HSC Exam Practice

Sample response. A gene pool is the total collection of alleles present across all individuals in a population. It is the population-level inventory of genetic variation against which allele frequencies are measured.

Marking notes. 1 mark for "total collection of alleles" (or equivalent — all alleles for all genes in the population); 1 mark for identifying that it is a population-level concept (not an individual genotype). Both required for full marks.

Sample response. Mutation introduces new alleles into the gene pool by changing the DNA sequence — it is the only source of genuinely novel variation. Gene flow does not create alleles; it transfers existing alleles between populations through migration of individuals and their subsequent breeding. Mutation therefore changes which alleles can exist in the gene pool, while gene flow changes the distribution of alleles between populations.

Marking notes. 1 mark for mutation = new alleles via DNA-sequence change; 1 mark for gene flow = transfer of existing alleles between populations via migration + reproduction; 1 mark for explicit contrast (creation vs transfer).

Sample response. Genetic drift is the change in allele frequency caused by chance variation in which individuals reproduce. In a small population, each individual contributes a large proportion of the next generation's alleles, so chance events (e.g. one carrier failing to breed) cause large proportional changes in allele frequency. In a large population, chance variation among different individuals tends to cancel out across many reproductive events, so the proportional change in allele frequency between generations is small. Drift is therefore strongest where the sample of "who reproduces" is small relative to the gene pool.

Marking notes. 1 mark for identifying drift as chance variation in reproduction / sampling of alleles; 1 mark for proportional argument (one individual contributes more in small N); 1 mark for "cancelling out" / law-of-large-numbers reasoning in large N.

Sample response. Both are forms of genetic drift in which chance produces a non-representative sample of the parent gene pool. The founder effect occurs when a small number of individuals establishes a new population (e.g. a few birds blown off course onto an island), so the new population's allele frequencies reflect that small founding sample. The bottleneck effect occurs when an existing population is sharply reduced in size by an event such as a fire, disease or hunting, so the surviving frequencies are a chance sample of the pre-event population (e.g. cheetahs after a Pleistocene population crash).

Marking notes. 1 mark for identifying both as forms of genetic drift; 1 mark for correctly distinguishing founder (new population from few colonists) vs bottleneck (existing population reduced sharply); 1 mark for one valid named example per type (or two reasonable examples one of which is correctly assigned).

Sample response. Genetic drift is a random change in allele frequency caused by chance in which individuals reproduce. An allele that becomes common by drift has not been favoured because it suits the environment — it has been over-represented by accident. "Adaptive" implies that allele frequencies are being shaped by environmental fit (i.e. selection), which is a different mechanism. Describing drift as adaptive confuses random change with directional change.

Marking notes. 1 mark for identifying drift as random / chance-based; 1 mark for explicitly contrasting it with adaptation / fit-driven change.

Sample response (a). The mainland frequency of allele b is essentially flat across all 12 generations at approximately 0.05. The island frequency starts at ~0.36 in generation 0 and rises overall to ~0.51 by generation 12, but it does not rise smoothly — it fluctuates between generations (e.g. 0.32 in generation 2, 0.46 in generation 3, 0.40 in generation 1), with clear inter-generational volatility. By generation 12 the two populations differ by approximately 0.46 in allele frequency.

Sample response (b). The dominant process is genetic drift. The initial gap (mainland 0.05 vs island 0.36 in generation 0) is consistent with a founder effect — the 14 founders carried a non-representative sample of the mainland's alleles. The ongoing inter-generational jitter and the further rise toward 0.51 reflect continuing drift in the small island population (N ≈ 30), where chance variation in which individuals reproduce has a large proportional effect on allele frequency. By contrast, the mainland's much larger population size means chance variation in different individuals' reproduction cancels out, producing the flat reference line at ~0.05.

Marking notes. Part (a) — 1 mark for describing mainland as flat at ~0.05 with a quoted value; 1 mark for describing the island as starting at ~0.36 and rising/fluctuating to ~0.51 with at least one quoted value and a comment on volatility. Part (b) — 1 mark for identifying the founder effect / drift as the source of the initial gap; 1 mark for ongoing drift continuing in the small island population; 1 mark for explicit population-size reasoning explaining why mainland is flat and island fluctuates.

Sample response. The claim that genetic drift is always the most important process changing allele frequencies overstates the case. The relative importance of mutation, gene flow and genetic drift depends on the population's size, its isolation and the timescale considered, so a single "winner" cannot be assigned without context. Mutation is the only process that introduces a genuinely new allele into a gene pool — for example, a single nucleotide change at a haemoglobin gene in a human germ cell can give rise to the sickle-cell allele. Without mutation, neither drift nor gene flow has any new variation to act on, so over very long timescales mutation is indispensable, but per-generation it changes population frequencies very slowly. Gene flow dominates when populations regularly exchange individuals — for example, mainland deer populations that exchange ~5% of bucks per year across a low ridge have their allele frequencies homogenised by migration, regardless of any drift inside each sub-population. Gene flow does not create new alleles but it transfers existing ones, so when migration rates are high the receiving population's allele frequencies track the source rather than wandering. Genetic drift dominates when the population is small and isolated — the lesson's anchor example is an island population founded by only a few individuals, where a rare allele can become common simply because the founders happened to carry it. In a population of N ≈ 30 every chance event has a large proportional effect on allele frequency, whereas in N ≈ 10 000 the same chance events average out. The honest evaluative judgement is therefore conditional: drift dominates in small isolated populations (especially after founder events or bottlenecks), gene flow dominates where migration is regular, and mutation is essential for long-term novelty. Drift is not "always" most important — it is most important when population size is small and migration is low. The claim is therefore rejected as a universal statement and should be reformulated as: "Drift is the most important process changing allele frequencies in small, isolated populations; in larger or exchanging populations, gene flow tends to dominate short-term change while mutation supplies long-term novelty."

Marking notes. 1 mark — explicit overall judgement that rejects "always". 1 mark — correctly identifies mutation as the only source of new alleles with a valid example. 1 mark — correctly identifies gene flow as the transfer of existing alleles with a valid example (migration / interbreeding between populations). 1 mark — correctly identifies genetic drift as random change in allele frequencies, strongest in small populations, with a valid example (e.g. founder effect on an island, bottleneck). 1 mark — explicit reasoning linking population size to the relative importance of drift (small N → drift dominates; large N → drift weak). 1 mark — explicit reasoning linking migration rate to the relative importance of gene flow (high migration → gene flow dominates; isolated → gene flow negligible). 1 mark — reaches a clear context-dependent reformulation of the claim in precise lesson terminology (mutation / gene flow / drift), avoiding a one-process ranking.