CourseCourseQ BankQuestion BankDocsDocuments

A3.2.4 (HL)—Clades

A clade contains a common ancestor and all descendants, identified through shared derived traits, molecular evidence, nodes, and cladogram structure.

Syllabus
First assessment 2025
Objective
A3.2.4
Level
HL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1

Common command terms

  • Define
  • Label
  • Identify
  • State
  • Explain
  • Outline

Scoring notes

Common mistake
Selecting a partial branch group as a clade even though it excludes descendants.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ122[ 1 ]A3.2.4 (HL)—Clades
May 2025Paper2 ["HL"] · TZ37(c)[ 1 ]A3.2.4 (HL)—Clades
May 2022Paper1 ["HL"] · TZ217[ 1 ]A3.2.4 (HL)—Clades
May 2017Paper3 ["HL"] · TZ19(b)[ 1 ]A3.2.4 (HL)—Clades
May 2016Paper2 ["HL"] · TZ04(a)[ 3 ]A3.2.4 (HL)—Clades
Practice this objective

Coverage 2014–2025 · Updated 15 Jul 2026

A Clade Contains an Ancestor and All Its Descendants

HL only

A clade is a group consisting of a common ancestor and all of its descendants.

Clade membership is supported by shared characteristics inherited from the common ancestor. Base sequences of genes and amino-acid sequences of proteins provide especially objective comparisons; homologous morphology can provide additional evidence.

To test a proposed clade: locate one ancestral node, include every branch descending from it, and identify shared derived evidence. Excluding a descendant makes the group paraphyletic; combining separate lineages by superficial similarity makes it polyphyletic.

If several taxa share a derived DNA sequence change absent from the outgroup, that change supports their placement in one clade when the simplest tree assigns it to their common ancestor.

A group of organisms with similar habitats or lifestyles may be an ecological category without being a clade.

Clades exam focus

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Define / Label / Identify.

Command terms

Define / Label / Identify / State / Explain / Outline

What earns marks

Build the answer around this relationship: A clade includes a common ancestor and all of its descendants.

Watch for

Selecting a partial branch group as a clade even though it excludes descendants.

Representative question

Question 1

[Maximum number: 3]

Outline the types of evidence that can be used to place a species in a particular clade.

Read And Defend A Cladogram

HL only

A3.2 exam answers are strongest when they sound like evidence arguments. Classification organizes diversity, but fixed ranks and morphology can mislead. Evolutionary classification should match phylogeny using monophyletic clades supported by synapomorphies. Molecular clocks estimate divergence time from calibrated sequence differences. Cladograms are built from aligned sequence data and interpreted by nodes, not tip positions. Cladistics can reclassify old taxa, and rRNA evidence supports the three-domain system.

  • Classification should reveal relationships, not just names.
  • Fixed ranks and convergence can mislead.
  • Monophyletic clades include an ancestor and all descendants and are supported by synapomorphies.
  • Molecular clocks need calibration and can vary in rate.
  • Cladograms are built from aligned sequences using computer analysis and parsimony.
  • Read relatedness from most recent common ancestors, not tip positions.
  • Figwort and three-domain examples show molecular evidence changing classification.

Concept essentials

  • A clade includes a common ancestor and all of its descendants.
  • Nodes represent common ancestors or divergence points.
  • Shared derived traits help identify clades.
  • Molecular and homologous anatomical evidence can support clade membership.
  • The closest relatives share the most recent common ancestor.