Mutation, gamete variation and the source of new alleles: HSC Biology Module 6

Address all four mechanisms and tie each to the data. Sample answer: Mutation is likely the underlying origin of the allele, but mutation alone is unlikely to produce such large frequency differences unless environmental mutagens differed between populations (most prevalent in South Asia, ~60%). Natural selection occurs when certain genotypes are more likely to survive and reproduce; if a population was challenged by the virus causing severe inflammation, selection could reduce the allele frequency, as seen in Africans and East Asians. Gene flow: if mixing between populations was rare, limited gene flow (little migration across wide geographic areas) would maintain the different frequencies. Genetic drift: allele frequencies can change by chance, especially in small populations where which individuals breed can shift frequencies; over large geographic areas chance effects average out. Marks: 7 = extensive understanding with thorough explanations of all four processes, using the stimulus; 6 = thorough; 4-5 = sound understanding of processes affecting gene pools; 2-3 = some understanding; 1 = relevant information. Common error: not distinguishing gene flow from genetic drift and not using cause-and-effect tied to the table.

Source: NESA 2022 HSC Biology examination and marking guidelines.

Evaluate both factors against population size and gene pool, using the timeline and a judgement. Sample answer (key points): The Mt Buller population was already low (~90) in 1996, so the gene pool had low allele diversity. Bushfires in 1998, 2000 and 2002 further reduced the population and restricted the alleles present; the surviving population formed a new gene pool in which some allele frequencies were lost or amplified - this can cause genetic drift / a bottleneck effect, reducing the population further (2002-2007). Introducing 6 males from the distant, isolated Mt Bogong population produced gene flow, bringing new/different alleles, increasing genetic diversity and the number of suitable adaptations, and the population rose from about 8 individuals in 2007 to ~150 in 2016. Judgement: the increased diversity (genetic rescue) improved the sub-population's capacity to adapt, whereas bushfires reduced both size and diversity. Marks: 7 = extensive understanding of the relationships AND an informed judgement; 5-6 = sound understanding and suitable judgement; 3-4 = understanding of bushfires OR males on size/gene pool; 2 = identifies one relationship; 1 = relevant information. Common error: not using scientific terminology to explain trends or not engaging with all the stimulus.

Source: NESA 2023 HSC Biology examination and marking guidelines.

Compare gene flow's effect in small vs large populations and reach an overall judgement. Sample answer: Small populations have limited genetic diversity. Gene flow can benefit the small gene pool by introducing genetic variation; in favourable conditions the new allele can spread quickly and improve adaptability, but in unfavourable conditions it is rapidly removed by natural selection - shown in the small-population graph where the introduced allele's frequency fluctuates dramatically. In the larger population the impact is less pronounced because high genetic variation is already present; gene flow generally still increases diversity, but the graph shows the allele frequency changes little because the introduced allele is a smaller proportion of the gene pool. Judgement: the effect of gene flow can be positive or negative depending on population size and environmental conditions. Marks: 4 = evaluates effects for both small and large populations; 3 = describes effects for both; 2 = describes an effect for either; 1 = relevant information. Common error: misstating the direction of gene flow's effect, or not explaining how population size influences the impact.

Source: NESA 2025 HSC Biology examination and marking guidelines.

Use natural selection with a clear selective pressure plus a DNA-level (mutation) explanation. Sample answer: The map shows lactose-digestion ability varies between populations (e.g. much lower in Australia than in Northern Europe). This variation is likely due to natural selection where the presence of milk in the diet is the selective pressure. A mutation to the lactase gene causes continued production of lactase past age five. Adults with this mutation have an increased chance of survival because of the extra nutrition, so they reproduce and pass the mutation on, making it more common in dairying populations. Populations that remained largely lactose-intolerant were less likely to have milk available, so the mutation offered no advantage and did not become common. Marks: 5 = identifies the variation from the stimulus AND gives reasons with detailed reference to evolution and DNA; 4 = some reference to evolution and DNA; 3 = reference to evolution OR DNA; lower bands = less detail. Common error: invoking Lamarckian inheritance instead of natural selection.

Source: NESA 2019 HSC Biology examination and marking guidelines.

1 mark - gene pool. The gene pool is the total collection of all alleles of all genes present in all individuals of a population at a given time.

1 mark - allele frequency. An allele frequency is the proportion (fraction or percentage) of a particular allele among all copies of that gene in the gene pool.

Both terms must be tied to the population level. Defining "gene pool" as one organism's genes, or "allele frequency" as a count rather than a proportion, does not earn the mark.

1 mark - mutation

Mutation is a heritable change in the DNA sequence; it is the only process that introduces a completely new allele into the gene pool, though it changes frequencies very slowly on its own.

1 mark - gene flow

Gene flow is the movement of alleles between populations by the migration of individuals (or gametes/pollen) that then breed; it adds or removes existing alleles and tends to make populations more genetically similar.

1 mark - genetic drift

Genetic drift is the random change in allele frequencies between generations due to chance; its effect is strongest in small populations and it tends to reduce genetic diversity.

The mark for each hinges on the distinguishing feature: mutation = new allele (source), gene flow = transfer between populations, drift = random chance (strongest when small). Blurring gene flow and drift is the most penalised error.

1 mark - the only origin of new alleles. Meiosis (crossing over, independent assortment, random fertilisation), gene flow and genetic drift can only rearrange, transfer or resample alleles that already exist; only mutation creates a brand-new allele that was not present before.

1 mark - it seeds the other processes. Once mutation has supplied a new allele, selection, gene flow and drift can then act on it; without mutation the pool of variation would eventually be exhausted and evolution would stall.

The answer must contrast "creates new" (mutation) with "redistributes existing" (everything else). Saying mutation is fast or common is incorrect and loses the mark.

1 mark - founder effect identified

The 8 founders carry only a small, random subset of the mainland gene pool's alleles, so by chance the island's starting allele frequencies differ from the mainland's and some alleles are absent altogether (a founder effect, a form of genetic drift).

1 mark - ongoing drift in a small population

Because the population is small, chance events (which lizards survive and breed each generation) cause allele frequencies to fluctuate randomly and some alleles to be lost (fixed or eliminated), reducing genetic diversity over generations.

1 mark - gene flow effect

If lizards (or their eggs) occasionally arrive from the mainland again, this gene flow introduces alleles back into the island pool, increasing diversity and making the island population more similar to the mainland.

1 mark - net contrast

Drift tends to erode and randomise the small island gene pool, whereas recurring gene flow tends to replenish and homogenise it; the balance between them shapes the island's variation.

Full marks require the founder effect named as drift, the small-population sensitivity, AND gene flow correctly described as introducing existing alleles between populations (not creating new ones).

1 mark - bottleneck defined

A bottleneck is a sharp reduction in population size in which the survivors are, in effect, a small random sample of the original gene pool.

1 mark - effect on allele frequencies

The survivors carry only a subset of the original alleles; rare alleles are likely lost entirely and the frequencies of surviving alleles shift by chance (genetic drift), so the gene pool has reduced diversity / lower heterozygosity.

1 mark - why it persists

When the population grows again, it can only multiply the alleles the survivors carried - recovery in numbers does not restore the lost alleles, because only mutation (slow) or gene flow can introduce new/absent alleles.

1 mark - consequence

The recovered population therefore remains genetically depleted, with less raw material for adaptation and a higher risk from inbreeding.

The discriminator is recognising that numerical recovery and genetic recovery are different: a fast rebound in size leaves the reduced allelic diversity intact unless mutation or gene flow restores it.

"Evaluate" requires a judgement about relative importance in each scenario, not just three definitions.

Definitions (1-2 marks)

Mutation: a heritable change in DNA sequence; the only source of new alleles. Gene flow: movement of alleles between populations via migrating individuals or gametes; transfers existing alleles and makes populations more alike. Genetic drift: random change in allele frequencies from one generation to the next due to chance, strongest in small populations.

(a) Small, isolated population (1-2 marks)

Genetic drift dominates - chance has a large effect when numbers are small, rapidly fixing or losing alleles and eroding diversity (founder and bottleneck effects). Gene flow is minimal because the population is isolated, so it does little to counter drift. Mutation still supplies the only new alleles but acts too slowly to offset rapid drift, so diversity tends to fall.

(b) Large, well-connected population (1-2 marks)

Drift is weak because chance effects average out over many individuals, so allele frequencies are relatively stable. Gene flow can be significant, keeping the population genetically similar to its neighbours and topping up diversity. Mutation contributes a steady trickle of new alleles that large numbers help retain.

Judgement (1 mark)

A supported conclusion: drift is the most powerful agent of change in small, isolated populations, whereas in large, connected populations gene flow (and, over long timescales, mutation) matters more while drift is negligible - so the same three processes have very different relative weights depending on population size and connectivity.

An answer that defines the three but does not weigh their relative importance separately for small vs large populations cannot reach the top band.

Target a sequenced response that defines each process, then argues that selection is necessary but not sufficient, reaching a justified judgement.

Frame the claim (1 mark)

Acknowledge that natural selection is a powerful, non-random driver - it changes allele frequencies by favouring variants that improve survival and reproduction - but evaluate whether it is the only important one.

Mutation (1-2 marks)

Selection cannot act on variation that does not exist. Mutation is the ultimate source of all new alleles, so without it selection would have nothing new to favour and the gene pool would eventually stop changing - selection depends on mutation.

Gene flow (1-2 marks)

Gene flow (migration of individuals/gametes between populations) changes allele frequencies independently of fitness: it can introduce or remove alleles and make populations more similar, sometimes counteracting local selection (e.g. swamping a locally favoured allele).

Genetic drift (1-2 marks)

Genetic drift changes frequencies by chance, especially in small populations (founder and bottleneck effects); it can fix even neutral or mildly harmful alleles regardless of selection, and erodes diversity that selection might otherwise use.

Judgement (1 mark)

A justified conclusion: the claim is false - natural selection is important but is one of four interacting agents; mutation supplies the raw material, gene flow transfers alleles between populations, and drift randomly reshapes small gene pools, so all four must be considered to explain how a real gene pool changes.

Band 6 answers explicitly relate each process to whether it is random (drift, mutation, gene flow direction) or non-random (selection), and use a worked example (e.g. a small population where drift outweighs selection) to support the judgement.