A3.1.3—Binomial system
Binomial nomenclature gives each species a universal two-part scientific name that identifies genus first and species second in a standard format.
- Syllabus
- First assessment 2025
- Objective
- A3.1.3
- Level
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–3
Common command terms
- Outline
- Identify
- State
- Suggest
Scoring notes
- Common mistake
- Using the second word of a binomial to infer closest relatedness when the genus differs.
Recent exam appearances
May 2025Paper1B ["HL"] · TZ31(a)[ 1 ]A3.1.3—Binomial system
November 2023Paper2 ["HL"] · TZ28(c)[ 3 ]A3.1.3—Binomial system
November 2023Paper2 ["HL"] · TZ18(c)[ 3 ]A3.1.3—Binomial system
May 2021Paper2 ["HL"] · TZ23(c)[ 1 ]A3.1.3—Binomial system
Practice this objective
Coverage 2021–2025 · Updated 15 Jul 2026
Species boundaries and binomial names
A species is a group of organisms that share characteristic inherited features and, in the biological species concept, can normally interbreed to produce fertile offspring. Variation is expected within that boundary, so appearance alone is not a complete test.
Biologists use a binomial name to identify a species precisely: the genus starts with a capital letter, the species name is lower case, and the two words are italicised (for example, Homo sapiens). This avoids the ambiguity of common names and links an organism to a classification system.
When deciding whether a description supports one species, separate the evidence: compare morphology or DNA, consider reproductive compatibility and geographic context, and state which species concept is being used. Closely similar organisms may be different species, while visibly different populations may still exchange genes. Hybridisation, asexual reproduction and fossil organisms show why the biological species concept is useful but not universal.
Binomial system
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using Outline / Identify / State.
Command terms
Outline / Identify / State / Suggest
What earns marks
Build the answer around this relationship: The genus is the first word of a binomial name.
Watch for
Using the second word of a binomial to infer closest relatedness when the genus differs.
Representative question
Question 1
Outline the binomial system of classification.
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
- The genus is the first word of a binomial name.
- The specific epithet is the second word and is not a species name by itself.
- Species sharing a genus are usually more closely related than species in different genera.
- Scientific names reduce ambiguity caused by local common names.