Classification and Extinction

This lesson covers how organisms are classified into groups and the causes of extinction. For AQA GCSE Biology, you need to understand the Linnaean classification system, the three-domain system proposed by Carl Woese, and the factors that can lead to species becoming extinct.

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What Is Classification?

Classification is the process of organising living organisms into groups based on their shared characteristics. It helps scientists to:

• Identify organisms
• Understand evolutionary relationships between species
• Communicate clearly about organisms using a universal system
• Predict characteristics of newly discovered species based on their classification group

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The Linnaean Classification System

The classification system used today was developed by the Swedish scientist Carl Linnaeus in the 18th century. He organised organisms into a hierarchy of groups, from the largest and most general to the smallest and most specific.

The Classification Hierarchy

Level	Description	Human example
Kingdom	Largest group; broad characteristics	Animalia
Phylum	Major body plan	Chordata (animals with backbones)
Class	Further subdivision	Mammalia
Order	More specific grouping	Primates
Family	Group of related genera	Hominidae (great apes)
Genus	Group of closely related species	Homo
Species	Most specific level; organisms that can interbreed to produce fertile offspring	sapiens

A useful mnemonic for remembering the order: King Philip Came Over For Good Spaghetti

graph TD
    A[Kingdom - broadest] --> B[Phylum]
    B --> C[Class]
    C --> D[Order]
    D --> E[Family]
    E --> F[Genus]
    F --> G[Species - most specific]

Exam Tip: You must be able to list the seven levels of classification in order. Use the mnemonic: King Philip Came Over For Good Spaghetti. AQA also expects you to know the human classification as an example.

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Binomial Naming System

Linnaeus also introduced the binomial naming system, which gives every species a unique two-part Latin name:

• The first part is the genus (always capitalised)
• The second part is the species (always lower case)
• The name is always written in italics (or underlined when handwritten)

Organism	Binomial name
Human	Homo sapiens
Domestic dog	Canis familiaris
Oak tree	Quercus robur
Common daisy	Bellis perennis
E. coli bacterium	Escherichia coli

Why use binomial names?

• They are universal — understood by scientists worldwide regardless of language
• They are unique — no two species share the same binomial name
• They show evolutionary relationships — species in the same genus are closely related

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The Three-Domain System [H]

In the 1990s, American scientist Carl Woese proposed a new classification system based on analysis of ribosomal RNA (rRNA). This system organises life into three domains, which sit above the kingdom level.

Domain	Characteristics	Examples
Archaea	Prokaryotic; no nucleus; chemically distinct cell walls and membranes; often found in extreme environments (e.g. hot springs, salt lakes)	Methanogens, thermophiles, halophiles
Bacteria	Prokaryotic; no nucleus; different cell wall chemistry from Archaea	E. coli, Staphylococcus, Streptococcus
Eukarya	Eukaryotic; cells have a nucleus and membrane-bound organelles	Animals, plants, fungi, protists

Why was the three-domain system needed?

Previously, all prokaryotic organisms were grouped together in the kingdom Prokaryotae (or Monera). However, Woese's analysis of rRNA showed that Archaea and Bacteria are fundamentally different at the biochemical level — they are as different from each other as either is from eukaryotes.

graph TD
    A[All Life] --> B[Domain Archaea]
    A --> C[Domain Bacteria]
    A --> D[Domain Eukarya]
    D --> E[Kingdom Animalia]
    D --> F[Kingdom Plantae]
    D --> G[Kingdom Fungi]
    D --> H[Kingdom Protista]

Exam Tip: The key reason for the three-domain system is that chemical analysis of rRNA showed that Archaea and Bacteria are fundamentally different, even though they both appear similar under a microscope. This is a common Higher Tier question.

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Classification Changes Over Time

Classification systems are not fixed — they change as new evidence is discovered:

Reason for change	Explanation
New species discovered	As expeditions explore new habitats, previously unknown species are found and need to be classified
DNA analysis	Comparing DNA sequences reveals that some species are more or less closely related than their physical appearance suggests
Reclassification	Organisms may be moved to different groups as understanding improves (e.g. fungi were once classified as plants)
Microscopy advances	Electron microscopy reveals structural differences not visible with light microscopes
Chemical analysis	Analysis of rRNA, proteins, and other molecules provides more accurate evolutionary information

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What Is Extinction?

Extinction occurs when there are no remaining individuals of a species left alive anywhere on Earth. The species is permanently lost.

Causes of Extinction

Cause	Explanation	Example
Environmental change	Climate change, habitat loss, or changes in conditions that a species cannot adapt to	Woolly mammoth (climate warming after Ice Age)
New predators	Introduction of a predator that a species has no defence against	Dodo hunted by introduced rats, cats, and pigs on Mauritius
New diseases	A new pathogen to which a species has no immunity	Red squirrel population decline due to squirrel pox carried by grey squirrels
Competition	A new species outcompetes the existing species for food, territory, or other resources	Grey squirrels outcompeting red squirrels in the UK
Catastrophic events	Volcanic eruptions, asteroid impacts, or other sudden events that rapidly change the environment	Dinosaurs (asteroid impact ~66 million years ago)
Habitat destruction	Deforestation, urbanisation, pollution, and drainage of wetlands destroy the places species live	Orangutan habitat loss due to palm oil plantations
Hunting and overexploitation	Humans hunting a species to extinction or harvesting it unsustainably	Passenger pigeon (hunted to extinction by 1914)

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Mass Extinction Events

A mass extinction is an event in which a large proportion of all species on Earth become extinct in a relatively short period of geological time. The most famous mass extinction events include:

Event	When	Cause	Impact
End-Ordovician	~443 million years ago	Ice age, sea level changes	~85% of marine species lost
End-Permian ("The Great Dying")	~252 million years ago	Volcanic activity, climate change	~96% of all species lost
End-Cretaceous	~66 million years ago	Asteroid impact, volcanic activity	~76% of species lost, including all non-avian dinosaurs

Scientists are concerned that human activity (habitat destruction, pollution, climate change) may be causing a sixth mass extinction today.