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A3.1.4—Biological species concept

The biological species concept defines species by reproductive compatibility and fertile offspring, while morphology, genetics, and exceptions complicate boundaries in practice.

Syllabus
First assessment 2025
Objective
A3.1.4
Level
SL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–3

Common command terms

  • Discuss
  • Outline
  • State
  • Define

Scoring notes

Common mistake
Using interbreeding alone without stating that offspring must be fertile.

Recent exam appearances

November 2025Paper1A ["SL"] · TZ313[ 1 ]A3.1.4—Biological species concept
May 2025Paper2 ["SL"] · TZ22(a)[ 1 ]A3.1.4—Biological species concept
May 2025Paper1A ["SL"] · TZ314[ 1 ]A3.1.4—Biological species concept
November 2019Paper2 ["SL"] · TZ01(a)[ 1 ]A3.1.4—Biological species concept
November 2016Paper2 ["SL"] · TZ04(c)[ 2 ]A3.1.4—Biological species concept
Practice this objective

Coverage 2012–2025 · Updated 15 Jul 2026

Biological species concept and limits

Bacterial horizontal gene transfer by transduction, transformation and conjugation.

The biological species concept defines a species as organisms that can interbreed and produce fertile, viable offspring. Their shared reproductive potential allows gene flow within the species; different species are expected to be reproductively isolated.

The definition has limits:

  • It cannot be tested directly for fossils or populations that are geographically isolated.
  • It is difficult to apply to asexual organisms and bacteria, which do not form species by interbreeding and may exchange genes horizontally.
  • Closely related species may hybridize, but the hybrid may be infertile.
  • Ring species can show gene flow between neighbouring populations even when the two end populations cannot interbreed.
  • Speciation is gradual: isolated populations may diverge for a long time before a clear boundary appears.

Therefore, “same species” is a useful reproductive hypothesis, not a perfect rule for every organism. In a question, state the definition first, then name the specific limitation shown by the evidence.

Biological species concept

Assessment in practice

1–3 marks
How it is assessed

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

Command terms

Discuss / Outline / State / Define

What earns marks

Build the answer around this relationship: Members of the same biological species can produce fertile offspring.

Watch for

Using interbreeding alone without stating that offspring must be fertile.

Representative question

Question 1

[Maximum number: 3]

Outline the criteria that should be used to assess whether a group of organisms is a species.

SL Transfer: Classify With Evidence

For SL, A3.1 is really one evidence map. Variation explains why individuals differ. Morphology and binomial nomenclature help group and name organisms. The biological species concept uses interbreeding and fertile offspring, but boundaries can be difficult during gradual speciation. Chromosomes and karyograms add cellular evidence. Genomes, SNPs, genome size, and whole genome sequencing add molecular evidence. The skill is choosing the right evidence for the question.

  • Variation supports natural selection.
  • Morphology groups by structure; binomial nomenclature names species universally.
  • Biological species concept uses interbreeding and fertile offspring.
  • Karyograms compare chromosome number and structure.
  • Genome evidence includes SNPs, between-species differences, genome size, and sequencing uses.

Concept essentials

  • Members of the same biological species can produce fertile offspring.
  • Gene flow helps maintain genetic similarity within a species.
  • Hybridisation can make species boundaries difficult to draw.
  • Asexual organisms, fossils, and isolated populations limit the biological species concept.
  • Morphological and genetic evidence can support species classification when breeding evidence is unavailable.
ConceptIB Biology SL