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A4.1.9 (HL)—Adaptive radiation

Adaptive radiation diversifies one ancestral lineage into multiple related forms adapted to different niches, often leaving homologous structures as evidence.

Syllabus
First assessment 2025
Objective
A4.1.9
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper2
Typical marks3

Common command terms

  • Outline

Scoring notes

Common mistake
Describing adaptive radiation as unrelated species becoming similar rather than related lineages diverging.

Recent exam appearances

May 2024Paper2 ["HL"] · TZ24(b)(i)[ 3 ]A4.1.9 (HL)—Adaptive radiation
November 2022Paper2 ["HL"] · TZ08(a)[ 3 ]A4.1.9 (HL)—Adaptive radiation
Practice this objective

Coverage 2022–2024 · Updated 15 Jul 2026

Adaptive Radiation Fills Many Ecological Roles

HL only

Adaptive radiation is rapid diversification of one ancestral lineage into multiple species adapted to different ecological niches.

When competitors are absent or new habitats become available, populations experience different selection pressures. Divergence in feeding, timing or habitat use can reduce gene flow and produce a cluster of related species.

Look for the combination:

  • one ancestral lineage
  • several ecological opportunities
  • divergent traits
  • reproductive separation

Finches colonizing islands may evolve different beaks for seeds, insects or nectar, with mating patterns that maintain the new lineages.

Many species in one area are not automatically an adaptive radiation; shared ancestry and niche divergence are required.

Adaptive radiation

HL only

Assessment in practice

3 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline.

Command terms

Outline

What earns marks

Build the answer around this relationship: Adaptive radiation produces multiple related species from one ancestral species.

Watch for

Describing adaptive radiation as unrelated species becoming similar rather than related lineages diverging.

Representative question

Question 1

[Maximum number: 5]

The human hand is an example of adaptive radiation. Outline adaptive radiation.

Routes, Niches, And Barriers

HL only

HL speciation questions usually ask you to choose the right layer. Route layer: allopatric means geographic isolation; sympatric means no spatial separation. Niche layer: adaptive radiation produces many related species from one ancestor as they occupy different niches, as in Darwin’s finches. Barrier layer: prezygotic barriers act before fertilization, postzygotic barriers act after hybrid formation. Chromosome layer: plant polyploidy can abruptly create reproductive isolation and fertile new lineages.

  • Allopatric speciation involves geographic isolation.
  • Sympatric speciation occurs without spatial separation.
  • Adaptive radiation produces many related species from one ancestor in different niches.
  • Prezygotic barriers prevent mating/fertilization; postzygotic barriers include infertile hybrids.
  • Polyploidy can cause abrupt plant speciation by chromosome-number change.

Concept essentials

  • Adaptive radiation produces multiple related species from one ancestral species.
  • Different niches impose different selection pressures on diverging groups.
  • Homologous structures can reveal adaptive radiation from common ancestry.
  • Niche specialization can reduce competition while promoting divergence.