Classification

This lesson covers the classification of living organisms — the system used to group and name species — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the Linnaean system, the three-domain system, binomial naming and how evolutionary trees show relationships between organisms.

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Why Do We Classify Organisms?

Classification is the process of sorting living organisms into groups based on their shared characteristics. Scientists classify organisms to:

• Organise the enormous diversity of life (over 8 million known species).
• Show evolutionary relationships between organisms.
• Make it easier to identify and study organisms.
• Allow scientists worldwide to use a universal system of naming.

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The Linnaean System

Carl Linnaeus (1735) developed the first widely accepted classification system. He grouped organisms into a hierarchy of increasingly specific categories:

Level	Example (Human)	Example (Domestic dog)
Kingdom	Animalia	Animalia
Phylum	Chordata	Chordata
Class	Mammalia	Mammalia
Order	Primates	Carnivora
Family	Hominidae	Canidae
Genus	Homo	Canis
Species	sapiens	familiaris

Remembering the Order

A common mnemonic: 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)"]

Key points:

• As you move down the hierarchy, the groups become smaller and organisms share more characteristics.
• Organisms in the same species are the most similar.
• Organisms in the same kingdom share only very broad features.

Exam Tip: You must know the seven levels in order from Kingdom to Species. The mnemonic "King Philip Came Over For Good Spaghetti" is reliable and quick to recall.

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Binomial Naming

Linnaeus also introduced binomial nomenclature — a two-part Latin naming system for every species:

1. The first name is the genus (capitalised).
2. The second name is the species (lowercase).
3. The name is always written in italics (or underlined if handwritten).

Examples

Common name	Binomial name
Human	Homo sapiens
Domestic dog	Canis familiaris
Common daisy	Bellis perennis
Lion	Panthera leo
House cat	Felis catus

Why Use Binomial Names?

• Universal — the same name is used worldwide, regardless of local language.
• Precise — avoids confusion (a "robin" in the UK and USA are completely different species).
• Shows relationships — organisms in the same genus are closely related.

Exam Tip: When writing binomial names, always italicise (or underline) and capitalise only the genus. Writing "homo sapiens" or "Homo Sapiens" is incorrect.

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The Five Kingdoms

Traditionally, organisms were classified into five kingdoms:

Kingdom	Examples	Key features
Animalia	Mammals, fish, insects	Multicellular, no cell wall, heterotrophic
Plantae	Flowering plants, ferns, mosses	Multicellular, cellulose cell wall, autotrophic (photosynthesis)
Fungi	Mushrooms, yeast, moulds	Cell wall made of chitin, heterotrophic (absorb nutrients), some unicellular
Protista	Amoeba, algae, Plasmodium	Mostly unicellular, eukaryotic
Prokaryotae (Monera)	Bacteria	Unicellular, prokaryotic (no nucleus), cell wall of peptidoglycan

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The Three-Domain System

More recently, Carl Woese (1990) proposed the three-domain system based on analysis of ribosomal RNA (rRNA) sequences. This system sits above the kingdom level:

Domain	Contains	Cell type
Archaea	Extremophiles (organisms living in extreme environments, e.g. hot springs, salt lakes)	Prokaryotic
Bacteria	True bacteria	Prokaryotic
Eukarya	Animals, plants, fungi, protists	Eukaryotic

graph TD
    A["Three Domains of Life"] --> B["Archaea (prokaryotic)"]
    A --> C["Bacteria (prokaryotic)"]
    A --> D["Eukarya (eukaryotic)"]
    D --> E["Animalia"]
    D --> F["Plantae"]
    D --> G["Fungi"]
    D --> H["Protista"]

Why the Change?

• Chemical analysis of rRNA and cell membrane composition showed that Archaea are more different from Bacteria than previously thought.
• The old five-kingdom system grouped all prokaryotes together, but Archaea and Bacteria are now recognised as fundamentally different.
• The three-domain system better reflects evolutionary relationships.

Exam Tip: The three-domain system was developed using molecular evidence (rRNA analysis), not just physical characteristics. This is an important distinction — it shows how advances in technology have changed classification.

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Evolutionary Trees (Phylogenetic Trees)

An evolutionary tree (or phylogenetic tree) is a diagram that shows the evolutionary relationships between organisms.

How to Read an Evolutionary Tree

Feature	Meaning
Branch point (node)	A common ancestor
Branch length	Can represent time or amount of genetic change
Tips (ends)	Living species (or sometimes extinct ones)
Closer together	More closely related (share a more recent common ancestor)

How Evolutionary Trees Are Constructed

Evolutionary trees can be constructed using:

1. Morphological data — comparing physical structures (e.g. bone structure, body plans).
2. DNA sequence analysis — species with more similar DNA are more closely related.
3. Protein comparisons — similar proteins suggest closer evolutionary relationships.

Scientists increasingly use DNA and molecular data because they are more objective and accurate than physical appearance alone.

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Difficulties with Classification

Classification is not always straightforward:

Challenge	Example
Hybridisation	A mule (horse x donkey) does not fit neatly into one species
Ring species	Populations around a geographic barrier can interbreed with neighbours but not with distant populations
Convergent evolution	Unrelated species may look similar (e.g. dolphins and sharks) but are not closely related
New discoveries	DNA analysis sometimes reveals that organisms classified together are actually not closely related

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Summary