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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

Classify Species and Write Binomial Names

Morphology and naming rules.

The morphological species concept groups organisms as one species when they share a characteristic set of observable, inherited structural traits.

This original Linnaean approach is practical for field observations, preserved specimens and fossils. However, natural variation, life stages and sexual dimorphism can make members of one species look different, while unrelated species can look similar.

Binomial nomenclature gives every species a universal two-part name: the genus begins with a capital letter, the second word is lowercase, and both words are italicized when typed or underlined when handwritten. Species placed in the same genus share similar traits.

In Panthera leo and Panthera tigris, Panthera is the shared genus and the lowercase second words distinguish lion from tiger.

A shared appearance supports a morphological classification but does not by itself demonstrate interbreeding. The biological species concept uses reproductive evidence instead.

Binomial system

Assessment in practice

1–2 marks
How 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

[Maximum number: 4]

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.