# Explain why a taxon that is defined only by shared primitive characters (symplesiomorphies) is considered invalid in cladistic classification.

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

## Mark scheme (4 marks)

1. Symplesiomorphies are ancestral characters inherited from a distant common ancestor and therefore shared by many groups beyond the taxon in question.
2. A valid clade (monophyletic group) must be defined by shared derived characters (synapomorphies) that originated in the most recent common ancestor of the group.
3. A group defined only by symplesiomorphies would be paraphyletic or polyphyletic, excluding descendants that also inherited those characters or including distantly related taxa.
4. Cladistics requires groups to reflect true evolutionary (phylogenetic) relationships / common descent, so a grouping not supported by shared derived characters does not represent a real evolutionary lineage.

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