Explain how a clade differs from a traditional taxonomic group, and why paraphyletic groups are not accepted in cladistic classification.

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)

A clade is a monophyletic group that contains an ancestor and ALL of its descendants, whereas a traditional taxonomic group can be based on overall similarity or shared morphological traits and may not reflect true evolutionary relationships.

A paraphyletic group contains a common ancestor but only SOME of its descendants. In cladistics this is unacceptable because it does not represent a complete, natural evolutionary lineage and therefore gives an inaccurate picture of phylogeny.

Examiner tips

Use the term ‘monophyletic’ to describe a clade; mention ancestor + all descendants. Explain that traditional groups rely on similarity, not phylogeny. State that paraphyletic groups omit some descendants and are rejected because they misrepresent evolutionary relationships.

Common mistakes

Confusing a clade with a traditional group by saying both include all descendants. Forgetting to mention that traditional groups are based on similarity rather than ancestry. Saying paraphyletic groups are acceptable because they are easier to define.

Mark scheme (4 marks)

A clade includes an ancestor and ALL of its descendants
Traditional taxonomic groups can be defined by overall similarity or shared morphological features, and may not reflect true evolutionary relationships
A paraphyletic group includes a common ancestor and SOME but not all of its descendants
Paraphyletic groups are rejected in cladistics because they do not represent a complete/natural evolutionary lineage, making the classification inaccurate in reflecting phylogeny

Key terms in this question

clade · paraphyletic