HSC Exam Practice

Homologous structures share the same underlying anatomical origin but may perform different functions; evidence of common ancestry (e.g. pentadactyl limb: human arm and whale flipper) [1]. Analogous structures perform the same function but have different evolutionary origins, produced by convergent evolution (e.g. bird wing and insect wing, or shark body and dolphin body) [1]. Clear distinguishing statement: homologous = same origin, analogous = same function [1].

The pentadactyl limb shows the same underlying bone arrangement (humerus, radius, ulna, carpals and phalanges) in the forelimbs of all vertebrates [1]. For example, a human arm and a whale flipper share this arrangement despite performing very different functions — manipulation versus swimming [1]. This shared structural plan implies inheritance from a common vertebrate ancestor; each lineage then modified the plan for different functions through divergent evolution, rather than each evolving the same bone arrangement independently [1].

A vestigial structure is a reduced, non-functional remnant of a structure that was functional in ancestors [1 for definition]. Example 1: human coccyx — implies humans descended from ancestors with functional tails; retained as a reduced remnant [1]. Example 2: whale pelvic bones (or python pelvic spurs) — whale pelvic bones imply whales descended from land-dwelling ancestors with functional hind limbs; python spurs imply descent from limbed ancestors [1 for each, up to 2 marks for examples with implications].

Early embryos of fish, reptiles, birds and mammals share structural features including pharyngeal arches (transient gill-arch-like structures) and a tail [1]. These features appear across very different adult forms, suggesting they are inherited from a common vertebrate ancestor [1]. The shared early developmental patterns indicate conserved developmental pathways — similar genetic instructions for early development were inherited and retained across all these groups, consistent with descent from a shared ancestor [1].

Convergent evolution produces analogous structures — similar-looking features in unrelated lineages responding to the same selective pressures [1]. If classification is based on physical appearance alone, convergent organisms could be grouped as close relatives when in fact their similarity reflects independent adaptation, not shared ancestry; this would produce an incorrect phylogeny [1].

(i) 2 marks. Humans, bats, whales and horses all possess the same set of bones (humerus, radius, ulna, carpals, metacarpals and phalanges) in their forelimbs, though the proportions and specialisation of individual bones differ [1]. These are homologous structures [1].

(ii) 3 marks. Bat forelimbs contain the same vertebrate bone arrangement as other mammals (humerus, radius, ulna, carpals and elongated digit bones) — they share a mammalian ancestral forelimb that was modified for flight [1]. Butterfly wings have no such bone arrangement at all; they are composed of chitinous veins, not modified vertebrate limb bones [1]. The internal structures are fundamentally different, demonstrating that bat wings and butterfly wings are analogous — independently evolved for flight under similar selection pressures, not inherited from a common ancestor [1].

(iii) 2 marks. Conclusion: humans, bats, whales and horses all descended from a common vertebrate ancestor that possessed the pentadactyl limb plan [1]. Justification: the shared bone arrangement is most parsimoniously explained by inheritance from this common ancestor rather than by independent, identical evolution of the same complex bone arrangement in four separate lineages [1].

Marking criteria (1 mark each):

• Identifies and explains homologous structures as strong evidence for common ancestry (same underlying anatomy, different function).
• Explains that analogous structures are a limitation of appearance-based classification because they reflect convergent evolution, not common ancestry.
• Explains vestigial structures as evidence for descent with modification from ancestral body plans (with a named example).
• Explains comparative embryology as evidence of shared developmental pathways and common ancestry.
• Explains when anatomical evidence is insufficient and why molecular evidence is needed (to distinguish convergent analogy from homology).
• Reaches an explicit evaluative judgement: physical appearance is useful but insufficient on its own; it must be supplemented by structural, developmental and molecular evidence.
• Quality mark: coherent, precise terminology, logical argument.