria in later generations are resistant.
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- The bacteria vary: 4 carry an allele for resistance, which arose by mutation.
- The antibiotic acts as a selection pressure and kills all non-resistant cells.
- The 4 resistant cells survive and reproduce, passing on the allele.
- Each new cell from them carries the allele, so the proportion of resistant bacteria rises to almost the whole population.
The antibiotic did not cause the resistance. It selected cells that already had it.
Q9 (harder). A student writes: “The fictional moths turned dark because they needed to hide from predators.” Identify the error and write a corrected explanation.
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The error is that need does not cause a change, and it treats one moth changing colour as if it changed the population. Corrected: “Moths varied in colour because of different alleles. Predators found pale moths more easily on dark bark, so dark moths survived and reproduced more. They passed on the allele for dark colour, so its proportion rose over generations.”
Q10 (harder). Two fictional fern populations are sampled. Sample X has frond lengths of 24, 26, 28, 30, 32 cm. Sample Y has 28, 30, 32, 34, 36 cm. Calculate both means and the difference. A student says every fern in Y is longer than every fern in X. Evaluate the claim.
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X mean: (24 + 26 + 28 + 30 + 32) ÷ 5 = 140 ÷ 5 = 28 cm. Y mean: (28 + 30 + 32 + 34 + 36) ÷ 5 = 160 ÷ 5 = 32 cm. The difference is 4 cm. The claim is wrong: the ranges overlap from 28 to 32 cm, and the 32 cm fern in X is longer than the 28 cm and 30 cm ferns in Y. A difference in means describes the groups, not every individual, and the small samples limit what can be concluded.
If you got these wrong
| What went wrong | Where to go |
|---|---|
| Mixed up continuous and discontinuous, or chose the wrong graph (Q1 to Q3) | Distinguish continuous and discontinuous variation |
| Named the wrong source of variation, or blamed genes for an environmental difference (Q4 to Q6) | Explain a source of genetic variation |
| Lost marks on the percentage or the order of the selection chain (Q7, Q8) | Trace natural selection across generations |
| Used “needed” or “wanted” as a cause (Q9) | Compare selection with an organism choosing to adapt |
| Claimed too much from group means (Q10) | Interpret a distribution without biological stereotypes |
Mark yourself on the method as well as the answer. If two or more questions from one row went wrong, reread that lesson’s worked example before retrying.
What next?
If this set felt steady, move on to ecology and energy flow, or return to the Biology learning guide for the full module list. If the same row keeps catching you, a teacher in online one-to-one Biology tuition can work through your own answers and find the pattern behind them.