HSC Exam Practice

Sample response. Allele frequency is the proportion of a specific allele in a population's gene pool, expressed as a decimal or percentage. Saying evolution is a change in allele frequency over generations means that evolution is measured by tracking how the proportion of an allele rises or falls across successive generations in the population, rather than by changes in any single individual.

Marking notes. 1 mark for defining allele frequency as the proportion of an allele in the gene pool. 1 mark for explaining the population-and-generations basis of evolution.

Sample response. Mutation creates new alleles by changing the DNA sequence. Genetic recombination shuffles existing alleles into new combinations during meiosis. (Gene flow, which moves alleles between populations by migration, is also acceptable.)

Marking notes. 1 mark per correctly identified process with its effect (max 2). Accept any two of mutation (creates alleles), recombination (shuffles alleles), gene flow (moves alleles between populations).

Sample response. Natural selection is a non-random change in allele frequency because the environment consistently favours certain variants for a fitness reason. Genetic drift is a random change in allele frequency caused by chance events, with no adaptive reason, and is strongest in small populations.

Marking notes. 1 mark for identifying selection as non-random/fitness-linked. 1 mark for identifying drift as random/chance-based.

Sample response. Mutation changes the DNA sequence itself and so produces entirely new alleles that did not previously exist. Recombination only rearranges existing alleles into new combinations; it cannot create a new allele. Therefore the new alleles that recombination later shuffles must originally have come from mutation, making mutation the ultimate source.

Marking notes. 1 mark for stating that mutation creates new alleles by changing DNA. 1 mark for explaining that recombination only shuffles existing alleles and cannot create new ones.

Sample response. The antibiotic/pesticide acts as a selection pressure that kills susceptible bacteria but not those carrying the resistance allele. The resistant survivors reproduce, and because the resistance allele is heritable, more of the next generation carry it. Repeated over several generations of exposure, the resistance allele's frequency rises in the gene pool until resistant individuals dominate the population.

Marking notes. 1 mark for identifying the selection pressure killing susceptible individuals while resistant ones survive. 1 mark for inheritance of the resistance allele to offspring. 1 mark for the rise in allele frequency over successive generations.

Sample response. Evolution is defined as a change in allele frequencies across a population over generations, not a change within one organism. An individual's genotype is fixed for life, when it survives a selection pressure it does not change its own DNA; it simply lives or dies. What actually evolves is the population: as individuals with favourable alleles survive and reproduce more, the frequency of those alleles increases across generations. So a surviving individual has not evolved, its survival contributes to the population's evolution.

Marking notes. 1 mark for stating evolution is a population-level change in allele frequency. 1 mark for explaining that an individual's genotype is fixed and it cannot evolve within its lifetime. 1 mark for explaining how differential survival/reproduction drives population change across generations.

Sample response (a). The resistance-allele frequency increases steadily and rapidly across all four generations, from 0.04 at generation 0 to 0.71 at generation 3. The rise is large in every generation of pesticide exposure, showing a strong directional increase.

Sample response (b). Increase = 0.71 − 0.04 = 0.67, i.e. 71% − 4% = 67 percentage points.

Sample response (c). The pesticide is a selection pressure that kills susceptible insects but not resistant ones, so resistant individuals survive and reproduce more, and because resistance is inherited the allele rises in frequency each generation. This is natural selection rather than genetic drift because the change is non-random with respect to fitness, the environment consistently favours the resistant variant in a directed way, rather than allele frequencies changing by chance as they would under drift.

Marking notes. (a) 1 mark for the increasing trend; 1 mark for quoting at least two specific values. (b) 1 mark for the correct subtraction (0.67 / 67 percentage points); 1 mark for correct expression/working. (c) 1 mark for identifying the selection pressure and differential survival; 1 mark for inheritance and rising frequency; 1 mark for explaining why it is selection not drift (non-random/fitness-linked).

Sample response. The claim that natural selection is the only mechanism that changes allele frequencies must be rejected: allele frequencies change through four mechanisms, mutation, gene flow, natural selection and genetic drift, and selection is only one of them. Mutation alters allele frequency by introducing entirely new alleles into the gene pool; although the rate is low, Grant and Grant measured ongoing beak-gene mutation in Galápagos finches and showed it is what replenishes the variation that selection draws down. Gene flow changes allele frequency by the migration of individuals between populations, adding or removing alleles locally and reducing differences between populations. Genetic drift changes allele frequency by chance, particularly in small populations: for example, if a storm randomly kills many individuals on a small island, the survivors' allele frequencies may differ greatly from the original population purely by chance, with no fitness advantage involved, a clear case of allele-frequency change with no selection. Natural selection certainly is a major mechanism, a pesticide-resistance allele rising from 4% to 71% under repeated pesticide exposure is a non-random, fitness-linked change, but it is not the only one. The most defensible conclusion is that selection is the only mechanism that produces consistent adaptation, but mutation, gene flow and drift all change allele frequencies independently of fitness, so the original claim is false: identifying which mechanism is responsible requires evidence beyond the frequency change itself.

Marking criteria. 1 mark, explicit judgement rejecting the claim and naming the four mechanisms. 1 mark, explains how mutation changes allele frequency (introduces new alleles). 1 mark, explains how gene flow changes allele frequency (migration). 1 mark, explains how genetic drift changes allele frequency (chance, strongest in small populations). 1 mark, provides a worked example of a non-selection mechanism (e.g. drift after a storm/bottleneck). 1 mark, acknowledges selection as a real mechanism with a worked example (e.g. pesticide resistance). 1 mark, reaches a synthesis judgement (selection is unique in producing adaptation, but is not the only driver; evidence beyond frequency change is needed).