HSC Exam Practice

Sample response. A selection pressure is an environmental factor that affects an organism's chance of surviving and reproducing.

Marking notes. 1 mark for a definition that links an environmental factor to survival and/or reproduction.

Sample response. A biotic selection pressure comes from a living component of the environment, for example predation, competition or disease. An abiotic selection pressure comes from a non-living physical or chemical condition, for example temperature, water availability or salinity. The key difference is living (biotic) versus non-living (abiotic).

Marking notes. 1 mark for the living/non-living distinction; 1 mark for a valid biotic example; 1 mark for a valid abiotic example.

Sample response. A selection pressure does not change an individual lizard's body during its lifetime. Variation in water-storing ability already exists in the population. During the drought, lizards that happen to store water better are more likely to survive and reproduce (differential survival), and because the trait is heritable, the proportion of water-storing lizards increases over generations. The population changes, not the individual.

Marking notes. 1 mark for stating the variation is pre-existing and the individual does not change its own genes. 1 mark for explaining the change through differential survival and inheritance over generations.

Sample response. Biological control is the use of a living organism (such as a predator, parasite or disease) to reduce a pest population [1]. Example: the Cactoblastis moth was released to control the prickly pear cactus [1].

Marking notes. 1 mark for the definition (living organism used to reduce a pest); 1 mark for a valid example from the lesson (Cactoblastis moth / prickly pear; cane toad was an attempted control agent).

Sample response. The selection pressure is the Cactoblastis moth, specifically its larvae feeding on the cactus [1]. It is a biotic (living) pressure [1].

Marking notes. 1 mark for identifying the moth/larvae as the pressure; 1 mark for classifying it as biotic.

Sample response. After the moth is released, prickly pear cover falls rapidly (around 90% is destroyed within about 7 years) [1], then levels off and remains at a low, stable amount [1].

Marking notes. 1 mark for describing the rapid decline; 1 mark for noting the levelling off at a low, stable level.

Sample response. As the cactus is destroyed, the moth's food supply runs low [1]. With less food, the moth population also declines, so the feeding pressure on the remaining cactus eases [1]. This allows a small prickly pear population to survive and persist at a low, stable level rather than being driven to complete local extinction [1].

Marking notes. 1 mark for noting the moth's food runs low; 1 mark for the moth population declining and the pressure easing; 1 mark for linking this to survival of a small, stable cactus population.

Sample response. An introduced species can act as a selection pressure, an environmental factor affecting survival and reproduction, on both itself and the native species around it. The cane toad (Rhinella marina) was introduced to Queensland in 1935 to control cane beetles. It failed to control the beetles but bred and spread rapidly across northern Australia.

The cane toad population increased because it faced very weak selection pressures in Australia: it is toxic, so it had almost no effective predators, and with abundant food it survived and reproduced at a very high rate, expanding its population and range.

At the same time, the toxic cane toad acts as a strong biotic selection pressure on native predators such as quolls, goannas and snakes. Predators that attempt to eat the toad are poisoned and die. Through differential survival, individuals unable to avoid eating toads suffer high mortality, so the populations of several native predators have declined where the toad has spread. In some predator populations, individuals that avoid eating toads are now favoured, an emerging selective response.

Over time, therefore, the same mechanism, a selection pressure acting on a population, produced an increase in the cane toad population and a decrease in affected native predator populations, because the strength of the pressure differed for each population.

Marking criteria.

• 1 mark States the cane toad was introduced (1935) and spread rapidly.
• 1 mark Explains the toad population increased due to weak/absent predation pressure (toxicity, no effective predators).
• 1 mark Links high survival and reproduction to the population increase.
• 1 mark Identifies the toad as a biotic selection pressure ON native predators.
• 1 mark Explains predators are poisoned and die when they eat the toad.
• 1 mark Uses differential survival to explain the decline in predator populations.
• 1 mark States the change occurs over time / over generations.
• 1 mark Reaches a clear conclusion that one mechanism produced opposite outcomes for the two populations, using precise vocabulary.