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This lesson covers how organisms are classified into groups based on their characteristics, how classification systems have changed over time, and why species become extinct. These are key topics in the AQA GCSE Combined Science Trilogy specification (8464).
Classification is the process of organising living organisms into groups based on their similarities and differences. It helps scientists to:
Carl Linnaeus (1707–1778) developed a hierarchical system of classification that is still used today. Organisms are sorted into groups within groups, from the largest (kingdom) to the smallest (species):
graph TD
A["Kingdom<br/>(largest group)"] --> B["Phylum"]
B --> C["Class"]
C --> D["Order"]
D --> E["Family"]
E --> F["Genus"]
F --> G["Species<br/>(smallest group)"]
style A fill:#bbdefb,stroke:#1565c0
style B fill:#c8e6c9,stroke:#2e7d32
style C fill:#fff9c4,stroke:#f9a825
style D fill:#ffccbc,stroke:#d84315
style E fill:#e1bee7,stroke:#6a1b9a
style F fill:#b2dfdb,stroke:#00796b
style G fill:#ef9a9a,stroke:#c62828
You can remember the order using: King Philip Came Over For Good Spaghetti
| Level | Example (Human) | Example (Lion) |
|---|---|---|
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Mammalia |
| Order | Primates | Carnivora |
| Family | Hominidae | Felidae |
| Genus | Homo | Panthera |
| Species | sapiens | leo |
Linnaeus also introduced the binomial naming system, where every species has a two-part Latin name:
Examples: Homo sapiens, Panthera leo, Quercus robur (English oak)
Exam Tip: AQA (8464) may ask you to use or interpret binomial names. Remember: the genus comes first (capital letter) and the species second (lower case). Always write in italics.
The traditional five-kingdom classification has been updated. Carl Woese proposed the three-domain system based on analysis of ribosomal RNA (chemical analysis rather than physical characteristics):
| Domain | Includes | Key Features |
|---|---|---|
| Archaea | Primitive bacteria-like organisms | No nucleus; often found in extreme environments (e.g. hot springs, salt lakes); chemically different cell walls from bacteria |
| Bacteria | True bacteria | No nucleus; peptidoglycan cell walls; most familiar prokaryotes |
| Eukarya | All organisms with cells containing a nucleus | Includes protists, fungi, plants and animals |
The three-domain system was proposed because chemical analysis of ribosomal RNA showed that Archaea are fundamentally different from Bacteria, even though they look similar under a microscope.
Exam Tip: AQA (8464) specifically requires you to know about the three-domain system and why it was proposed. The key answer is: analysis of the genes (RNA/DNA) of organisms revealed that what were previously classified as bacteria actually fell into two very different groups — Archaea and Bacteria.
| Era | Basis of Classification |
|---|---|
| Linnaeus (18th century) | Physical characteristics (observable features) |
| Modern classification | Evolutionary relationships based on DNA analysis, protein comparison and ribosomal RNA analysis |
| Three-domain system | Chemical analysis of ribosomal RNA revealed Archaea and Bacteria are fundamentally different |
Advances in technology (particularly genome sequencing) have allowed scientists to classify organisms more accurately based on their evolutionary relationships rather than just their appearance.
Extinction is the permanent loss of a species — it no longer exists anywhere on Earth.
| Cause | Explanation | Example |
|---|---|---|
| Environmental change | Climate change or habitat destruction makes an area unsuitable | Ice ages caused many extinctions |
| New predators | A new predator arrives and hunts the species to extinction | Dodo hunted by introduced animals on Mauritius |
| New diseases | A new pathogen kills large numbers | Chytrid fungus threatening amphibians worldwide |
| New competitors | A more successful species outcompetes for resources | Grey squirrels outcompeting red squirrels in the UK |
| Catastrophic events | Volcanic eruptions, asteroid impacts | Asteroid impact contributed to dinosaur extinction |
| Human activity | Hunting, habitat destruction, pollution, climate change | Passenger pigeon hunted to extinction |
Fossils are the preserved remains or traces of organisms that lived millions of years ago. They provide evidence for:
| Method | Description |
|---|---|
| Mineralisation | Hard parts (bones, shells) are gradually replaced by minerals |
| Casts and moulds | Organisms leave impressions in sediment that harden into rock |
| Preservation in amber | Insects trapped in tree resin are perfectly preserved |
| Preservation in ice | Organisms frozen in glaciers or permafrost are preserved |
| Preservation in peat | Acidic, anaerobic conditions prevent decomposition |
| Reason | Explanation |
|---|---|
| Soft tissue decay | Many organisms have no hard parts to fossilise |
| Geological activity | Tectonic movements, erosion and volcanic activity can destroy fossils |
| Conditions not right | Fossils only form under specific conditions; most dead organisms decompose |
| Not yet discovered | Many fossils remain buried and undiscovered |
Exam Tip: AQA (8464) often asks why the fossil record is incomplete. Give at least two reasons and explain each one — simply listing reasons may not earn full marks.
| Mistake | Correction |
|---|---|
| Saying the Linnaean system starts with species | It starts with kingdom (largest) and ends with species (smallest) |
| Confusing Archaea and Bacteria | Archaea are chemically different from Bacteria despite looking similar |
| Saying extinction only happens because of humans | Natural causes (climate change, volcanic eruptions, disease) also cause extinction |
| Saying all fossils are bones | Fossils include impressions, traces, preserved organisms in amber/ice and mineralised remains |
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