Explain how the construction of a cladogram differs from classical Linnaean classification in the way that shared characteristics are used to group organisms.

IB DP Biology Higher Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Cladistics groups organisms on shared derived traits (synapomorphies) that arose in a common ancestor and are exclusive to its descendants, so only these characters define the branches of a cladogram. Classical Linnaean classification, by contrast, may group organisms on overall similarity or on ancestral traits (symplesiomorphies) that are not unique to a lineage. Linnaean ranks (kingdom, phylum, class, order, family, genus, species) are fixed and may not reflect true evolutionary relationships, whereas a cladogram shows the branching pattern of descent without assigning ranks. Finally, Linnaean groupings can be misled by convergent or analogous traits, whereas cladistics distinguishes homologous derived characters from analogous ones, giving a more accurate picture of common ancestry.

Examiner tips

  • Use the terms synapomorphy and symplesiomorphy; explain exclusivity of derived traits
  • Mention fixed ranks vs. unranked clades
  • Show how convergence is avoided in cladistics

Common mistakes

  • Confusing synapomorphy with symplesiomorphy
  • Assuming Linnaean ranks always reflect phylogeny
  • Overlooking that cladograms are unranked and purely evolutionary

Mark scheme (4 marks)

  1. Cladistics groups organisms based on shared derived characteristics (synapomorphies), whereas Linnaean classification may use overall similarity / shared ancestral characteristics (symplesiomorphies).
  2. In cladistics, only characteristics that arose in a common ancestor and are shared exclusively by its descendants (defining a clade) are used to determine branching points / nodes.
  3. Linnaean classification assigns organisms to hierarchical ranked taxa (kingdom, phylum, class, order, family, genus, species) based on morphological / physiological criteria that may not reflect evolutionary relationships, whereas cladograms represent evolutionary / phylogenetic relationships without requiring fixed ranks.
  4. Convergent / analogous characteristics can lead to incorrect groupings in classical classification, whereas cladistics avoids this by distinguishing homologous derived characters from analogous ones, making groups more reflective of true common ancestry.

Key terms in this question

cladogram · Linnaean classification

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