Explain how cladistics differs from traditional Linnaean classification in the way organisms are grouped together.

IB DP Biology Standard 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 according to shared derived traits (synapomorphies) that indicate common ancestry, so the groups are monophyletic.

Linnaean classification groups organisms mainly on overall morphological similarity, which can combine species that are not closely related.

Because cladistics uses only true evolutionary relationships, it avoids paraphyletic or polyphyletic groups that can arise in Linnaean systems.

Cladistics relies on both morphological and molecular (DNA) data to build cladograms, giving a more objective, evidence‑based classification.

Examiner tips

Use the term "synapomorphies" and "monophyletic" to show understanding of cladistics. Explain the difference in grouping criteria (evolutionary vs. overall similarity). Mention the avoidance of paraphyletic/polyphyletic groups. Show that cladistics uses molecular data for objectivity.

Common mistakes

Confusing cladistics with phylogenetic trees without mentioning monophyly. Using the word "convergent evolution" incorrectly. Failing to note that Linnaean groups can be paraphyletic or polyphyletic.

Mark scheme (4 marks)

Cladistics groups organisms based on shared derived characteristics (synapomorphies) / evolutionary relationships, whereas Linnaean classification groups organisms based on overall morphological / physical similarity.
Cladistics produces clades, which include an ancestor and all of its descendants (monophyletic groups), ensuring groups reflect true evolutionary history.
Linnaean classification can produce paraphyletic or polyphyletic groups that do not accurately reflect evolutionary relationships / may group unrelated organisms together based on convergent evolution.
Cladistics uses molecular / genetic / DNA sequence data as well as morphological evidence to construct cladograms, making the classification more objective and evidence-based.

Key terms in this question

cladistics · Linnaean classification