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A3.1.15 (HL)—Environmental DNA barcodes

DNA barcoding identifies species using short diagnostic DNA sequences, including sequences recovered from environmental DNA samples in habitats and ecosystems.

Syllabus
First assessment 2025
Objective
A3.1.15
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1

Recent exam appearances

May 2025Paper1A ["HL"] · TZ322[ 1 ]A3.1.15 (HL)—Environmental DNA barcodes
Practice this objective

Coverage 2025–2025 · Updated 15 Jul 2026

Dichotomous keys and DNA identification

HL only
A split workflow visual. Left side: a branching dichotomous key built from observable local traits. Right side: an eDNA barcoding pipeline showing environmental sample, PCR amplification, barcode sequence, and database match.

Choose the identification tool that matches the available evidence.

  • Dichotomous key: repeatedly choose between paired statements based on observable traits. Use reliable, relatively immutable features such as structures or reproductive processes; size, colour and behaviour may vary with age or environment. Each choice should lead to one narrower group until the specimen is identified.
  • DNA barcoding: amplify and sequence a short standard region of DNA from a specimen, then compare it with a reference database to identify the species.
  • Environmental DNA (eDNA): collect DNA traces from water, soil or another habitat, amplify barcode regions by PCR and match sequences to a database; it can detect organisms that are not directly observed.

Keys are useful when visible features are intact and reliable. Barcoding or eDNA is more useful when specimens are damaged, partial, cryptic or absent. Local plant or animal species can be used to build and test a key, but a database match is evidence of sequence similarity, not automatically proof that every individual in the habitat was sampled.

Environmental DNA barcodes

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice.

What earns marks

Build the answer around this relationship: DNA barcodes are short diagnostic sequences used for species identification.

Representative question

Question 1

[Maximum number: 1]

What is one application of DNA barcoding?

A

Selective breeding

B

Preserving an endangered species

C

Analysing environmental DNA

D

Sequencing a genome

Boundaries And Identification

HL only

The HL extension connects two problems: species boundaries and species identification. Bacteria challenge the biological species concept because they are asexual and exchange genes horizontally. Chromosome number can help define boundaries in many sexual species because meiosis conserves it and mismatches can produce infertile hybrids. Dichotomous keys identify visible organisms using paired choices. DNA barcoding and eDNA identify species using short sequences, PCR, and databases when visible traits are limited.

  • Bacterial boundaries are blurred by mutation and horizontal gene transfer.
  • Chromosome number is usually conserved within sexual species by meiosis.
  • Different chromosome numbers can prevent fertile hybrids, as in mules.
  • Dichotomous keys use observable traits and paired choices.
  • DNA barcoding/eDNA use PCR and database comparison for damaged, partial, hidden, or difficult specimens.
  • Local plant or animal species can be used to build and test dichotomous keys.

Concept essentials

  • DNA barcodes are short diagnostic sequences used for species identification.
  • Environmental DNA can reveal organisms present in a habitat.
  • PCR and database comparison are central to barcode-based identification.
  • DNA barcoding is different from sequencing an entire genome.