Biology • Year 12 • Module 5 • Lesson 1
Reproduction and Continuity of Species
Apply sexual vs asexual reproduction to real population data, real pathogen scenarios, and a diagram critique.
1. Interpret population-recovery data after a pathogen outbreak
A research team modelled population sizes of two plant populations of the same crop species over five growing seasons. Population X reproduces almost entirely by vegetative propagation (asexual); Population Y reproduces almost entirely by sexual outcrossing. At the start of Season 2, a new fungal pathogen is introduced to both populations. 7 marks
| Season | Pop. X — clonal (plants) | Pop. Y — sexual (plants) | Event |
|---|---|---|---|
| 1 | 1000 | 1000 | Baseline, no pathogen |
| 2 | 180 | 620 | Pathogen introduced |
| 3 | 120 | 540 | Pathogen still present |
| 4 | 95 | 610 | Pathogen still present |
| 5 | 80 | 720 | Pathogen still present |
1.1 Describe the trend in population size for Pop. X and Pop. Y from Season 1 to Season 5. 2 marks
1.2 Using lesson content, explain why the clonal population (X) was hit harder by the pathogen than the sexual population (Y). 3 marks
1.3 Predict what would happen to Pop. Y if a different pathogen, to which no member of the current population has any resistance, were introduced in Season 6. Justify your prediction. 2 marks
2. Interpret graph — variation and survival under environmental change
The figure below is a stylised model showing what proportion of offspring in two populations survive to reproduce when the environmental temperature shifts away from the long-term mean. One population reproduces asexually (low variation); the other reproduces sexually (high variation). 6 marks
Stylised survival model — illustrative of the trade-off described in Card 4 of the lesson.
2.1 At 0 °C shift (long-term mean), which population has higher offspring survival, and by approximately how much? 2 marks
2.2 At a +3 °C shift, which population has higher offspring survival, and approximately what is the difference? 2 marks
2.3 Use the shape of the two curves to explain the lesson's claim that "successful now does not always mean resilient later". 2 marks
3. Diagram critique — what's wrong with this student's diagram?
A Year 12 student has drawn the diagram below to explain how sexual reproduction maintains continuity of species. There are three biological errors in the diagram. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a new scenario — strawberry runners on a grazed farm
A farmer notices that one wild strawberry plant in a paddock produces fruit that birds rarely take. The farmer wants to multiply this exact "bird-resistant" plant as fast as possible by encouraging it to send out runners (vegetative propagation, an asexual mechanism). A neighbour argues this is risky for long-term farming. 5 marks
4.1 Explain one short-term advantage of the farmer's plan, using lesson terms. 2 marks
4.2 Explain one long-term risk of the farmer's plan, using lesson terms. 2 marks
4.3 The neighbour suggests also growing seedlings from cross-pollinated seeds of the same plant alongside the runner clones. Briefly justify why this hybrid strategy supports continuity better than runners alone. 1 mark
Q1.1 — Trend description
Both populations are equal in Season 1 (1000 plants). After the pathogen is introduced in Season 2, Pop. X (clonal) falls sharply and continues to decline each season (1000 → 180 → 120 → 95 → 80) — a roughly 92% reduction by Season 5. Pop. Y (sexual) also drops in Season 2 (1000 → 620) but stabilises and partially recovers (540 → 610 → 720), ending Season 5 at about 72% of baseline.
Q1.2 — Why the clonal population is hit harder (3 marks)
Pop. X reproduces asexually, so its individuals are genetically near-identical (clones) [1]. This means almost every plant has the same susceptibility to the pathogen, so the pathogen can spread through the whole population without meeting resistance [1]. Pop. Y reproduces sexually, so its individuals carry a range of allele combinations, and some plants by chance carry alleles conferring partial or full resistance — these survive, reproduce, and the population recovers [1].
Q1.3 — Prediction for Pop. Y (2 marks)
Pop. Y would likely also crash initially because no current member has any resistance to the new pathogen [1]. However, because sexual reproduction continues to generate new allele combinations each generation, the chance of producing offspring with novel resistance is higher than in a clonal population, so recovery over multiple generations is more likely than for Pop. X [1]. Accept also: prediction of crash with reasoning that variation alone is not a guarantee.
Q2.1 — Survival at 0 °C shift (2 marks)
The asexual population has higher survival (≈95–100%) than the sexual population (≈75%) [1]. Difference is roughly 20–25 percentage points [1]. Accept ±5 points.
Q2.2 — Survival at +3 °C shift (2 marks)
The sexual population has higher survival (≈40–50%) than the asexual population (≈5–10%) [1]. Difference is roughly 30–40 percentage points in favour of the sexual population [1].
Q2.3 — "Successful now ≠ resilient later" (2 marks)
The asexual curve is a tall narrow peak — survival is highest at zero shift but collapses quickly when conditions move away from the long-term mean [1]. The sexual curve is broader and lower — survival is never as high at the mean, but it remains moderate across a much wider range of temperatures. So the asexual population looks "more successful" under unchanged conditions but is much more vulnerable when conditions shift, exactly the trade-off described in Card 4 [1].
Q3 — Diagram critique (6 marks)
3.1 Error 1 ("no DNA transfer"): Sexual reproduction does transfer DNA — it is the central function of reproduction. Correction: replace the label with "DNA / gametes transferred from each parent into the zygote". [1 + 1]
3.2 Error 2 ("new cell"): The structure formed by fusion of two gametes is a zygote, not just a generic "new cell"; the diagram also fails to show that this zygote develops into the offspring. Correction: label the central circle "zygote" and draw arrows from the zygote down to the offspring (not from each parent separately). [1 + 1]
3.3 Error 3 (caption + offspring identical to one parent): Sexual reproduction does not produce clones; offspring inherit genetic information from both parents and are genetically varied. Correction: redraw the offspring as a mixture of colours (varied) inheriting from both parents, and rewrite the caption as "Sexual reproduction produces genetically varied offspring carrying alleles from both parents." [1 + 1]
Q4.1 — Short-term advantage (2 marks)
Vegetative propagation is asexual, so the farmer can rapidly produce a large number of offspring plants that are genetic clones of the bird-resistant individual [1]. This preserves the desirable genotype without having to find a mate, and gives a uniform, fast-growing crop in the short term [1].
Q4.2 — Long-term risk (2 marks)
Because all the runner-grown plants are genetically near-identical, the whole population shares the same susceptibility to any future stressor (e.g. a new fungal disease, drought, a pest that bypasses bird-resistance) [1]. As shown by the Cavendish banana example in the lesson, low genetic variation in a clonal crop can let one pathogen damage the entire population at once — long-term continuity is reduced [1].
Q4.3 — Why the hybrid strategy is better (1 mark)
Adding sexually produced seedlings introduces genetic variation, so if a new stressor appears at least some plants are more likely to carry resistance alleles — the population is more resilient to environmental change, while the runner clones still capture the short-term efficiency benefits. [1]