Classification and Taxonomy
Discover how scientists organise the millions of living species on Earth using a hierarchical system of domains, kingdoms, and binomial nomenclature.
The Three Domains and Six Kingdoms
All life on Earth is classified into three domains based on cell structure and molecular evidence. These domains are further divided into six kingdoms. This hierarchical system allows biologists to categorise organisms logically, grouping those that share common ancestry and characteristics.
The Three Domains of Life
Bacteria
Prokaryotic, unicellular, no membrane-bound nucleus
Kingdom: Eubacteria
Archaea
Prokaryotic, unicellular, extremophiles, distinct biochemistry
Kingdom: Archaebacteria
Eukarya
Eukaryotic cells with membrane-bound organelles
Kingdoms: Protista, Fungi, Plantae, Animalia
The Taxonomic Hierarchy
Domain
Broadest grouping (e.g. Eukarya)
Kingdom
e.g. Animalia
Phylum
e.g. Chordata
Class
e.g. Mammalia
Order
e.g. Carnivora
Family
e.g. Felidae
Genus
e.g. Felis
Species
e.g. catus
Memory aid: Dear King Philip Came Over For Good Spaghetti
Key idea: As you move down the hierarchy from Domain to Species, the groups become smaller and more specific. Organisms within the same species share the most characteristics and can interbreed to produce fertile offspring.
Binomial Nomenclature
Swedish naturalist Carl Linnaeus developed a universal naming system called binomial nomenclature. Each species receives a unique two-part Latin name consisting of the genus (capitalised) and the specific epithet (lowercase). The name is always italicised (or underlined when handwritten).
Naming Rules
- 1. Names are written in Latin
- 2. Genus is capitalised: Homo
- 3. Species is lowercase: sapiens
- 4. Always italicised or underlined
- 5. Can abbreviate genus after first use: H. sapiens
Australian Examples
- Macropus rufus -- Red kangaroo
- Phascolarctos cinereus -- Koala
- Ornithorhynchus anatinus -- Platypus
- Eucalyptus globulus -- Blue gum
Why use Latin names? Common names vary between languages and regions. Binomial nomenclature provides a single, universally understood name for every species, preventing confusion in the international scientific community.
Phylogenetic Trees
A phylogenetic tree (also called a cladogram) is a branching diagram showing the evolutionary relationships among species. The branching points (nodes) represent common ancestors, and the tips of the branches represent existing or extinct species. Modern phylogenetic trees are built using DNA and protein sequence comparisons.
Reading a Phylogenetic Tree
Fish
Amphibians
Reptiles
Mammals
Common Ancestor
Species closer together on the tree share a more recent common ancestor and are more closely related.
Key Features of Phylogenetic Trees
Nodes (branch points): Represent where lineages diverged from a common ancestor. Each node is a speciation event.
Tips (terminal nodes): Represent current species or groups. They sit at the ends of branches.
Root: The base of the tree, representing the most ancient common ancestor of all groups shown.
Branch length: Can represent time (millions of years) or the amount of genetic change between species.
Key Vocabulary
Taxonomy
The science of naming, describing, and classifying organisms into groups based on shared characteristics.
Binomial Nomenclature
The two-part Latin naming system for species, consisting of the genus and specific epithet (e.g. Homo sapiens).
Phylogenetic Tree
A branching diagram showing the inferred evolutionary relationships among organisms based on similarities and differences.
Clade
A group of organisms that includes an ancestor and all of its descendants, forming a single branch on the tree of life.
Worked Examples
Classify a domestic cat using the full taxonomic hierarchy.
Domain: Eukarya (eukaryotic cells)
Kingdom: Animalia (multicellular, heterotrophic)
Phylum: Chordata (has a backbone)
Class: Mammalia (produces milk, has fur)
Order: Carnivora (meat-eating adaptations)
Family: Felidae (cat family)
Genus: Felis
Species: Felis catus
Two organisms belong to the same Order but different Families. How closely related are they?
Step 1: They share the same Domain, Kingdom, Phylum, Class, and Order -- so they have many shared characteristics.
Step 2: They diverge at the Family level, meaning they differ in more specific features.
Conclusion: They are moderately related. For example, dogs (Family Canidae) and cats (Family Felidae) are both in Order Carnivora. They share a distant common ancestor but have evolved distinct features.
Using a phylogenetic tree, which two organisms are most closely related: Species A and B share a recent node, while Species C branches off earlier?
Step 1: On a phylogenetic tree, species that share a more recent common ancestor (node) are more closely related.
Step 2: Species A and B share a recent node, meaning they diverged from each other more recently.
Answer: Species A and B are more closely related than either is to Species C. Species C diverged earlier, meaning it has been evolving independently for a longer period.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What is the correct order of the taxonomic hierarchy from broadest to most specific?
Question 2
Which of the following is correctly written using binomial nomenclature?
Question 3
Which domain includes organisms with eukaryotic cells?
Question 4
On a phylogenetic tree, what does a node (branch point) represent?
Question 5
Two organisms share the same genus but have different species names. Which statement is true?
Key Concepts Summary
- ●Taxonomy is the science of classifying organisms into hierarchical groups based on shared characteristics.
- ●The three domains of life are Bacteria, Archaea, and Eukarya, further divided into six kingdoms.
- ●The hierarchy from broadest to most specific: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.
- ●Binomial nomenclature gives each species a unique two-part Latin name (genus + specific epithet).
- ●Phylogenetic trees show evolutionary relationships; species sharing a more recent common ancestor are more closely related.