Gene Pools — Mutation, Gene Flow and Genetic Drift

Q1 — Sample Band 6 response (8 marks), annotated

A gene pool is the total collection of alleles present in a population. The fundamental population-genetics problem revealed by this scenario is that the species has very low allelic diversity (~2–2.5 alleles per locus, down from a pre-bottleneck ~7) and exists as four small isolated sub-populations of 1 500–2 500 — exactly the conditions in which genetic drift is strongest. [1 — defines gene pool + identifies underlying problem]

Under strategy (i) (in situ, no translocation) the projection shows mean alleles per locus declining from 2.5 to 2.0 across 25 years, while the inbreeding coefficient F climbs steeply from 0.10 to ~0.19. Under strategy (ii) (translocation) allelic diversity rises from 2.5 to ~3.6 and F barely changes (0.10 → 0.11). The two strategies produce divergent gene-pool trajectories on both metrics. [1 — describes data on alleles per locus + 1 — describes data on F, both quoted]

Mechanistically, in strategy (i) each isolated sub-population continues to lose alleles by genetic drift — chance variation in who reproduces each generation reduces the rarer alleles to zero, especially with N ≈ 1 500–2 500, and inbreeding rises because individuals are increasingly related. [1 — drift explanation linked to small N] In strategy (ii), translocations physically move individuals (and their alleles) between sub-populations — by the lesson's definition this is gene flow, which transfers existing alleles between populations and counteracts drift's tendency to fix or lose alleles, simultaneously slowing the rise in F. [1 — gene flow explicitly named as the mechanism] Note that mutation does also add new alleles at a slow background rate, but per-generation mutation cannot match the pace of allele loss by drift in N ≈ 2 000, so it cannot rescue strategy (i) on its own. [1 — mutation acknowledged but evaluated as insufficient]

Strategy (ii) is therefore better supported by the data and by population-genetics theory for the 25-year window: it slows drift, maintains allelic diversity, and keeps inbreeding low. The recommendation, however, is conditional: if the sub-populations were locally adapted to different conditions (e.g. different parasite loads), unrestricted translocation could disrupt that local adaptation and the optimal solution might be carefully limited gene flow rather than free mixing. [1 — justified recommendation that names conditions changing the answer]

Marking criteria.

• 1 mark — Defines gene pool correctly and identifies the bottleneck-derived problem (low allelic diversity + small sub-populations vulnerable to drift).
• 1 mark — Uses the data to compare the two strategies on allelic diversity, with at least one quoted value (e.g. 2.0 vs 3.6 at 25 yr).
• 1 mark — Uses the data to compare the two strategies on inbreeding coefficient F, with at least one quoted value (e.g. 0.19 vs 0.11 at 25 yr).
• 1 mark — Identifies genetic drift as the process driving the gene-pool decline in strategy (i), and links it explicitly to small population size.
• 1 mark — Identifies gene flow as the process introduced by translocation in strategy (ii) and explains its effect on allele frequencies.
• 1 mark — Correctly evaluates the role (or insufficiency) of mutation over the 25-year window.
• 1 mark — Reaches a clear evaluative recommendation that prefers strategy (ii) on the data shown.
• 1 mark — Names at least one condition under which the recommendation could change (e.g. local adaptation, disease transfer risk, cost), avoiding a one-winner ranking.

Q2 — Sample Band 6 response (8 marks), annotated

From 1990 to 2020, the mainland population's frequency of allele S is essentially flat at ~0.10, with negligible year-to-year change. Over the same window the island population starts at 0.45 — already much higher than the mainland — and rises further, in jagged steps, to ~0.62 by 2020. The two populations therefore diverge in frequency (0.10 vs ~0.62) and the island series shows clear inter-generational volatility that the mainland series does not. [1 — uses the data to describe difference + 1 — notes volatility]

The most likely explanation is genetic drift, beginning as a founder effect when the 1950s release used only a handful of birds (so the initial island allele frequencies were a chance sample of the mainland gene pool and over-represented S), and continuing as ongoing drift in N ≈ 120 — chance variation in which individuals reproduce produces the visible inter-generational jitter that is absent from the much larger mainland population. [1 — names drift / founder effect with correct mechanism] Mutation cannot explain a frequency shift this large in 30 years, and there is no current gene flow between the populations.

Strategy (a) — the "genetic time capsule" — preserves whatever allele combinations happen to be present on the island. This has value if those alleles are genuinely unique (the lesson's framing: gene flow is the only process that can transfer alleles between populations, so without it the island's gene pool is uniquely its own). [1 — fair evaluation of (a) on uniqueness] However, with N ≈ 120, the island remains highly vulnerable to ongoing drift — alleles are still being lost or fixed by chance, so the "time capsule" is actually still changing over time and is at long-term risk of an inbreeding-related crash. [1 — drift risk evaluation of (a)]

Strategy (b) — periodic translocation of ~10 mainland birds every 5 years — introduces deliberate gene flow. This raises effective population size and replenishes allelic diversity, reducing the impact of drift. The trade-off is that mainland alleles will dilute the island's distinctive frequencies; over time the island gene pool will look more like the mainland's, and the historical "snapshot" feature is lost. [1 — fair evaluation of (b) on gene-flow benefit + dilution trade-off] Practical cost is moderate (small numbers of birds, infrequent movements) and lower than restoring a lost gene pool would be after a future drift-driven crash. [1 — cost criterion explicitly applied]

My recommendation is conditional on which value the conservation programme is prioritising. If the goal is to preserve a unique historic gene pool, strategy (a) is more consistent — but it must be paired with very close monitoring and a contingency translocation plan for when drift starts to fix damaging alleles. If the goal is to maintain a viable wild population in the long term, strategy (b) is the better fit, because the lesson is explicit that drift dominates in small populations and only gene flow (or, much more slowly, mutation) can replenish a gene pool. Neither strategy is universally correct — the right answer is environment-dependent and value-dependent. [1 — explicit, conditional recommendation that refuses a one-winner ranking and uses lesson framing]

Marking criteria.

• 1 mark — Describes the mainland frequency of S as essentially flat at ~0.10 and the island frequency as rising from ~0.45 to ~0.62, quoting at least one value.
• 1 mark — Notes the inter-generational volatility / jitter visible in the island series but not in the mainland series.
• 1 mark — Identifies genetic drift (with at least one of founder effect or "small population") as the most likely explanation, and rules out mutation / current gene flow.
• 1 mark — Evaluates strategy (a) on its preservation of unique island alleles.
• 1 mark — Evaluates strategy (a) on its long-term drift / inbreeding risk in N ≈ 120.
• 1 mark — Evaluates strategy (b) explicitly as introducing gene flow, raising diversity but diluting island distinctiveness.
• 1 mark — Applies a practical / cost criterion to at least one of the strategies.
• 1 mark — Reaches a conditional, value-dependent recommendation in precise lesson terminology (mutation / gene flow / drift), rejecting a one-winner ranking.