What is 1. Independent assortment?

During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With n chromosome pairs, this produces 2^n possible combinations.

What is 2. Crossing over?

During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the 2^n figure an underestimate; the actual number of unique gametes is astronomically larger.

What is 3. Random fertilisation?

Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

What is semi-conservative replication?

Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.

What is mismatch repair?

After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in 10^10.

What are mutation - the source of new alleles?

A mutation is a heritable change in the DNA sequence. It is the only process that puts a brand-new allele into the gene pool. On its own it changes frequencies extremely slowly (new mutations are rare), but it is indispensable: without it, the other three processes would have nothing new to work with.

What are gene flow - moving alleles between populations?

Gene flow (also called migration) is the transfer of alleles from one population to another, carried by individuals that move and then breed, or by gametes such as wind-blown pollen. It moves existing alleles around; it does not create new ones. Gene flow tends to introduce alleles a population lacked (raising diversity) and to make connected populations more genetically similar to each other.

What is genetic drift - random change by chance?

Genetic drift is the random change in allele frequencies from one generation to the next, simply because of which individuals happen to survive and breed. Unlike selection, it is not based on fitness - it is chance. Its effect is strongest in small populations, where a few lucky or unlucky events sway a large fraction of the pool; in large populations chance effects average out.

What is tie every mechanism to the stimulus?

Gene-pool questions are almost always data-based (a table, map or graph). State the trend you see, then name the mechanism and give cause and effect - do not describe the mechanisms in the abstract.

What is match depth to the command word?

"Define" wants a one-line meaning; "distinguish" wants the contrasting feature of each; "explain" wants cause and effect; "evaluate" wants a judgement weighing the relative importance of the mechanisms (e.g. drift dominates in small populations, gene flow in connected ones).

What is use "random" precisely?

Genetic drift and mutation are random; the direction of natural selection is not. Markers reward candidates who label each process random or non-random correctly.

§-Syllabus dot point

NSWBiologySyllabus dot point

Inquiry Question 1: How does mutation introduce new alleles into a population?

Investigate the causes of genetic variation relating to the changes and conservation of the DNA sequence including: variations in gametes due to crossing over and segregation in meiosis, the cell replication processes that allow the conservation, variation and mutation of DNA, and the contribution of mutation to genetic variation and evolution

A focused answer to the HSC Biology Module 6 dot point on the sources of genetic variation. Meiotic shuffling (independent assortment, crossing over, random fertilisation), DNA replication fidelity, mutation as the ultimate source of new alleles, and the link to natural selection and evolution.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-21

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section

What this dot point is asking
The answer
Examples in context

What this dot point is asking

NESA wants you to explain where genetic variation comes from: the shuffling that happens in meiosis, the fidelity of DNA replication, and the role of mutation as the ultimate source of new alleles. The evolutionary significance ties Modules 5 and 6 together.

The answer

Genetic variation has two distinct sources: recombination of existing alleles in meiosis and fertilisation, and new alleles introduced by mutation.

Variation from meiosis

Meiosis produces haploid gametes from a diploid parent, reducing chromosome number by half. Three processes generate variation.

1. Independent assortment: During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With $n$ chromosome pairs, this produces $2^n$ possible combinations. In humans (n = 23), that is $2^{23}$ = 8,388,608 combinations per gamete.
2. Crossing over: During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the $2^n$ figure an underestimate; the actual number of unique gametes is astronomically larger.
3. Random fertilisation: Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

These mechanisms generate enormous variation within a generation, but they all act on existing alleles. They cannot create alleles that are not already present in the parents.

Variation from DNA replication and conservation

Semi-conservative replication: Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.
Proofreading: DNA polymerase has a 3' to 5' exonuclease activity that removes incorrectly added bases during synthesis, reducing the error rate from about 1 in $10^5$ (unaided) to 1 in $10^7$ .
Mismatch repair: After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in $10^{10}$ .

The net effect is that DNA replication is extraordinarily faithful. This conservation is essential for maintaining the genetic information across generations and across the trillions of cell divisions within a single body.

Variation from mutation

Even with proofreading and repair, mistakes accumulate. Mutagens (UV, chemicals, viruses) further increase the rate. In humans, each newborn carries roughly 60 to 100 new mutations not present in either parent. Most are in non-coding regions and have no effect; some are mildly deleterious; a small fraction are advantageous.

Mutation is the only source of completely new alleles. Meiosis can only shuffle what already exists.

The link to evolution

Natural selection requires three things: heritable variation, differential reproduction and inheritance of the advantageous variant.

Meiosis generates the variation natural selection acts on in the short term.
Mutation supplies new alleles for selection to act on in the long term.
Selection, drift and gene flow then change allele frequencies across generations.

Without mutation, evolution would stall once the existing alleles were sorted by selection. With mutation, the genetic toolkit is continually replenished and new traits (and new species) can arise.

Changing the gene pool: mutation, gene flow and genetic drift

The gene pool is the total collection of all alleles in a population. Evolution, at its simplest, is a change in allele frequencies in that pool over time. Four processes change those frequencies: mutation, natural selection, gene flow and genetic drift. NESA asks you to define and distinguish the first, third and fourth of these (selection is dealt with separately), so be precise about what each one does and does not do.

Mutation - the source of new alleles: A mutation is a heritable change in the DNA sequence. It is the only process that puts a brand-new allele into the gene pool. On its own it changes frequencies extremely slowly (new mutations are rare), but it is indispensable: without it, the other three processes would have nothing new to work with. Mutation increases variation.
Gene flow - moving alleles between populations: Gene flow (also called migration) is the transfer of alleles from one population to another, carried by individuals that move and then breed, or by gametes such as wind-blown pollen. It moves existing alleles around; it does not create new ones. Gene flow tends to introduce alleles a population lacked (raising diversity) and to make connected populations more genetically similar to each other. Cut off the migration (isolation) and the populations are free to diverge.
Genetic drift - random change by chance: Genetic drift is the random change in allele frequencies from one generation to the next, simply because of which individuals happen to survive and breed. Unlike selection, it is not based on fitness - it is chance. Its effect is strongest in small populations, where a few lucky or unlucky events sway a large fraction of the pool; in large populations chance effects average out. Drift tends to reduce genetic diversity, fixing some alleles and losing others. Two special cases are the founder effect (a new population started by a few individuals carries only a small random subset of the original alleles) and the bottleneck effect (a population crash leaves survivors who are a small random sample, so rare alleles are lost - and growing the numbers back does not restore them).

So the three are easy to confuse but genuinely different: mutation creates, gene flow transfers between populations, and drift randomly resamples - with drift mattering most when the population is small.

Summary table

Source	Mechanism	Scale	New alleles?
Independent assortment	Random alignment in metaphase I	$2^{23}$ in humans	No
Crossing over	Chiasmata exchange in prophase I	Multiplies meiotic variation	No
Random fertilisation	Any sperm meets any egg	Multiplies variation	No
DNA replication errors	Mis-incorporation by polymerase	60 to 100 per human generation	Yes
Mutagens	UV, chemicals, viruses, ROS	Variable; high in some environments	Yes

Distinguishing the three agents of gene-pool change

Agent	What it is	New alleles?	Effect on diversity	Where it matters most	Random?
Mutation	Heritable change in DNA sequence	Yes (only source)	Increases (slowly)	Everywhere, over long timescales	Random
Gene flow	Movement of alleles between populations	No (transfers existing)	Increases; homogenises	Well-connected populations	Direction not fitness-based
Genetic drift	Random change in frequencies by chance	No (resamples existing)	Reduces; loses alleles	Small / isolated populations	Random

Worked exam answers

Worked example 1 - distinguish the three agents (3 marks)

Question. Distinguish between mutation, gene flow and genetic drift as agents that change a population's gene pool.

Model answer. Mutation is a heritable change in the DNA sequence and is the only process that introduces a completely new allele into the gene pool. Gene flow is the movement of existing alleles between populations by migrating individuals or gametes that then breed, which tends to make populations more genetically similar. Genetic drift is the random change in allele frequencies between generations due to chance, and its effect is strongest in small populations, where it reduces diversity.

Marker's note: 1 mark each for a correct, distinguishing definition of mutation (creates a new allele), gene flow (transfers existing alleles between populations) and genetic drift (random change, strongest in small populations). The most common cap is blurring gene flow and drift - the marker looks for "between populations" (flow) versus "by chance" (drift).

Worked example 2 - mutation supplies the raw material (4 marks)

Question. A population of beetles is genetically uniform in colour, carrying only the "green" allele, when the forest bark gradually darkens. Explain why this population can only adapt if mutation occurs.

Model answer. Meiosis (crossing over, independent assortment and random fertilisation) can shuffle the beetles' chromosomes, but every gamete still carries only the green allele, so the population has no variation in colour for selection to act on and cannot adapt to the darker bark. A rare mutation in the DNA produces a new "brown" allele - the only way a never-before-seen allele can enter the gene pool. On dark bark, brown beetles are better camouflaged, survive predation more often and reproduce, so the brown allele rises in frequency over generations. Natural selection acts on the new allele that mutation introduced, producing adaptive evolution.

Marker's note: 1 mark for recognising meiosis only recombines existing alleles, 1 mark for mutation as the source of the new allele, 1 mark for the selective advantage on dark bark, 1 mark for the frequency change over generations linking mutation to selection. Saying meiosis "creates" the brown allele caps the response.

Worked example 3 - bottleneck and genetic drift (4 marks)

Question. A population crashes when a disease kills 95% of individuals, then recovers in number. Explain how the bottleneck changes the gene pool and why the effect persists after recovery.

Model answer. A bottleneck is a sharp reduction in population size, so the survivors are a small, random sample of the original gene pool. By chance the survivors carry only a subset of the original alleles - rare alleles are likely lost entirely and the frequencies of the surviving alleles shift (genetic drift), reducing genetic diversity. When the population grows again it can only multiply the alleles the survivors carried, so numerical recovery does not restore the lost alleles; only slow mutation or gene flow from another population can reintroduce them. The recovered population therefore remains genetically depleted, with less raw material for future adaptation.

Marker's note: 1 mark for defining the bottleneck as a random reduction, 1 mark for loss of alleles / shifted frequencies (drift), 1 mark for explaining persistence (recovery multiplies survivors only), 1 mark for the consequence (reduced diversity, only mutation/gene flow restores it). Real examples that fit this model include the northern elephant seal and the cheetah.

Conservation and variation: a balance

A genome that mutates too much loses its information; a genome that mutates too little cannot adapt. Real organisms balance the two with high-fidelity replication (conservation) plus a small residual rate of mutation (variation). The mutation rate is itself an evolved property.

Common traps

Saying meiosis creates new alleles. It does not. Meiosis recombines existing alleles into new combinations; only mutation creates new alleles.

Forgetting random fertilisation. It is a separate source of variation distinct from independent assortment and crossing over.

Saying DNA replication is "perfect." It is extremely faithful (1 in $10^{10}$ ) but not perfect. The residual errors are an essential source of new alleles.

Treating mutation as always harmful: Most mutations are neutral; some are mildly deleterious; a small fraction are beneficial. The beneficial ones drive adaptive evolution.
Confusing gene flow with genetic drift: This is the most penalised error on the gene-pool dot point. Gene flow is migration between populations (alleles move in or out); genetic drift is random change by chance within a population. They are different mechanisms - do not use one term for the other.
Saying genetic drift "selects" alleles: Drift is random - it is not based on fitness. An allele can be lost or fixed by drift even if it is harmful or beneficial. Only natural selection is fitness-based and non-random.
Saying gene flow or drift creates new alleles: Only mutation creates new alleles. Gene flow transfers existing alleles between populations; drift only resamples the alleles already present.
Skipping the evolutionary link: This dot point explicitly asks for the link to evolution; not making it loses marks.

Exam technique

Define all three when asked to "distinguish": Mutation = creates a new allele; gene flow = transfers existing alleles between populations; genetic drift = random change by chance (strongest in small populations). A question that names all three (like the 2022 HSC) awards marks per mechanism - leaving one out, or merging gene flow and drift, costs marks directly.
Tie every mechanism to the stimulus: Gene-pool questions are almost always data-based (a table, map or graph). State the trend you see, then name the mechanism and give cause and effect - do not describe the mechanisms in the abstract.
Match depth to the command word: "Define" wants a one-line meaning; "distinguish" wants the contrasting feature of each; "explain" wants cause and effect; "evaluate" wants a judgement weighing the relative importance of the mechanisms (e.g. drift dominates in small populations, gene flow in connected ones).
Use "random" precisely: Genetic drift and mutation are random; the direction of natural selection is not. Markers reward candidates who label each process random or non-random correctly.

Examples in context

Example 1. Peppered moth and industrial melanism. Before the Industrial Revolution, almost all Biston betularia (peppered moth) populations near Manchester were the pale "typica" form, well camouflaged on lichen-covered tree bark. A spontaneous mutation in the cortex gene (originally arising in a single moth in the early 1800s, dated by SNP analysis in 2016) produced a melanic "carbonaria" form. As coal soot killed lichens and darkened trees, the carbonaria form became cryptic against soot-stained bark and pale moths became conspicuous. From 1848 to 1900, the carbonaria allele rose from under 2 percent to over 95 percent. This is the canonical example of how a single new mutation, combined with strong directional selection, drives rapid evolution.

Example 2. Bacterial antibiotic resistance in Sydney hospitals. Methicillin-resistant Staphylococcus aureus (MRSA) emerged when a spontaneous mutation in the mecA gene of a Staphylococcus strain altered the penicillin-binding protein's structure so beta-lactam antibiotics no longer bound. NSW Health surveillance data show MRSA bloodstream infection rates rose from negligible in the 1970s to a peak of about 1.4 per 10 000 patient-days by 2005 before infection-control measures pushed them down. Every dose of antibiotic in a Sydney ICU creates strong selection: susceptible bacteria die; resistant bacteria carrying the mecA mutation survive and reproduce. The new allele was rare to begin with, but mutation supplied the variation and selection did the rest.

Note

Picture a population's gene pool as a giant jar of coloured marbles, where each colour is a different version of a gene (an allele). Evolution is just the mix of colours in the jar changing over time. Three things can change that mix. Mutation is when a marble spontaneously changes to a brand-new colour no one had before - it is the only way a new colour ever appears, and it happens rarely. Gene flow is when someone carries marbles between two jars, so the jars end up looking more alike. Genetic drift is pure luck: if you scoop out only a few marbles to start a new jar (or a disaster leaves only a handful), the colours you happen to grab might not match the original mix, and rare colours can vanish for good - which is why small populations lose variety so easily. Mutation makes new colours, gene flow shares them around, and drift is the random luck of the draw.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2022 HSC7 marksAn allele of a gene is associated with increased lung inflammation and increased chance of death from a virus. Its frequency is South Asian 60.3%, European 15.1%, African 2.4%, East Asian 1.8%. Explain how mutation, natural selection, genetic drift and gene flow could have led to these differences in the gene pools of populations with differing ancestry.

Show worked answer →

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.

2023 HSC7 marksThe mountain pygmy possum at Mt Buller is critically endangered. A graph shows its population following bushfires (1998-2002) and the introduction of 6 males from Mt Bogong (2007 and 2012). Evaluate how bushfires and the introduction of males from other locations have affected the population size and gene pool of the Mt Buller population.

Show worked answer →

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.

2025 HSC4 marksTwo graphs show changes in the frequency of an introduced allele in a small population (n = 20) and a large population (n = 2000) over 50 generations. Evaluate the effects of gene flow on the gene pools of the two populations.

Show worked answer →

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.

2019 HSC5 marksA map shows the percentage of adult Indigenous populations able to digest lactose. The gene producing lactase is usually switched off between ages 2 and 5, but some people remain able to digest lactose for life. With reference to evolution and DNA, provide possible reasons for the distribution shown in the map.

Show worked answer →

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.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation2 marksDefine the gene pool of a population and state what is meant by an allele frequency.

Show worked solution →

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.

foundation3 marksDistinguish between mutation, gene flow and genetic drift as agents that change the allele frequencies of a population's gene pool.

Show worked solution →

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.

foundation2 marksExplain why mutation is described as the 'ultimate' source of variation, even though it changes allele frequencies only slowly.

Show worked solution →

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.

core4 marksA small island population of lizards is founded by 8 individuals blown over from a large mainland population. Explain how genetic drift (including the founder effect) and gene flow could each affect the island gene pool over time.

Show worked solution →

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).

core4 marksA population passes through a severe bottleneck when a disease kills 95% of individuals. Using the term 'allele frequency', explain how the bottleneck changes the gene pool and why the effect persists after the population recovers in number.

Show worked solution →

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.

exam6 marksDefine mutation, gene flow and genetic drift, and evaluate their relative contributions to changing the gene pool of (a) a small, isolated population and (b) a large, well-connected population.

Show worked solution →

"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.

exam7 marksEvaluate the claim that natural selection is the only important driver of changes to a population's gene pool, with reference to mutation, gene flow and genetic drift.

Show worked solution →

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.