Classification and Taxonomy

Classification is the organisation of living organisms into groups based on shared characteristics. Taxonomy is the science of naming and classifying organisms. A robust classification system reflects evolutionary relationships and allows biologists to communicate clearly about the diversity of life.

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Why Classify Organisms?

• To organise the enormous diversity of living organisms into a manageable system.
• To identify organisms accurately and consistently.
• To communicate about organisms without ambiguity — a species with different common names in different regions has only one scientific name.
• To reveal evolutionary relationships — organisms that are closely related share more characteristics and a more recent common ancestor.
• To aid in conservation — classifying organisms helps identify species at risk and understand ecosystem structure.

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The Linnaean Hierarchy

Carl Linnaeus (1707–1778) established the hierarchical classification system still used today. Organisms are classified into a series of nested groups (taxa), each more inclusive than the last.

The eight principal taxonomic ranks, from broadest to narrowest:

Rank	Example (Humans)	Example (Dog)
Domain	Eukarya	Eukarya
Kingdom	Animalia	Animalia
Phylum	Chordata	Chordata
Class	Mammalia	Mammalia
Order	Primates	Carnivora
Family	Hominidae	Canidae
Genus	Homo	Canis
Species	Homo sapiens	Canis familiaris

Memory Aid: "Dear King Philip Came Over For Good Spaghetti" — Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Key Points

• Each level is called a taxon (plural: taxa).
• As you move down the hierarchy, the groups become smaller and more specific, and the organisms within them share more characteristics and are more closely related.
• Members of the same species share the most characteristics and have the most recent common ancestor.

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Binomial Nomenclature

Key Definition: Binomial nomenclature is the two-part naming system for species, devised by Linnaeus. Every species has a unique scientific name consisting of its genus name (capitalised) and its species name (lower case), both written in italics (or underlined when handwritten).

Examples:

• Homo sapiens (humans)
• Escherichia coli (a bacterium)
• Quercus robur (English oak)
• Panthera leo (lion)

Rules of Binomial Nomenclature

• The genus name always begins with a capital letter; the species name is always lower case.
• Both names are written in italics (or underlined if handwritten).
• After first use in a text, the genus name can be abbreviated to its initial (e.g., H. sapiens, E. coli).
• The system is universal — the same name is used worldwide, regardless of local language.

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The Five-Kingdom Classification

Traditionally, organisms were classified into five kingdoms (proposed by Robert Whittaker, 1969):

Kingdom	Cell Type	Cell Wall	Nutrition	Examples
Prokaryotae (Monera)	Prokaryotic	Peptidoglycan (murein)	Various (autotrophic and heterotrophic)	Bacteria
Protoctista (Protista)	Eukaryotic	Varies (cellulose, silica, or absent)	Various	Amoeba, algae, Plasmodium
Fungi	Eukaryotic	Chitin	Saprophytic (heterotrophic — digest extracellularly)	Mushrooms, yeast, moulds
Plantae	Eukaryotic	Cellulose	Autotrophic (photosynthesis)	Mosses, ferns, flowering plants
Animalia	Eukaryotic	Absent	Heterotrophic (ingest food)	Insects, fish, mammals

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The Three-Domain System

The three-domain system was proposed by Carl Woese in 1990, based on analysis of ribosomal RNA (rRNA) sequences. It replaced the five-kingdom system as the primary framework for classifying life.

The three domains are:

Domain	Cell Type	Key Features
Bacteria	Prokaryotic	Peptidoglycan cell walls; circular DNA; 70S ribosomes; no membrane-bound organelles
Archaea	Prokaryotic	No peptidoglycan in cell walls; unique membrane lipids (ether-linked); often found in extreme environments; 70S ribosomes
Eukarya	Eukaryotic	Membrane-bound nucleus and organelles; linear chromosomes; 80S ribosomes; includes all four eukaryotic kingdoms

Why Three Domains?

• Analysis of rRNA sequences revealed that the organisms formerly classified as "Prokaryotae" actually comprise two fundamentally different groups: Bacteria and Archaea.
• Although Bacteria and Archaea are both prokaryotic, their rRNA sequences, membrane chemistry, and gene expression mechanisms differ profoundly.
• In many molecular features, Archaea are more closely related to Eukarya than to Bacteria.
• The three-domain system reflects evolutionary relationships more accurately than the five-kingdom system.

Key Differences Between Bacteria and Archaea