State that organisms can be classified into groups by the features that they share.

1.2 Concept and Uses of Classification Systems

Learning Objective

State that organisms can be classified into groups by the features that they share.

Why Do We Classify?

• To organise the enormous diversity of living things.
• To make study, comparison and communication about organisms easier.
• To reveal relationships and evolutionary history – groups that share a recent common ancestor are placed together.

Core Concepts Required by the Syllabus

• Species – the basic unit of classification; a group of organisms that can produce fertile offspring when they mate.
• Binomial nomenclature – each species has a two‑part scientific name (genus + specific epithet), e.g. Homo sapiens.
• Dichotomous key – a step‑by‑step identification tool that presents paired statements; the user follows the path that matches the organism.

Key Features Used for Classification

Organisms are grouped according to characteristics that are similar. The main categories of features used in the IGCSE syllabus are:

• Cellular organisation – unicellular vs. multicellular.
• Cell type – prokaryote or eukaryote; presence or absence of a true nucleus and membrane‑bound organelles.
• Mode of nutrition – autotrophic, heterotrophic, parasitic, saprotrophic.
• Reproductive structures – spores, seeds, flowers, fruits, eggs, live birth, etc.
• Structural and physiological traits – presence of cell walls, type of cell wall (cellulose, chitin, peptidoglycan), presence of chloroplasts, type of locomotion, etc.
• Genetic and molecular similarities – DNA or RNA sequence similarity, presence of particular genes or proteins.

Evolutionary Basis of Modern Classification (Supplementary)

Modern taxonomy aims to reflect evolutionary relationships. Organisms that share a more recent common ancestor are placed in the same group because they:

• Share a larger proportion of their DNA sequences.
• Often possess shared derived characteristics (synapomorphies) that are not found in more distant groups.

A simple phylogenetic tree (branching diagram) can be used to visualise these relationships – the closer two branches are, the more recent their common ancestor.

DNA‑Sequence Based Classification (Supplementary)

• DNA bar‑coding compares a short, standardised gene region of an unknown organism with reference sequences in a database.
• Common barcode regions:
  • Animals – mitochondrial cytochrome c oxidase I (COI) gene.
  • Plants – chloroplast rbcL and matK genes.
  • Fungi – nuclear ribosomal internal transcribed spacer (ITS) region.
• High sequence similarity (typically > 98 %) indicates a recent common ancestor and allows placement of the organism into the correct taxonomic group.

Hierarchical Classification System

Living organisms are placed into a series of nested ranks. Each rank groups organisms that share particular features. The system used in the IGCSE syllabus is the three‑domain system (Archaea, Bacteria, Eukarya) followed by the traditional kingdom‑to‑species hierarchy.

Using a Dichotomous Key

Step 1: Read the first pair of statements.

Step 2: Choose the statement that matches the organism and follow the indicated number to the next pair.

Step 3: Continue until a final identification is reached.

Example – simple classroom key:

1. Organism multicellular? – Yes → go to 2; No → Protist
2. Has a nucleus? – Yes → go to 3; No → Bacteria
3. Cell walls contain cellulose? – Yes → Plant; No → Animal

Suggested Practical Activities

• Build a dichotomous key: In groups, choose 5–6 local organisms (e.g., moss, fern, oak, frog, beetle, mushroom). Record observable features and construct a key that leads to each organism’s identification.
• DNA‑barcoding simulation: Provide short DNA “sequences” (e.g., 8‑letter strings) for a set of organisms. Students calculate % similarity, discuss which organisms are most closely related, and place them on a simple phylogenetic tree.
• Phylogenetic tree construction: Using a set of DNA‑barcode data (real COI or rbcL fragments), students draw a branching diagram and label synapomorphies that justify each node.

How to State the Classification Principle (Exam Answer)

Organisms are classified into groups based on the features that they share.

Suggested Diagram

Check Your Understanding

1. Give two features that could be used to place an organism in the Kingdom Fungi.
   Answer example: cell walls made of chitin; heterotrophic nutrition by absorption of organic matter.
2. Explain why DNA sequencing has become an important tool in modern classification.
   Answer example: DNA sequences provide objective, quantifiable evidence of genetic similarity. Because DNA changes slowly over evolutionary time, closely related species have very similar sequences, allowing scientists to infer evolutionary relationships and place organisms in the correct taxonomic group.
3. Match the following organisms to the correct taxonomic rank (short answer):
   
   • Rose – Species (e.g., Rosa rubiginosa)
   • Salmon – Class (Actinopterygii)
   • Oak tree – Family (Fagaceae)