Variation and Allele Frequency

Q1, Sample Band 6 response (7 marks), annotated

Mutation is a permanent change in the DNA sequence; it creates entirely new alleles and is therefore the only process that increases the total allelic variety of a gene pool [1, definition + effect]. Genetic recombination shuffles existing alleles into new combinations during meiosis and sexual reproduction; it generates new genotypes but does not invent new alleles [1, definition + effect]. Gene flow moves alleles between populations by migration, adding or removing variation locally and reducing differences between populations [1, definition + effect].

Mutation is the ultimate source because both recombination and gene flow can only redistribute alleles that already exist; without mutation there would be no new alleles for them to shuffle or move [1, ultimate-source reasoning]. Grant and Grant measured the spontaneous mutation rate in Geospiza beak genes and calculated that, without this ongoing low-level input, standing variation would be exhausted within roughly 200–400 generations as selection drew it down [1, evidence that mutation replenishes variation].

All three processes are needed for long-term evolution: mutation supplies new raw material, recombination expands the range of genotypes selection can act on each generation, and gene flow maintains variation across populations and counteracts local loss by drift [1, why all three are needed]. In terms of relative importance, mutation is foundational because it is the sole origin of novelty, but on short timescales recombination and gene flow are the major day-to-day contributors to standing variation; the most defensible judgement is that they are complementary, with mutation indispensable over the long term [1, justified relative-importance judgement].

Marking criteria.

• 1 mark Defines mutation and its effect (creates new alleles).
• 1 mark Defines recombination and its effect (shuffles existing alleles).
• 1 mark Defines gene flow and its effect (moves alleles between populations).
• 1 mark Explains why mutation is the ultimate source.
• 1 mark Uses evidence that mutation replenishes variation (e.g. Grant & Grant).
• 1 mark Explains why all three are needed for long-term evolution.
• 1 mark Reaches a justified relative-importance judgement.

Q2, Sample Band 6 response (8 marks), annotated

Jump from 5% to 25% (eruption). This is genetic drift (a bottleneck/founder effect) [1]. The eruption killed most of the population at random, so the high dark-allele frequency among the 30 survivors arose by chance sampling, not because dark colour conferred a survival advantage at that point, the change is random with respect to fitness [1].

Rise from 25% to 78% (20 generations). This is natural selection [1]. Once the island darkened with ash, birds preferentially ate pale beetles, so dark beetles survived and reproduced more successfully; because colour is inherited, the dark-colour allele rose in frequency for a fitness reason, a non-random, directional change driven by a consistent selection pressure [1].

Role of ongoing mutation. The continuing low rate of new colour-gene mutations replenishes the pool of variation that selection and drift draw down [1]. Without it, once an allele is fixed or lost the population could not regain that variation, and over the long term the gene pool would converge on a single uniform genotype with no raw material for future adaptation [1].

Evaluating "the beetles evolved to need dark colour". The claim is flawed. Individuals do not develop a trait because they "need" it; the dark-colour allele already existed in the population through prior mutation, and selection simply increased the frequency of carriers because they survived better [1]. A defensible statement is that "the population evolved as the dark-colour allele's frequency rose across generations under selection", evolution is a population-level change in allele frequency, not individuals acquiring a needed trait [1].

Marking criteria.

• 1 mark Identifies drift/bottleneck for the 5%–25% jump.
• 1 mark Justifies it as random with respect to fitness.
• 1 mark Identifies natural selection for the 25%–78% rise.
• 1 mark Justifies it as a non-random, fitness-linked change driven by a selection pressure.
• 1 mark Explains ongoing mutation replenishes variation.
• 1 mark Explains the long-term consequence if mutation stopped (convergence on uniformity).
• 1 mark Corrects the "need" misconception (allele pre-existed; selection raised its frequency).
• 1 mark Reformulates as a population-level change in allele frequency.

Q3, Sample Band 6 response (6 marks)

The claim is largely flawed. [1, evaluative judgement]

What is wrong:

• "Whenever an allele frequency changes, natural selection must be responsible." Allele frequencies also change through mutation, gene flow and genetic drift; in small populations drift can cause large random changes with no selective advantage, so a frequency change alone does not prove selection [1, refutes "must be selection"].
• "Individuals that need a trait develop it and pass it on." This is the inheritance-of-acquired-characteristics fallacy. Variation arises from random mutation before any need exists; individuals cannot rewrite their DNA in response to need, and an individual's genotype is fixed for life [1, refutes the "need" mechanism].
• "Genetic drift is irrelevant to real evolution." Drift is a genuine evolutionary mechanism, especially important in small populations where it can fix or lose alleles by chance and reduce genetic diversity, for example, after a population bottleneck [1, refutes the dismissal of drift].

Defensible reformulation: "Allele frequencies in a population change through several mechanisms, mutation, gene flow, natural selection and genetic drift. Natural selection is the non-random, fitness-linked mechanism, whereas drift is random change that is strongest in small populations. Identifying which mechanism is responsible requires evidence beyond the frequency change itself, and variation always arises before need, not because of it." [1, defensible reformulation] The claim's only sound element is that selection is one real driver of allele-frequency change, but it is not the only one, and its proposed mechanism is incorrect [1, concedes the valid kernel while correcting it].

Marking criteria.

• 1 mark States an overall evaluative judgement.
• 1 mark Refutes "must be selection" by naming other mechanisms (drift/mutation/gene flow).
• 1 mark Refutes the "need" mechanism (variation precedes need; no acquired characteristics).
• 1 mark Refutes the dismissal of drift, with the small-population point.
• 1 mark Reformulates into a defensible statement listing the mechanisms correctly.
• 1 mark Concedes the valid kernel (selection is one real driver).