HSC Exam Practice

Sample response. Divergent evolution is when related species descended from a common ancestor become increasingly different over time as they adapt to different environments or niches. Convergent evolution is when unrelated species independently evolve similar features because they live in similar environments and face similar selection pressures.

Marking notes. 1 mark for each correct definition. Key idea for divergent: common ancestor, becoming more different. Key idea for convergent: unrelated, becoming more similar.

Sample response. Homologous structures have the same evolutionary origin but possibly different functions, for example, the pentadactyl limb in humans, whales, bats and cats. Analogous structures have similar function but different evolutionary origins, for example, the wings of a bird and the wings of an insect (or the streamlined bodies of sharks and dolphins). Homologous structures are evidence of divergence (shared ancestry); analogous structures are evidence of convergence.

Marking notes. 1 mark for correctly defining homologous with an example. 1 mark for correctly defining analogous with an example. 1 mark for clearly distinguishing them (same origin vs different origin / divergence vs convergence).

Sample response. Adaptive radiation is the rapid divergence of one ancestral species into many species that fill different ecological niches. For example, Darwin's finches diverged from one ancestral finch into many species with different beaks suited to different foods; or about 500 cichlid species formed from one ancestor in Lake Victoria.

Marking notes. 1 mark for the concept (rapid divergence of one ancestor into many species filling different niches). 1 mark for a named example from the lesson (Darwin's finches; Lake Victoria cichlids; Australian marsupials).

Sample response. (i) Sharks and dolphins, convergent. (ii) Darwin's finch beaks, divergent. (iii) Bird and insect wings, convergent. Justification for one: sharks and dolphins are convergent because they are unrelated (a fish and a mammal) yet independently evolved similar streamlined bodies in response to the same selection pressure (fast swimming in water), so they have become more similar despite not sharing recent ancestry.

Marking notes. 1 mark for all three classified correctly. 1 mark for a valid justification of one classification referencing related/unrelated origin and similar/different selection pressures.

Sample response. Gradualism proposes that evolutionary change is slow, steady and continuous over long periods. Punctuated equilibrium proposes that species remain largely unchanged for long periods (stasis), with change concentrated in short, rapid bursts, often linked to a sudden change in selection pressure.

Marking notes. 1 mark for describing gradualism (slow, steady, continuous change). 1 mark for describing punctuated equilibrium (long stasis interrupted by short rapid bursts).

Sample response. Lineage P best supports punctuated equilibrium and Lineage Q best supports gradualism [1]. Justification (Lineage P): the fossil record shows a long period of stasis (about 9 million years of almost no change) followed by a short, rapid burst of change in which a new form appears, which is the characteristic pattern of punctuated equilibrium [1].

Marking notes. 1 mark for correctly assigning both lineages. 1 mark for a valid justification of one lineage that references the pattern (stasis then rapid change, or slow continuous change).

Sample response. A sudden change in selection pressure could trigger the rapid burst of change [1]. Examples include a rapid environmental or climate change, a new predator or competitor, a new food source, or movement into a new habitat with open niches, any event that changes which traits are favoured by natural selection [1].

Marking notes. 1 mark for identifying a sudden change in selection pressure / environmental change as the trigger. 1 mark for a relevant specific example.

Sample response. These two lineages show two different patterns at the same time: Lineage P shows long stasis then a rapid burst (punctuated equilibrium), while Lineage Q shows slow continuous change (gradualism) [1]. Because different lineages can show different patterns, the fossil record provides evidence for both models [1]. The two models therefore describe different tempos of the same underlying process of evolution by natural selection, rather than competing theories where only one can be true, so they are not mutually exclusive [1].

Marking notes. 1 mark for noting both patterns occur (one lineage each). 1 mark for stating the fossil record supports both. 1 mark for explaining that both describe the same process at different tempos, so they are not mutually exclusive.

Sample response. The claim that similar features always indicate a close evolutionary relationship is incorrect. Divergent evolution is when related species from a common ancestor become increasingly different, producing homologous structures (same origin, possibly different function). Convergent evolution is when unrelated species independently evolve similar features in similar environments, producing analogous structures (different origin, similar function).

Because of convergent evolution, similar appearance does not prove close relationship. For example, sharks (a fish, Class Chondrichthyes) and dolphins (a mammal, Class Mammalia) both have streamlined, torpedo-shaped bodies, yet they belong to completely different vertebrate classes. The similarity arose because both are active aquatic predators facing the same selection pressure, efficient movement through water, so each lineage independently evolved a streamlined body. These are analogous structures, not evidence of recent shared ancestry.

A further example is the wings of birds, bats and insects: all are used for flight but evolved from different origins, so they too are analogous. By contrast, true evidence of close relationship comes from homologous structures, such as the pentadactyl limb shared by humans, whales, bats and cats, which have the same underlying bone structure even where their functions differ.

Appearance alone is therefore a weak basis for inferring evolutionary relatedness, because similar environments can repeatedly select for similar phenotypes in unrelated organisms. Reliable conclusions about relatedness require homologous structures and ideally agreement with other evidence such as molecular (DNA) data.

The claim is false: convergent evolution shows that similar features can arise independently in unrelated organisms, so similarity of appearance does not automatically indicate close ancestry.

Marking criteria.

• 1 mark Correct definition of divergent evolution.
• 1 mark Correct definition of convergent evolution.
• 1 mark Distinguishes homologous from analogous structures.
• 1 mark Names at least one specific example of convergence (sharks/dolphins; bird/bat/insect wings).
• 1 mark Identifies the selection pressure for the named example (aquatic locomotion / flight).
• 1 mark Explains that homologous structures (not appearance) are the correct evidence of relatedness, with an example.
• 1 mark Evaluates the strength of appearance alone as evidence (weak; needs homology / molecular data).
• 1 mark Reaches an explicit evaluative conclusion that the claim is false, using precise vocabulary (convergent evolution, analogous, homologous, selection pressure).