Spec Mapping — OCR H420 Module 4.2.2 — Classification and evolution, content statements covering the five-kingdom classification of Whittaker (1969), the three-domain system of Woese (1977/1990), the molecular evidence (rRNA, lipid biochemistry, cell wall composition) supporting the three-domain framework, and the reasons classification systems change over time (refer to the official OCR H420 specification document for exact wording). This lesson is high-yield for AO1 recall and AO3 evaluation questions on why scientific frameworks evolve.
As biology advanced, the simple plant/animal division of Linnaeus gave way first to the Five Kingdom system (1969) and then to the Three Domain system (1977/1990). Each revision reflected new evidence about the diversity of life — especially the discovery that what look like "bacteria" actually comprise two ancient and distantly related groups. OCR A-Level Biology A Module 4.2.2 requires you to know both systems and explain why the Three Domain system has replaced the older Five Kingdom framework.
The two architects to remember are Robert Whittaker (American ecologist, 1969 five-kingdom synthesis) and Carl Woese (American microbiologist, 1977 three-domain proposal based on rRNA sequencing). Woese's recognition of Archaea as a separate domain was initially controversial but is now textbook orthodoxy. Lynn Margulis's endosymbiotic theory (developed in the 1960s and 1970s) is the other big idea sitting alongside this lesson: the eukaryotic cell is a chimera, with mitochondria and chloroplasts descended from once-free-living bacteria taken in by an archaeal-like host.
Key Definitions:
Kingdom — a major division of life.
Domain — an even broader category above kingdom.
Prokaryote — organism with no membrane-bound nucleus.
Eukaryote — organism with a membrane-bound nucleus.
Archaea — single-celled prokaryotes distinct from bacteria at the molecular level.
The Five Kingdom System (Whittaker, 1969)
flowchart TD
A[All Life] --> B[Prokaryotae]
A --> C[Protoctista]
A --> D[Fungi]
A --> E[Plantae]
A --> F[Animalia]
In 1969, the American ecologist Robert Whittaker proposed five kingdoms to capture the diversity revealed by 20th-century biology:
1. Prokaryotae (= Monera)
Prokaryotic cells (no nucleus, no membrane-bound organelles).
Circular DNA; may have plasmids.
Peptidoglycan cell wall (in bacteria; archaea use different molecules).
Reflected the major differences in cell structure (prokaryotic vs eukaryotic) and mode of nutrition (autotrophic vs heterotrophic).
Separated fungi from plants, which had previously been lumped together.
Gave single-celled eukaryotes their own kingdom rather than wedging them into Plantae or Animalia.
Limitations
Prokaryotes turned out to be deeply divided into two evolutionarily distant groups (bacteria and archaea), which the Five Kingdom system failed to recognise.
Protoctista is clearly not a natural group — it lumps together organisms more closely related to plants, animals and fungi than to each other.
Based on morphology and biochemistry rather than molecular data.
The Three Domain System (Woese, 1990)
In the late 1970s, the American microbiologist Carl Woese pioneered the use of ribosomal RNA (rRNA) to reconstruct evolutionary relationships. Because rRNA is present in every cell and changes slowly over time, differences in its sequence reveal deep evolutionary splits. Woese's analysis showed something astonishing: "prokaryotes" were not a single group at all. Certain single-celled organisms previously lumped with bacteria were in fact as distantly related from bacteria as they were from animals and plants. Woese named them Archaea.
He proposed a new, higher level — the Domain — above kingdom:
flowchart TD
A[Life] --> B[Bacteria]
A --> C[Archaea]
A --> D[Eukarya]
B --> B1[E. coli, Cyanobacteria, Lactobacillus]
C --> C1[Methanogens, Thermophiles, Halophiles]
D --> D1[Protoctista]
D --> D2[Fungi]
D --> D3[Plantae]
D --> D4[Animalia]
1. Domain Bacteria
True bacteria — the familiar prokaryotes. Characteristics:
Include pathogens (Salmonella, Vibrio), symbionts (gut flora) and environmental species (cyanobacteria, nitrogen-fixing bacteria).
2. Domain Archaea
Previously called "archaebacteria". At first glance they look like bacteria — single cells, no nucleus — but their molecular biology is strikingly different:
No peptidoglycan in cell walls (instead pseudopeptidoglycan, S-layers, or no wall).
Ether-linked membrane lipids (more stable at extreme temperatures).
Different ribosomal RNA sequences.
More eukaryote-like RNA polymerase (often with 8+ subunits) and DNA replication machinery.
Histone-like proteins package DNA in many archaea.
Not sensitive to most antibacterial antibiotics.
Many archaea are extremophiles:
Thermophiles live in hot springs and hydrothermal vents (Pyrococcus survives above 100 °C).
Halophiles live in saturated salt (Halobacterium in the Dead Sea).
Acidophiles live at very low pH.
Methanogens produce methane in anaerobic environments (cow rumens, marshes, rice paddies).
But not all archaea are extremophiles — recent discoveries show they are abundant in soils, oceans and even the human gut.
3. Domain Eukarya
All organisms with nucleated cells:
Protoctista
Fungi
Plantae
Animalia
Molecular evidence shows Eukarya are more closely related to Archaea than to Bacteria; eukaryotes may even have evolved from within the Archaea through endosymbiosis. The membrane lipids, DNA replication and transcription machinery all resemble archaeal systems more than bacterial ones.
Evidence for Three Domains
Woese's proposal was controversial at first but is now universally accepted because of multiple lines of evidence:
rRNA sequences — the original data. Archaeal and bacterial rRNAs are as different from each other as either is from eukaryotes.
Membrane lipid chemistry — ether vs ester linkages.
Cell wall composition — peptidoglycan (bacteria) vs various alternatives (archaea).
Antibiotic sensitivity — archaea are insensitive to most antibacterials.
DNA replication and transcription machinery — archaea use eukaryote-like enzymes.
Histones — archaea often have histone-like proteins; true bacteria do not.
Why the Three Domain System Replaced Five Kingdoms
The Three Domain system is now the accepted framework because:
It reflects true evolutionary relationships. Five Kingdoms failed to recognise the deep split between bacteria and archaea.
It is based on molecular evidence (rRNA and whole genomes) rather than morphology.
It accommodates the enormous diversity of microbial life, which dominates the tree of life.
It matches genomic and biochemical data from many different kinds of analysis.
In most modern textbooks the Five Kingdoms are preserved as sub-divisions within the Eukarya domain, with Bacteria and Archaea getting their own status at the top.
Exam Tip: OCR exam questions often ask "Why have classification systems changed over time?" The key points are: new evidence (especially molecular), new techniques (DNA sequencing, microscopy), better understanding of evolutionary relationships, and a move towards monophyletic groupings.
Other Classification Systems
Some scientists propose alternative systems:
Six Kingdoms — splits Prokaryotae into Eubacteria and Archaebacteria, keeping the kingdom level familiar.
Three Domains, Six Kingdoms — combines both.
Two Empires (Prokaryota and Eukaryota) — a cruder division used less often today.
Eukaryotic supergroups — some taxonomists abandon kingdoms within Eukarya altogether, using seven or eight supergroups based on rRNA (e.g. Opisthokonta, which includes animals and fungi).
OCR examines the Five Kingdom and Three Domain systems specifically, so focus your revision there.
Case Study: Why Archaea Matter
Archaea are not just esoteric. They:
Underpin biotechnology. The enzyme Taq polymerase (used in PCR) was originally isolated from the thermophilic bacterium Thermus aquaticus, but related enzymes from archaea such as Pyrococcus furiosus (Pfu) are now used for high-fidelity PCR.
Produce most of the world's methane. Methanogens in wetlands, cattle rumens and landfills produce a huge fraction of atmospheric methane (a powerful greenhouse gas).
Cycle nitrogen in the oceans. Marine archaea perform ammonia oxidation at scales rivalling bacteria.
Illuminate early life. As extremophiles, many archaea may resemble the earliest living organisms, giving clues about life's origins.
Common Exam Mistakes
Confusing Archaea with bacteria. They look similar but are as distantly related as you are from a tree.
Saying archaea are "primitive". They are not — they are as evolved as any other modern organism, just different.
Assuming all archaea are extremophiles. Many archaea live in ordinary environments including the human gut.
Forgetting why systems changed. Always mention new molecular evidence (rRNA, genome sequencing).
Getting the Five Kingdoms in the wrong order. Prokaryotae usually comes first; the order of the eukaryotic kingdoms is less fixed.
Quick Recap
The Five Kingdom system (Whittaker, 1969): Prokaryotae, Protoctista, Fungi, Plantae, Animalia.
The Three Domain system (Woese, 1990): Bacteria, Archaea, Eukarya.
Woese used ribosomal RNA to show that archaea are as distantly related to bacteria as either is to eukaryotes.
Key differences: membrane lipids (ester vs ether), cell wall chemistry, DNA replication machinery, antibiotic sensitivity.
Classification systems change as new evidence (especially molecular) becomes available.