Selection Pressures & Population Change

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

The selection pressure is the colder climate, which is an abiotic (non-living) factor. [1, identifies and classifies the pressure]

A selection pressure does not change individual animals during their lives; instead it acts on the variation that already exists in the population. In this population, individuals already differ in fur thickness, and that variation is heritable. [1, pressure acts on existing variation]

In a colder climate, individuals with thicker fur lose less body heat, so they are more likely to survive the cold and live long enough to reproduce. Individuals with thin fur are more likely to die before reproducing. This is differential survival and reproduction. [1, describes differential survival] [1, links thicker fur to higher survival/reproduction]

Because fur thickness is heritable, the survivors pass the thick-fur trait to their offspring. [1, heritability of the favoured trait]

Over many generations, the proportion of thick-furred individuals in the population increases, so the average fur thickness of the population rises. [1, trait proportion increases over generations]

Population abundance may temporarily fall (because thin-furred individuals die), but if enough thick-furred individuals survive and reproduce, the population can recover; if no individuals could cope, the result would be local extinction. [1, states the effect on abundance, including possible local extinction]

Marking criteria.

• 1 mark Identifies the colder climate as the selection pressure and classifies it as abiotic.
• 1 mark States the pressure acts on existing heritable variation in the population (not on the individual).
• 1 mark Describes the mechanism of differential survival.
• 1 mark Links thicker fur to greater survival and reproductive success in the cold.
• 1 mark States the favoured trait is heritable and passed to offspring.
• 1 mark States the proportion of the favoured trait increases over generations.
• 1 mark States the effect on population abundance (may fall, recover, or reach local extinction).

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

A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing. It acts on the variation already present in a population so that, over generations, advantageous traits become more common and population abundance can rise or fall. [1, definition and general effect]

The cane toad population increased because it faced very weak selection pressures in Australia. It is toxic, so native predators that ate it were poisoned, meaning it had almost no effective predators. [1, weak/absent predation pressure on the toad] With abundant food and no control, it survived and reproduced at a very high rate, so its population and range expanded rapidly. [1, high survival and reproduction drives the increase]

At the same time, the toxic cane toad acts as a strong biotic selection pressure ON the native predators. Predators that attempt to eat the toad are poisoned and die, so their populations decline where the toad has spread. [1, toad is a biotic pressure that reduces predator populations]

The prickly pear population decreased because the introduced Cactoblastis moth acted as a strong biotic selection pressure on the cactus. Its larvae fed on and killed the cactus, so cactus cover collapsed by about 90% within 7 years before stabilising at a low level. [1, moth is a biotic pressure that reduces the cactus] [1, accurate description of the collapse and stabilisation]

The same mechanism produces opposite outcomes because the effect depends on the strength of the pressure on each population. A population facing little or no pressure (the toad) grows, while a population facing intense pressure (the cactus, or the toad's predators) shrinks. [1, links outcome to the strength of the pressure on each population]

In conclusion, a selection pressure can raise abundance where individuals survive and reproduce freely, and lower abundance where individuals are killed faster than they can reproduce; whether a population increases or decreases depends on which population the pressure is acting on. [1, justified conclusion using precise vocabulary]

Q3, Sample Band 6 response (6 marks)

The claim is partly defensible but overstated and unsafe as a general rule. [1, overall evaluative judgement]

What is defensible: biological control can be highly effective and self-sustaining. The Cactoblastis moth caused the prickly pear to collapse across millions of hectares and held it at a low, stable level without ongoing chemical spraying, and a well-chosen agent can be specific to the target pest. [1, concedes the genuine benefit with the prickly pear example]

What is flawed: the word “always.” A control organism is itself an introduced species, and it can become a worse pest or attack non-target native species. The cane toad was introduced in 1935 as a control agent for cane beetles, failed to control them, and instead spread uncontrollably and poisoned native predators. [1, refutes “always” using the cane toad failure] Outcomes can be hard to predict and effectively irreversible once an organism is released. [1, identifies unpredictability and irreversibility as risks]

Therefore releasing a control organism “whenever we have a pest” is irresponsible without testing. [1, links the flaw to the management recommendation]

Defensible reformulation: “Biological control can be a powerful and sustainable way to manage an invasive species, as the Cactoblastis moth showed with the prickly pear, but only when the control agent is carefully tested for host-specificity beforehand; used carelessly it can become a worse pest, as the cane toad did.” [1, biologically responsible reformulation]

Marking criteria.

• 1 mark States an overall evaluative judgement (partly defensible, overstated).
• 1 mark Identifies the defensible benefit using the prickly pear example.
• 1 mark Refutes “always” using the cane toad as a failed control agent.
• 1 mark Notes unpredictability and irreversibility as risks of releasing a living agent.
• 1 mark Links the flaw to the unsafe management recommendation.
• 1 mark Reformulates the claim into a biologically responsible statement (control can work but requires prior host-specificity testing).