Types of Natural Selection

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

Directional, stabilising and disruptive selection are three distinct patterns of natural selection, each differing in which phenotypes are favoured and how they change the distribution of traits in a population. [1 — framing statement]

Directional selection occurs when one extreme phenotype has the highest fitness and the distribution shifts sideways toward that extreme. The population mean moves in one direction. A classic example is industrial melanism in peppered moths: pollution darkened tree bark, making pale moths more visible to predators, so dark moths were favoured and became more common. Directional selection shifts variation rather than increasing or decreasing it substantially — it changes the mean but maintains a range. [1 — directional correctly defined + example + effect on distribution]

Stabilising selection occurs when the intermediate phenotype has the highest fitness and both extremes are selected against. The distribution becomes narrower around the mean and variation decreases. Human birth weight is the classic example: both very low and very high birth weight carry higher risks, while intermediate birth weight has the best survival. Stabilising selection reduces variation over time by removing the extremes. [1 — stabilising correctly defined + example + effect on distribution]

Disruptive selection occurs when both extreme phenotypes have higher fitness than the intermediate, and the intermediate is selected against. The distribution can split into two peaks (bimodal), and variation increases. The oyster-size example in the lesson illustrates this: very small and very large oysters survive better than mid-sized ones, because predators ignore the smallest and cannot open the largest. Disruptive selection can increase variation and create divergence between the two favoured extremes. [1 — disruptive correctly defined + example + effect on distribution]

Comparing effects on variation: directional selection shifts and may eventually reduce variation around the new mean; stabilising selection narrows variation; disruptive selection increases variation and creates divergence. These are fundamentally different evolutionary outcomes. [1 — comparison of effects on variation across all three types]

Disruptive selection is most likely to contribute to speciation because it creates divergence between two favoured extreme phenotypes, increasing the probability that the two groups will eventually develop reproductive isolation. The lesson notes that if the favoured extreme groups become reproductively isolated, speciation can follow. Neither directional nor stabilising selection creates the same kind of within-population split that can drive divergence toward separate lineages. However, disruptive selection alone does not guarantee speciation — reproductive isolation is also required. [1 — evaluation of speciation link for disruptive selection, with the caveat that reproductive isolation is also needed]

In conclusion, all three selection types drive evolutionary change by altering allele frequencies, but they produce different outcomes: directional shifts the mean, stabilising narrows variation, and disruptive increases divergence and is most directly associated with the conditions that can lead to speciation. [1 — synthesised conclusion]

Marking criteria.

• 1 mark — Framing statement or overall comparison structure.
• 1 mark — Directional correctly defined (one extreme favoured; distribution shifts) + named example + effect on variation.
• 1 mark — Stabilising correctly defined (intermediate favoured; both extremes selected against; narrower distribution) + named example + effect on variation.
• 1 mark — Disruptive correctly defined (both extremes favoured; intermediate selected against; bimodal; variation increases) + named example + effect on variation.
• 1 mark — Comparison of effects on variation across all three types.
• 1 mark — Evaluates disruptive selection’s speciation link, with the caveat that reproductive isolation is also required.
• 1 mark — Synthesised conclusion with precise lesson vocabulary.

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

Pre-industrial period: directional selection favoured pale moths because pale bark covered in lichens made dark moths more visible to bird predators. The distribution would shift toward pale colouration over time, with dark moths decreasing in frequency. [1 — pre-industrial selection type identified with justification]

Industrial period: directional selection reversed direction and favoured dark moths because soot-darkened bark made pale moths more visible. The graph would shift from pale-dominant to dark-dominant, with the mean colouration moving toward the darker end of the spectrum. This is a textbook example of directional selection. [1 — industrial period selection type and graph change described correctly]

Post-clean-air period: as pollution decreased and pale lichens returned to tree bark, the selection pressure reversed again — pale moths became better camouflaged and dark moths more visible. Directional selection now favoured pale moths again, and pale moths increased in frequency in urban areas. [1 — post-clean-air period analysis]

The selection pressure in the industrial period was bird predation on a now-darkened background [1]. This created directional selection because bird predation rate was highest for pale moths (visible against dark bark), giving dark moths a strong survival advantage. Darker moths survived more, reproduced more and passed the dark-colour alleles to more offspring, shifting allele frequencies in the population toward the darker extreme. [1 — selection pressure identified and mechanism of directional selection explained]

The reversal of the trend after clean-air legislation reveals several key principles about natural selection. First, natural selection is not permanent or goal-directed — it changes allele frequencies in response to current environmental conditions, not toward any predetermined endpoint. When the environment changed (lighter bark returned), the fitness landscape reversed and different phenotypes were favoured. [1 — evaluates what the reversal shows about natural selection (environment-dependent; not permanent)]

Second, the reversal shows that variation must have been maintained in the population even during the industrial period. If pale moths had been completely eliminated, they could not have recovered after pollution decreased. This shows that natural selection works on existing variation; it reduces but does not instantly eliminate disfavoured alleles. [1 — notes that variation was maintained, enabling reversal]

Third, this example powerfully illustrates that directional selection is not a one-time event — it operates continuously as long as a consistent selection pressure exists, and its direction can change if the selection pressure changes. [1 — generalises the lesson of the reversal to the ongoing nature of natural selection]

In conclusion, the peppered moth example demonstrates directional selection operating in both directions in response to changing selection pressures, showing that evolution is a dynamic, ongoing and reversible process driven by the current relationship between heritable variation and environment. [1 — synthesised conclusion]

Marking criteria (8 marks).

• 1 mark — Pre-industrial period: directional selection favouring pale moths, with justification.
• 1 mark — Industrial period: directional selection favouring dark moths; graph shift described correctly.
• 1 mark — Post-clean-air period: selection pressure reversal; pale moths favoured again.
• 1 mark — Selection pressure in industrial period identified (bird predation on dark bark); mechanism of directional selection explained.
• 1 mark — Evaluates what the reversal shows (natural selection is environment-dependent; not permanent or goal-directed).
• 1 mark — Notes that variation was maintained during industrial period (enabled recovery).
• 1 mark — Generalises lesson: directional selection is ongoing and direction-dependent on selection pressure.
• 1 mark — Synthesised conclusion using precise vocabulary.

Q3 — Sample Band 6 response (6 marks)

The claim is partly defensible but largely flawed. [1 — overall evaluative judgement]

What is defensible: it is true that stabilising selection tends to maintain the existing distribution around the intermediate phenotype, meaning the mean does not shift as dramatically as in directional selection or become bimodal as in disruptive selection. In environments that are stable and well-suited to the current intermediate phenotype, stabilising selection can appear conservative. [1 — concedes the defensible element]

What is flawed: “It does not change the population.” Stabilising selection actively changes allele frequencies over time by eliminating alleles associated with extreme phenotypes. This is a form of evolution — the frequency of alleles at both extremes decreases over generations as those phenotypes are selected against. The lesson’s misconceptions box explicitly states: “Stabilising selection maintains variation in the population — it removes the extremes but preserves the intermediate range. It does not eliminate variation entirely; it narrows the phenotypic distribution.” Stabilising selection does change the population by reducing variation; it is a real evolutionary process. [1 — refutes “does not change the population” with the lesson’s misconception correction]

“Only directional and disruptive selection actually drive evolution.” All three selection types change allele frequencies — the defining criterion for evolution. Stabilising selection drives allele frequencies associated with extreme phenotypes downward and maintains those linked to the intermediate. This is evolutionary change. Claiming it is not evolution conflates “change in direction” with “evolution,” which is incorrect. [1 — refutes exclusion of stabilising selection from evolution by restating the definition of evolution as allele frequency change]

Additionally, stabilising selection can be critical for population persistence in stable environments by maintaining the optimal phenotype, which is arguably more important than directional or disruptive selection in many ecosystems. The human birth-weight example shows how stabilising selection preserves fitness even in the absence of directional change. [1 — positive case for significance of stabilising selection with example]

Defensible reformulation: “Stabilising selection changes allele frequencies over time by selecting against both extreme phenotypes, narrowing the distribution around the intermediate. While it does not shift the mean or split the distribution, it is a genuine evolutionary process. Directional, stabilising and disruptive selection each drive evolutionary change in different ways and all are significant, depending on the environment and selection pressures present.” [1 — biologically defensible reformulation]

Marking criteria.

• 1 mark — States an overall evaluative judgement.
• 1 mark — Identifies the defensible element.
• 1 mark — Refutes “does not change the population” using the lesson’s misconception correction.
• 1 mark — Refutes exclusion from evolution by restating the definition of evolution as allele frequency change.
• 1 mark — Makes a positive case for the significance of stabilising selection with an example.
• 1 mark — Reformulates the claim into a biologically accurate statement.