Classification Systems
Discover how scientists organise the diversity of life on Earth using taxonomy, the seven kingdoms, and the binomial naming system invented by Carl Linnaeus.
Why Do We Classify Living Things?
There are estimated to be over 8.7 million species of living things on Earth. To make sense of this incredible diversity, scientists use taxonomy — the science of naming, describing, and classifying organisms based on shared characteristics.
Classification helps scientists communicate clearly across different languages and countries, identify relationships between organisms, and predict the characteristics of newly discovered species.
Australian Curriculum Connection
This lesson aligns with AC9S7U01: "Classification systems and keys are used to organise and identify the diversity of organisms." Australian examples include the unique fauna of our continent — marsupials, monotremes, and endemic plant species.
The Taxonomic Hierarchy
Taxonomy organises life into a series of nested groups, from the broadest category down to the most specific. A useful mnemonic is: "Dear King Philip Came Over For Good Soup".
Example — Red Kangaroo: Domain: Eukaryota | Kingdom: Animalia | Phylum: Chordata | Class: Mammalia | Order: Diprotodontia | Family: Macropodidae | Genus: Osphranter | Species: Osphranter rufus
The Six Kingdoms
Modern classification recognises six kingdoms of life. Each is defined by key characteristics such as cell type, number of cells, and how the organism obtains energy.
Plantae
Multicellular, eukaryotic. Produce own food via photosynthesis. e.g. eucalyptus, grass, ferns.
Animalia
Multicellular, eukaryotic. Cannot make own food; consume other organisms. e.g. kangaroos, fish, insects.
Fungi
Mostly multicellular, eukaryotic. Absorb nutrients from dead matter. e.g. mushrooms, moulds, yeasts.
Protista
Mostly unicellular, eukaryotic. Diverse — includes algae, amoeba, paramecium.
Eubacteria
Unicellular, prokaryotic. True bacteria — no membrane-bound nucleus. e.g. E. coli, Lactobacillus.
Archaebacteria
Unicellular, prokaryotic. Extremophiles — live in hot springs, salt lakes, deep ocean vents.
Binomial Nomenclature
Swedish scientist Carl Linnaeus (1707–1778) devised the system of binomial nomenclature — giving every species a unique two-part Latin name. This system is still used worldwide today.
Rules for Writing Scientific Names
The first word is the genus name — always capitalised. e.g. Homo
The second word is the species name — always lowercase. e.g. sapiens
The full name is written in italics when typed, or underlined when handwritten. e.g. Homo sapiens
After the first use, the genus may be abbreviated to its first letter: H. sapiens
| Common Name | Scientific Name | Kingdom |
|---|---|---|
| Red Kangaroo | Osphranter rufus | Animalia |
| Koala | Phascolarctos cinereus | Animalia |
| River Red Gum | Eucalyptus camaldulensis | Plantae |
| Human | Homo sapiens | Animalia |
Key Vocabulary
Taxonomy
The science of naming, describing, and classifying organisms based on shared characteristics.
Binomial Nomenclature
The two-part Latin naming system for species: genus (capitalised) + species (lowercase), written in italics.
Eukaryote
An organism whose cells contain a membrane-bound nucleus (e.g. animals, plants, fungi, protists).
Prokaryote
An organism whose cells lack a membrane-bound nucleus (e.g. bacteria and archaea).
Worked Examples
Writing a scientific name correctly
A student writes "homo Sapiens" in their workbook. Identify and correct any errors.
Error 1: The genus "homo" must begin with a capital letter: Homo.
Error 2: The species "Sapiens" must be lowercase: sapiens.
Correct form: Homo sapiens (italicised, genus capitalised, species lowercase).
Classifying a mushroom
Which kingdom does a mushroom belong to, and why does it not belong to Plantae?
Kingdom: Fungi.
Reason it is not Plantae: Mushrooms cannot photosynthesise — they have no chlorophyll and no chloroplasts. Instead, they absorb nutrients from decaying organic matter (they are decomposers).
Conclusion: Mushrooms are classified as Fungi because they are multicellular eukaryotes that obtain energy by absorbing nutrients from their surroundings.
Using a dichotomous key
A scientist finds an organism. It is multicellular, has no cell wall, cannot photosynthesise, and moves under its own power. Use the kingdoms to identify it.
Step 1: Multicellular — eliminates Eubacteria, Archaebacteria (unicellular), and most Protista.
Step 2: No cell wall — eliminates Plantae and Fungi (which have cell walls).
Step 3: Cannot photosynthesise — eliminates Plantae.
Answer: The organism belongs to Kingdom Animalia.
Knowledge Check
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Key Concepts Summary
- ✓ Taxonomy is the science of classifying living things based on shared characteristics.
- ✓ The hierarchy from broadest to most specific: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
- ✓ There are six kingdoms: Animalia, Plantae, Fungi, Protista, Eubacteria, Archaebacteria.
- ✓ Binomial nomenclature gives each species a unique two-part Latin name (Genus species) written in italics.
- ✓ Eukaryotes have membrane-bound nuclei; prokaryotes do not.