HSC Exam Practice

Root: the oldest common ancestral lineage shown on the tree [1]. Node: a branching point representing divergence from a common ancestor — not a living species, but an ancestral population from which two lineages split [1]. Sister groups: two lineages sharing the most recent common ancestor on the tree [1].

Morphological evidence uses physical structures and body plans (e.g. homologous bone arrangement such as the pentadactyl limb) [1]. Molecular evidence uses DNA sequence comparison, protein comparison or mitochondrial DNA divergence (e.g. cytochrome c amino acid sequencing or BLAST alignment) [1]. The key difference: molecular evidence can reveal true relatedness hidden by convergent evolution — when morphological similarity arises from similar selective pressures rather than ancestry, molecular evidence distinguishes convergent similarity from shared ancestry [1]. Molecular evidence is more useful when organisms share similar physical features but may be unrelated, when morphology is absent or when the specimen is damaged or incomplete [1].

Parsimony is the principle that the most likely phylogenetic tree usually requires the fewest evolutionary changes to explain the observed shared traits [1]. It is used because multiple tree arrangements are often consistent with the same data; parsimony provides an objective criterion for selecting the most economical arrangement, avoiding unnecessary assumptions of extra independent evolutionary events [1].

A clade is a monophyletic group: it contains a specific common ancestor and every single one of its descendants, no more and no less [1]. A group that merely looks similar may include organisms that evolved the same features through convergent evolution without sharing a recent common ancestor. Requiring a common ancestor and all descendants ensures the group reflects actual evolutionary history rather than superficial similarity [1].

Visual similarity is insufficient because convergent evolution can independently produce similar-looking features in unrelated lineages under similar selective pressures — the similarity reflects environmental adaptation rather than common ancestry [1]. To confirm common ancestry, molecular evidence (DNA or protein sequence comparison) would be needed, as it reveals actual genetic relatedness independent of physical form [1].

(i) 2 marks. Taxon III and Taxon IV are sister groups [1]. They share the most recent common ancestor, represented by the rightmost node where their two branches directly diverge (the branching point closest to the tips of III and IV). This is determined by tracing each taxon back to the first shared branching point [1].

(ii) 3 marks. The outgroup O is most distantly related to Taxon I [1]. Starting from Taxon I and tracing back: Taxon I first shares a node with Taxon II, then that pair shares a node with the III+IV pair, then that entire group shares a node with the main ingroup. Finally, all of these share a node with O (the root node) [1]. Because O diverged earliest (from the root, the most ancient shared ancestor), it is most distantly related to all ingroup taxa including Taxon I [1].

(iii) 2 marks. A clade with exactly two taxa: Taxon III and Taxon IV [1]. Their shared node represents the ancestral population from which Taxon III and Taxon IV diverged — the last common ancestor they shared before their lineages split. This node is not a living species; it is a point in evolutionary history [1].

Marking criteria (1 mark each):

• States clearly that visual similarity alone is not sufficient (and explains why: convergent evolution).
• Explains convergent evolution: unrelated organisms can independently evolve similar appearances under similar selective pressures.
• Names a specific case from the lesson where molecular evidence changed the phylogenetic interpretation (whales and hippos; platypus; dolphins and sharks are all valid).
• Explains the role of morphological evidence in constructing trees (compares structures and body plans; useful when DNA is unavailable; captures anatomical and ecological context).
• Explains the role of molecular evidence in constructing trees (DNA comparison reveals true genetic relatedness; resolves convergent evolution ambiguity).
• Explains parsimony: choose the tree requiring the fewest evolutionary changes; provides an objective criterion when multiple arrangements are possible.
• Quality mark: coherent, well-structured argument using precise lesson terminology; conclusion follows logically from the analysis.