Modern Examples of Evolutionary Change

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

The 60-fold rise in MRSA frequency is a clear example of evolution by natural selection acting on a bacterial population. [1, frames the example as natural selection]

The heritable variation comes from random mutations that occur during DNA replication. Resistance mutations arise by chance and already exist at low frequency in the bacterial population before any antibiotic is ever used. They are heritable because, when a resistant bacterium divides, the resistance gene is copied into its offspring. [1, identifies the source of variation as random, pre-existing, heritable mutation]

The selection pressure is the antibiotic. When the antibiotic is applied, it kills the susceptible bacteria, which make up the great majority of the population, but the rare bacteria carrying a resistance mutation survive because the mutation disrupts the drug's mechanism of action. This is differential survival: survival depends on whether an individual carries the advantageous allele. [1, identifies the selection pressure and the differential survival it produces]

The antibiotic selects for resistance rather than causing it. It does not damage DNA to create the resistance mutation. The mutation must already be present for any bacterium to survive. The antibiotic simply removes the susceptible competitors, allowing the pre-existing resistant variants to reproduce and increase in frequency. [1, clear distinction between selecting and causing]

The surviving resistant bacteria reproduce and pass the resistance gene to their offspring, so over successive generations the frequency of the resistance gene rises. In hospitals, continued antibiotic use and horizontal gene transfer (bacteria sharing resistance genes directly) drove multi-drug resistance, producing MRSA. [1, links reproduction of survivors to the rise in allele frequency, with MRSA detail]

Bacteria evolve resistance very rapidly because their generation time is only about 20 minutes, so thousands of generations pass in a single year. Combined with an intense selection pressure (an antibiotic that kills all non-resistant cells) and the existence of heritable variation, this allows measurable evolutionary change within months rather than millions of years. [1, explains rapid evolution via short generation time + intense pressure + variation]

Overall, the MRSA data is direct evidence that natural selection is producing measurable change in real time: a heritable, advantageous trait increased in frequency because the individuals carrying it survived and reproduced more successfully under a strong selection pressure. [1, synthesises into an explicit evidence statement using precise vocabulary]

Marking criteria.

• 1 mark Frames the rise of MRSA as evolution by natural selection.
• 1 mark Identifies the heritable variation as random, pre-existing, heritable mutation.
• 1 mark Identifies the selection pressure (antibiotic) and the differential survival it causes.
• 1 mark Distinguishes clearly between the antibiotic selecting for resistance and causing it.
• 1 mark Links reproduction of survivors to the increase in resistance-gene frequency.
• 1 mark Explains rapid evolution using short generation time, intense pressure and existing variation.
• 1 mark Uses precise vocabulary throughout (heritable variation, selection pressure, differential survival, allele frequency).

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

The cane toad invasion is called a real-time evolution experiment because measurable evolutionary change can be observed in both the toads and their predators within a human lifetime. [1, explains why it is a real-time experiment]

Change in the toads. There is heritable variation in leg length within toad populations. At the invasion front, the selection pressure is the strong advantage of dispersal: toads with longer legs travel further per day and reach unoccupied territory with more food and fewer competitors. These longer-legged toads survive and reproduce more, so over successive generations the average leg length at the invasion front increases. [1, toad variation, selection pressure and outcome]

Change in the predators. Native predators such as some freshwater crocodiles and snakes show heritable variation in sensitivity to bufotoxin. The selection pressure is the toad toxin itself: predators that eat toads and happen to carry alleles reducing toxin sensitivity survive, while more sensitive individuals die. Surviving resistant predators reproduce, so the frequency of toxin-resistance alleles increases in high-exposure predator populations over the decades since the toads arrived. [1, predator variation, selection pressure and outcome]

Both changes illustrate natural selection acting on pre-existing heritable variation. Neither population was "designed" to respond to cane toads. In each case, individuals carrying an already-present advantageous variant (longer legs, or lower toxin sensitivity) survived and reproduced more than others, increasing the frequency of that variant over generations, exactly Darwin's mechanism. [1, links both to natural selection on existing variation]

Evaluating eradication. Eradicating cane toads would not immediately reverse the toxin resistance built up in predators. Removing the toads removes the selection pressure (the toxin), but it does not delete the resistance alleles already present in surviving predators, those individuals keep and pass on the gene. [1, recognises removing pressure does not delete alleles]

Resistance frequency would only decline slowly over many generations, and only if carrying the resistance trait imposed some fitness cost when toads were absent; if it were cost-free, the frequency could simply remain stable. [1, qualifies the prediction with the idea of a fitness cost]

In conclusion, the cane toad invasion is strong evidence for evolution because both the introduced species and the native fauna have changed measurably and predictably through natural selection on existing variation; the change is gradual at the population level and cannot be instantly undone by removing the selection pressure. [1, justified evaluative conclusion]

Marking criteria (8 marks).

• 1 mark Explains why this is a real-time / observable evolution experiment.
• 1 mark Toad change: variation in leg length; selection pressure of dispersal advantage; outcome.
• 1 mark Predator change: variation in toxin sensitivity; selection pressure of toad toxin; outcome.
• 1 mark Links both changes to natural selection acting on pre-existing heritable variation.
• 1 mark Recognises that eradication removes the selection pressure but does not delete existing alleles.
• 1 mark Qualifies the eradication prediction with the idea of a fitness cost / slow decline over generations.
• 1 mark Reaches a justified evaluative conclusion.
• 1 mark Uses precise lesson vocabulary throughout (heritable variation, selection pressure, differential survival, allele frequency).

Q3, Sample Band 6 response (6 marks)

The claim is partly defensible but contains a serious flaw. [1, overall evaluative judgement]

What is defensible: the peppered moth is genuinely strong evidence for natural selection, and it is true that the frequency of the dark form rose dramatically (to about 90%) once industrial pollution arrived. The example is well measured and the change is clearly linked to a specific, identified selection pressure (bird predation on poorly camouflaged moths). [1, concedes the defensible element with detail]

What is flawed: the claim that "dark moths appeared suddenly" and that "the environment can directly create new useful traits when they are needed" is incorrect. Both the pale and the dark (melanic) alleles already existed in the population before industrialisation, the dark form was simply rare (~1%) because dark moths were visible on pale lichen-covered trees and were eaten by birds. Pollution did not create the dark allele; it changed which form was better camouflaged, shifting the selection pressure so that the already-present dark allele became advantageous and rose in frequency. [1, refutes the "created on demand" idea by noting both alleles pre-existed]

The idea that an environment creates traits "because they are needed" is a Lamarckian misconception. Natural selection works on variation that already exists; it does not generate new advantageous variants in response to need. [1, names and rejects the Lamarckian misconception]

What actually makes the peppered moth so compelling is the part the claim omits: when the Clean Air Act reduced pollution and lichens returned, the selection pressure reversed and the pale form recovered. The same population responded predictably to both the imposition and the removal of the selection pressure, which is very strong evidence that selection, not chance or need, is driving the change. [1, identifies the reversal as the true strength of the evidence]

Defensible reformulation: “The peppered moth is strong evidence for natural selection because the frequency of the pre-existing dark allele rose when soot-blackened trees made dark moths better camouflaged, then fell again after the Clean Air Act restored pale lichen. This predictable, reversible change in allele frequency in response to an identified selection pressure shows selection acting on existing variation, not the environment creating new traits on demand.” [1, biologically defensible reformulation]

Marking criteria.

• 1 mark States an overall evaluative judgement (partly defensible but flawed).
• 1 mark Identifies the defensible element (well-measured rise in dark form linked to a real selection pressure).
• 1 mark Refutes the "trait created on demand" idea by explaining both alleles pre-existed.
• 1 mark Names and rejects the Lamarckian misconception (need does not create variation).
• 1 mark Identifies the reversal after the Clean Air Act as the true strength of the evidence.
• 1 mark Reformulates the claim into a biologically accurate statement.