Explain how the kingdom Protista is considered paraphyletic and discuss the implications of this for its status as a valid taxonomic group.

IB DP Biology Higher Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Protista is paraphyletic because it contains a common ancestor but excludes some of its descendants.

It is an artificial or convenience grouping that mixes phylogenetically unrelated or distantly related lineages.

A valid clade (monophyletic group) must include a common ancestor and all of its descendants; Protista fails to satisfy this.

Therefore Protista should be dissolved or replaced by several monophyletic groups, and it is no longer regarded as a valid kingdom in modern phylogenetic classification.

Examiner tips

Use the word ‘paraphyletic’ and explain the ancestor–descendant rule; mention that Protista lacks some descendants.
Show that it is an artificial grouping of unrelated lineages; link this to the need for monophyletic clades.
State the implication clearly: dissolution or replacement by monophyletic groups.
Keep the answer concise – 4 points, one sentence per point.

Common mistakes

Confusing ‘paraphyletic’ with ‘polyphyletic’; writing that Protista is monophyletic.
Failing to mention that Protista excludes descendants, e.g. animals, fungi, plants.
Giving extra detail about specific protist groups that distracts from the key points.

Mark scheme (4 marks)

Protista is paraphyletic because it includes a common ancestor but does not include all of its descendants
Protista is an artificial/convenience grouping that contains phylogenetically unrelated or distantly related lineages
A valid clade (monophyletic group) must include a common ancestor and ALL of its descendants, which Protista fails to satisfy
The implication is that Protista should be dissolved/replaced by multiple monophyletic groups, or at least that it is no longer considered a valid kingdom in phylogenetic classification systems

Key terms in this question

paraphyletic · Protista · kingdom