HSC Exam Practice

Sample response. The four conditions are: (1) variation — individuals differ in their traits; (2) heritability — differences are passed to offspring; (3) differential survival/reproduction — some individuals leave more offspring than others; (4) selection pressure — the environment favours some variants over others.

Marking notes. 1 mark for any two conditions correctly named and described. 2 marks for all four conditions correctly identified. Accept brief descriptions; exact lesson wording not required.

Sample response. In a bacterial population, some individuals already carry a resistance mutation before any antibiotic is used [1]. When antibiotics are applied, they act as a selection pressure that kills more susceptible bacteria while resistant bacteria survive and reproduce [1]. Over generations, the frequency of the resistance allele increases in the population [1]. The pre-existence of resistant variants before treatment is critical: it shows that the antibiotic does not create resistance but selects for it — consistent with Darwinian natural selection rather than Lamarckian adaptation.

Marking notes. 1 mark: resistant variants pre-exist (before treatment). 1 mark: antibiotic is selection pressure / kills susceptible bacteria / resistant bacteria survive and reproduce. 1 mark: resistance allele frequency increases over generations.

Sample response. Lamarck proposed that a giraffe stretches its neck during its lifetime and passes that longer neck to its offspring — acquired traits are inherited [1]. Darwin’s mechanism is different: some giraffes are born with slightly longer necks due to heritable variation already present in the population [1]. Those with longer necks feed more successfully and leave more offspring, so alleles associated with longer necks become more common over many generations — the population shifts without any individual changing its heritable traits during its lifetime [1].

Marking notes. 1 mark for Lamarckian view (acquired traits inherited; giraffe stretches and passes length on). 1 mark for Darwinian view (heritable variation already exists). 1 mark for explaining how selection shifts allele frequencies over generations rather than within individuals.

Sample response. An individual organism’s traits are determined by its genotype, which is fixed at birth [1]. Natural selection changes which alleles become more common by favouring individuals with certain traits — those individuals leave more offspring and pass on those alleles. This process accumulates across many generations, shifting allele frequencies in the population, not rewriting the genome of any individual organism [1].

Marking notes. 1 mark for the idea that individual genotypes are fixed (individuals do not rewrite their DNA). 1 mark for explaining that selection changes allele frequencies across populations over generations.

Sample response. Charles Darwin and Alfred Russel Wallace independently developed the idea of natural selection [1]. They presented the theory jointly to the Linnean Society in 1858. Darwin then expanded the theory in On the Origin of Species, published in 1859 [1].

Marking notes. 1 mark for both Darwin and Wallace independently developed natural selection. 1 mark for the joint 1858 presentation and/or Darwin’s 1859 publication.

Sample response. The key datum is that 3% resistant strains existed before treatment in patients with no prior antibiotic history [1]. This shows that resistance was already present due to random mutation — before any antibiotic was applied. Natural selection predicts that pre-existing resistant variants will be selected for when antibiotics are introduced; Lamarck would predict that exposure to the antibiotic causes bacteria to develop resistance. Because resistance existed before exposure, the data is consistent with natural selection and inconsistent with Lamarckian inheritance [1]. After treatment, resistance increased from 3% to 74%, showing that resistant bacteria survived and reproduced while susceptible bacteria were eliminated — confirming the differential reproduction component of natural selection [1].

Marking notes. 1 mark for identifying the key datum (3% resistance pre-existed before treatment). 1 mark for explaining why this distinguishes natural selection from Lamarckism (resistance pre-dates exposure; Lamarck predicts resistance created by exposure). 1 mark for explaining the post-treatment frequency increase as differential reproduction.

Sample response. The selection pressure is the antibiotic treatment [1]. It killed a much higher proportion of susceptible bacteria (from 97% to 26%) while resistant bacteria survived at much higher rates and reproduced, increasing from 3% to 74% of the population. The antibiotic thus shifted the relative proportions by strongly favouring resistant variants over susceptible ones [1].

Marking notes. 1 mark for identifying the antibiotic as the selection pressure. 1 mark for explaining its effect on proportions using the table data.

Sample response. The student’s argument is incorrect and unnecessary [1]. Natural selection predicts that resistant variants arise through random mutation and exist in populations before any selective pressure is applied. The lesson explicitly states that “resistant variants already existed due to mutation” and that this pre-existence is exactly what natural selection requires. A 3% baseline resistance in patients with no prior antibiotic history is expected: bacteria mutate randomly, and some mutations produce resistance alleles. These alleles would persist at low frequency in a population until a selection pressure (the antibiotic) increases their frequency. There is no need to invoke cross-contamination to explain the pre-existing resistance; it is the normal prediction of natural selection [1].

Marking notes. 1 mark for identifying the argument as incorrect. 1 mark for explaining why (natural selection predicts pre-existing variation from random mutation; 3% resistance is consistent with this and expected even without prior antibiotic use).

Sample response. The statement is incorrect. Natural selection acts on populations over generations by changing allele frequencies — not by transforming individual organisms during their lifetimes.

Natural selection requires four conditions: (1) Variation — individuals differ in their traits; (2) Heritability — differences are passed to offspring; (3) Differential survival and reproduction — some variants leave more offspring; (4) Selection pressure — the environment favours some variants over others. If all four conditions are met, allele frequencies shift across generations.

What changes is the frequency of alleles in a population, not the genotype of any individual. An organism’s traits are fixed by its genotype at birth. Selection favours individuals with certain traits, so those individuals leave more offspring and their alleles become more common. This requires multiple generations; it cannot occur within one organism’s lifetime.

This is fundamentally different from Lamarck’s proposal that organisms pass on characteristics acquired during their lifetimes. Lamarck would say a giraffe stretches its neck and passes that length to offspring. Darwinian natural selection says giraffes already varied in neck length; those with longer necks left more offspring; the population shifted over generations.

MRSA provides direct evidence. Before any antibiotic is applied, a small proportion of bacteria already carry resistance alleles due to random mutation. When antibiotics are used, susceptible bacteria die more often while resistant bacteria survive and reproduce. Over generations, the resistance allele frequency increases. The pre-existence of resistance before treatment demonstrates that selection acts on existing variation — it does not create the variation on demand. This is inconsistent with the statement’s claim that individual organisms adapt during their lifetimes, and consistent with Darwinian natural selection.

The statement is therefore false: populations evolve through natural selection because heritable variants that improve reproductive success become more common over generations — individual organisms do not change their heritable traits in response to environmental demands.

Marking criteria (8 marks).

• 1 mark — Explicit evaluative judgement: the statement is incorrect.
• 1 mark — All four conditions of natural selection stated correctly.
• 1 mark — Explains what natural selection actually changes (allele frequencies in populations over generations).
• 1 mark — Explains why it cannot act on individuals during their lifetimes (individual genotypes are fixed).
• 1 mark — Distinguishes Lamarck from Darwin clearly with an example.
• 1 mark — Uses MRSA as supporting evidence: resistant variants pre-exist; antibiotic is selection pressure; frequency increases over generations.
• 1 mark — Links the MRSA evidence back to the specific flaw in the original statement (individual adaptation vs population change).
• 1 mark — Synthesises a conclusion using precise lesson vocabulary throughout.