HSC Exam Practice

Sample response. A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing (e.g. an antibiotic, a predator, or pollution). Allele frequency is the proportion of a particular version of a gene within a population.

Marking notes. 1 mark for each term correctly defined. Accept paraphrasing provided the meaning is clear.

Sample response. Bacteria have a very short generation time (about 20 minutes), so thousands of generations pass each year [1]. With heritable variation already present and an intense selection pressure (an antibiotic that kills non-resistant cells), natural selection can produce measurable change in resistance frequency within months rather than over thousands of years [1].

Marking notes. 1 mark for short generation time / many generations per year. 1 mark for linking this to rapid change under intense selection on existing variation.

Sample response. The 30,000-year-old mammoth bone should be dated with Carbon-14 (¹⁴C). The 3-million-year-old volcanic rock should be dated with Potassium-Argon (K-Ar). Justification for one: Carbon-14 is appropriate for the mammoth bone because its effective range is up to about 50,000 years and the bone is recent organic material that originally contained carbon; at 30,000 years enough Carbon-14 remains to measure reliably.

Marking notes. 1 mark for Carbon-14 for the mammoth bone. 1 mark for Potassium-Argon (or Uranium-Lead) for the volcanic rock. 1 mark for a valid justification of one choice that references the effective range / half-life.

Sample response. Antibiotics do not create resistance mutations [1]. Resistance mutations arise randomly through DNA replication errors and already exist in the population before any antibiotic is used; the antibiotic acts only as a selection pressure that kills susceptible bacteria and allows the rare pre-existing resistant individuals to survive and reproduce [1].

Marking notes. 1 mark for stating that the resistance mutations pre-exist / are random. 1 mark for stating that the antibiotic selects for (not causes) resistance.

Sample response. Toads: invasion-front toads have evolved longer legs; the selection pressure is the advantage of faster dispersal into unoccupied territory [1]. Predators: some freshwater crocodiles and snakes have evolved reduced sensitivity to bufotoxin; the selection pressure is the toad's toxin, which kills sensitive individuals [1].

Marking notes. 1 mark for the toad change with its selection pressure. 1 mark for the predator change with its selection pressure.

Sample response. The dark allele frequency began very low (~1%) before industrialisation, then rose rapidly during the industrial period to a peak of about 90% around 1900 [1], and then began to fall again after the Clean Air Act of 1956 as pollution decreased, the trend rose and then reversed [1].

Marking notes. 1 mark for describing the rise to a peak (~1% to ~90%). 1 mark for describing the subsequent fall after the Clean Air Act.

Sample response. Before industrialisation, dark moths were poorly camouflaged on pale lichen-covered trees and were easily seen and eaten by bird predators, keeping the dark form rare [1]. Industrial soot killed the lichens and blackened the tree trunks, so now the dark moths were better camouflaged and the pale moths stood out and were eaten more [1]. Dark moths therefore survived and reproduced more successfully, so they passed on the dark allele and its frequency rose in the population through natural selection [1].

Marking notes. 1 mark for the pre-industrial situation (dark visible, eaten). 1 mark for the change in camouflage caused by soot. 1 mark for linking differential survival/reproduction to the rise in allele frequency.

Sample response. Pollution did not create the dark allele [1]. Both the pale and dark alleles already existed in the population before industrialisation, the dark allele was simply rare (~1%). Pollution only changed which form was better camouflaged, shifting the selection pressure so that the already-present dark allele became advantageous and increased in frequency [1].

Marking notes. 1 mark for stating the dark allele pre-existed. 1 mark for explaining that pollution changed the selection pressure rather than creating the allele.

Sample response. Modern examples and the fossil record are best seen as complementary lines of evidence rather than one being simply "stronger" than the other. Each addresses a different question, so combining them gives the most convincing case for evolution.

Antibiotic resistance is a direct, observable example of natural selection. Resistance mutations arise randomly and pre-exist in bacterial populations; when an antibiotic is applied it acts as a selection pressure that kills susceptible bacteria while the rare resistant individuals survive and reproduce. Because bacteria reproduce roughly every 20 minutes, the frequency of the resistance gene can rise measurably within months, as seen in the rise of MRSA from under 0.2% to about 12% of hospital infections in Australia between 1990 and 2020.

The peppered moth is a second strong example: the pre-existing dark allele rose from ~1% to ~90% when soot-blackened trees made dark moths better camouflaged, then fell again after the Clean Air Act. The reversal when the selection pressure reversed is especially compelling because the same population responded predictably to both the imposition and removal of the pressure.

These modern examples have the strength of being directly observed and measured, but they cover only short timescales. Radiometric dating supports the much longer fossil record by using the constant decay rates of isotopes (e.g. Carbon-14 up to ~50,000 years; Potassium-Argon and Uranium-Lead for millions to billions of years) to assign actual ages to fossils. These dates confirm that simpler organisms appear in older layers and more complex organisms appear progressively more recently, and that transitional fossils appear at the timepoints predicted by evolutionary theory.

Conclusion: the modern examples are not simply "stronger" than the fossil record; they are stronger for showing the mechanism of natural selection in real time, while radiometric dating and the fossil record are stronger for confirming the long-term pattern and timescale of evolution. The most convincing case comes from combining both, direct observation of the mechanism plus a dated fossil record showing the resulting pattern over deep time.

Marking criteria.

• 1 mark Explains a first modern example (e.g. antibiotic resistance) correctly using natural selection.
• 1 mark Explains a second modern example (e.g. peppered moth or cane toad) correctly using natural selection.
• 1 mark Identifies that these modern examples are directly observed/measured but cover short timescales.
• 1 mark Explains how radiometric dating assigns actual ages using isotope decay (with at least one named isotope and timescale).
• 1 mark Explains what the dated fossil record reveals (simpler to complex over time; transitional fossils at predicted timepoints).
• 1 mark Distinguishes the role of modern examples (mechanism in real time) from the fossil record (long-term pattern/timescale).
• 1 mark Reaches a justified conclusion about the value of combining lines of evidence.
• 1 mark Uses precise vocabulary throughout (selection pressure, heritable variation, allele frequency, half-life, transitional fossil).