Darwin, Wallace and Natural Selection

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

The claim that natural selection means organisms change because they need to survive is incorrect. [1 — explicit judgement]

Natural selection requires four specific conditions: (1) Variation — individuals in a population must differ in their traits, providing the raw material for selection. (2) Heritability — differences must be passed to offspring; if variation is not heritable, favourable traits cannot accumulate. (3) Differential survival and reproduction — some individuals must leave more offspring than others; this is the mechanism that changes allele frequencies. (4) Selection pressure — an environmental factor must favour some variants over others, determining which traits increase in frequency. [1 — all four conditions stated with brief explanation]

What natural selection actually changes is allele frequencies in populations — not the genes of individual organisms. Natural selection acts across generations: individuals whose traits improve survival or reproductive success leave more offspring, increasing the frequency of favourable alleles. This process is cumulative over many generations. A single organism’s traits are fixed by its genotype; natural selection cannot rewrite them in response to need. [1 — explains what changes and on what time scale (populations; generations)]

Darwin distinguished his view from Lamarck’s. Lamarck proposed that organisms could pass on characteristics acquired during their lifetime — that need or use creates heritable change. For example, Lamarck would say a giraffe stretches its neck and passes that length to offspring. Darwin’s view is that variation already exists within populations due to heritable differences, and natural selection acts on that existing variation. The giraffe population already contained individuals with slightly different neck lengths; those with longer necks fed more successfully and left more offspring, shifting the population over generations. [1 — clear Darwin vs Lamarck distinction with example]

The claim’s language (“because they need to”) is a Lamarckian idea. It implies organisms detect a need and respond by changing heritably — which contradicts the Darwinian mechanism where selection acts on pre-existing variation without any intentional direction. [1 — explicitly links the flaw to Lamarckian language and contrasts with Darwinian mechanism]

Darwin and Wallace independently developed the theory of natural selection, presenting it jointly to the Linnean Society in 1858 and Darwin expanding it in On the Origin of Species in 1859. Their contribution was precisely to replace need-based explanations with a mechanism based on differential reproductive success acting on existing heritable variation. [1 — historical context of Darwin and Wallace]

The claim is therefore flawed: natural selection does not mean organisms change because they need to. It means existing heritable variation is filtered by selection pressures over generations, changing allele frequencies in populations — not in individuals. [1 — synthesised conclusion with precise vocabulary]

Marking criteria.

• 1 mark — Explicit evaluative judgement: the claim is incorrect/flawed.
• 1 mark — All four conditions of natural selection stated with brief explanation.
• 1 mark — Explains what natural selection changes (allele frequencies in populations over generations; not individual organisms during their lifetimes).
• 1 mark — Clearly distinguishes Darwin from Lamarck with an example.
• 1 mark — Explicitly identifies the Lamarckian nature of the claim’s language.
• 1 mark — Notes the historical context of Darwin and Wallace.
• 1 mark — Synthesised conclusion using precise lesson vocabulary.

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

The stimulus data is consistent with natural selection and inconsistent with Lamarckian inheritance. [1 — overall evaluative statement]

Applying the four conditions: Variation — the stimulus reports that bacteria before treatment already showed “a small proportion of resistant strains”, confirming that resistant variants existed in the population before any antibiotic pressure. This is the essential variation condition. [1 — variation condition identified with stimulus evidence] Heritability — the persistence and increase of resistance genes across treated generations implies that resistance is heritable, passed from resistant bacteria to their descendants. [1 — heritability condition identified] Differential reproduction — after antibiotic treatment, resistant bacteria survive more often and reproduce, while susceptible bacteria die. The post-treatment population shows a much higher frequency of resistance genes, indicating that resistant bacteria left far more offspring. [1 — differential reproduction condition identified with stimulus data] Selection pressure — the antibiotic itself acts as the selection pressure, creating conditions where resistant variants have a strong survival and reproductive advantage over susceptible ones. [1 — selection pressure condition identified]

The critical piece of evidence for natural selection — not Lamarckian adaptation — is that resistant strains were already present before antibiotic treatment in patients who had never used that antibiotic. Natural selection predicts this: variation pre-exists and selection merely filters it. Lamarck would predict that exposure to the antibiotic directly causes bacteria to develop resistance — but if resistance only appeared after exposure, that would support a Lamarckian view. The pre-existence of resistant strains directly contradicts the Lamarckian prediction and supports Darwin’s mechanism. [1 — explains why pre-existing resistance is critical evidence against Lamarck]

Lamarck would have predicted that bacteria, after repeated exposure, would acquire resistance and pass it to offspring. The stimulus data falsifies this: resistance was not created by exposure — it existed beforehand as heritable variation. [1 — states Lamarckian prediction and explains why stimulus data is inconsistent with it]

Overall, this is high-quality evidence for natural selection: it identifies pre-existing variation, shows a clear selection pressure, demonstrates differential reproduction through frequency change, and can rule out the main alternative (Lamarckian inheritance) because resistance pre-dates treatment. The conclusion that MRSA is a product of natural selection acting on existing heritable variation is well-supported. [1 — evaluates quality of evidence and reaches a justified conclusion]

Marking criteria (8 marks).

• 1 mark — Overall evaluative statement (data consistent with natural selection; inconsistent with Lamarck).
• 1 mark — Variation condition identified and linked to stimulus (resistant strains pre-existed before treatment).
• 1 mark — Heritability condition identified (resistance passed to offspring).
• 1 mark — Differential reproduction condition identified with data (post-treatment frequency increase).
• 1 mark — Selection pressure condition identified (antibiotic is the selection pressure).
• 1 mark — Explains why pre-existing resistance is critical evidence distinguishing natural selection from Lamarckism.
• 1 mark — States Lamarckian prediction and explains why the stimulus data is inconsistent with it.
• 1 mark — Evaluates quality of evidence and reaches a justified conclusion.

Q3 — Sample Band 6 response (6 marks)

The claim is partly defensible but contains significant flaws. [1 — overall evaluative judgement]

What is defensible: the idea that individuals with traits that improve survival in their current environment tend to survive and reproduce more is a core part of natural selection. The phrase “survival of the fittest” captures the basic idea that some variants reproduce more successfully, and was originally used by Herbert Spencer to summarise Darwin’s mechanism. [1 — concedes the defensible element with context]

What is flawed: “Survival of the fittest” means the strongest and fastest. Fitness in biology means reproductive success — the ability to leave more offspring, not physical strength or speed. An organism that is physically small but survives and reproduces more than larger competitors has higher fitness. The lesson’s definition of fitness is “the reproductive success of an individual relative to others.” [1 — refutes “strongest and fastest” with correct definition of fitness]

“All harmful traits disappear.” Natural selection does not eliminate all harmful alleles from a population. Selection acts on traits as they affect fitness under current conditions; many mildly harmful alleles persist because they are recessive, because the environment changes, or because they have other effects. Variation is maintained in populations — selection does not produce uniformity. [1 — refutes elimination of all harmful traits with reasoning about how selection actually works]

“Every organism becomes perfectly adapted.” There is no endpoint of “perfect adaptation” in natural selection. Environments change continuously, so what is adaptive at one time may be less so later. Natural selection is ongoing and context-dependent; it does not produce a finished product. [1 — refutes “perfectly adapted” using environment-dependency argument]

Defensible reformulation: “Natural selection is a process in which individuals in a population that have heritable traits which improve their reproductive success under current conditions leave more offspring, causing favourable allele frequencies to increase over generations. It acts on existing variation and does not produce perfect adaptation because environments change and selection is ongoing.” [1 — biologically defensible reformulation]

Marking criteria.

• 1 mark — States an overall evaluative judgement.
• 1 mark — Identifies the defensible element (differential reproductive success is real; not the specific phrasing).
• 1 mark — Refutes “strongest and fastest” with the correct biological definition of fitness (reproductive success).
• 1 mark — Refutes elimination of all harmful traits with an explanation of how variation persists.
• 1 mark — Refutes “perfectly adapted” using environment-dependency argument.
• 1 mark — Reformulates the claim into a biologically defensible statement.