Classification

For most of the 20th century, biologists used a five-kingdom system (Robert Whittaker, 1969): Prokaryotae (or Monera), Protoctista, Fungi, Plantae, Animalia. All prokaryotes — bacteria, archaea, blue-green algae — were lumped together in Kingdom Prokaryotae.

In 1990, Carl Woese revolutionised classification. Using sequence comparisons of the small-subunit ribosomal RNA (rRNA) gene — a molecule present in all cellular life and slow-evolving enough to be aligned across very distantly related organisms — he showed that the "prokaryotes" actually contain two fundamentally different groups, as different from each other as either is from eukaryotes. He proposed a new rank above kingdom — the domain — and split life into three:

Domain 1 — Archaea. Prokaryotic cells (no nucleus). Distinguishing molecular features:

• No peptidoglycan in cell walls — instead they have pseudopeptidoglycan or protein S-layers.
• Membrane phospholipids have ether-linked isoprenoid chains (more stable, suited to high temperatures).
• Histone-like proteins package the DNA — like eukaryotes.
• Insensitive to penicillin (because they lack peptidoglycan).
• Often extremophiles: methanogens (anaerobic, make methane), halophiles (very salty water), thermophiles (boiling springs).

Domain 2 — Bacteria. Prokaryotic cells. Distinguishing features:

• Cell walls contain peptidoglycan (murein) — basis of Gram staining; target of penicillin.
• Membrane phospholipids have ester-linked fatty acid chains.
• No histones.
• Most familiar microbes: E. coli, Lactobacillus, Streptococcus, cyanobacteria.

Domain 3 — Eukarya. Eukaryotic cells. Distinguishing features:

• True nucleus with linear chromosomes packaged with histones.
• Membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, sometimes chloroplasts).
• Cell division by mitosis (and meiosis).
• Includes all multicellular life and many single-celled organisms.

Why the change? Woese's rRNA data showed that Archaea and Eukarya are more closely related to each other than either is to Bacteria. The old "Kingdom Prokaryotae" masked one of the deepest evolutionary splits in the tree of life. The three-domain system reflects the true molecular phylogeny.

In the modern view, Eukarya is split into four kingdoms (Protoctista, Fungi, Plantae, Animalia), and the older "Prokaryotae" is no longer a valid kingdom — it has been replaced by the two prokaryotic domains.

Within the domain Eukarya, four kingdoms are recognised. The five-kingdom scheme is still useful at this rank — it's only the prokaryotic group that has been replaced by the two domains Archaea + Bacteria.

Kingdom Protoctista. Eukaryotic organisms that don't fit into Fungi, Plantae or Animalia. Mostly single-celled, sometimes simple multicellular. Heterogeneous — includes:

• Amoeba — single-celled, moves by pseudopodia.
• Paramecium — ciliated protozoan in pond water.
• Plasmodium — protozoan parasite, causes malaria.
• Algae (sometimes placed here, sometimes in Plantae) — Chlamydomonas (unicellular), Spirogyra (filamentous), kelp (multicellular).

Kingdom Fungi. Eukaryotic, heterotrophic, with chitin cell walls. Mostly multicellular (hyphae forming a mycelium); some unicellular (yeasts). Reproduce by spores. Important decomposers; some are pathogens (e.g. Candida albicans, ringworm) or used in industry (yeast for brewing, Penicillium for penicillin).

Kingdom Plantae. Multicellular eukaryotic photoautotrophs. Distinguishing features:

• Cellulose cell walls.
• Chloroplasts containing chlorophyll for photosynthesis.
• Store glucose as starch.
• Reproduce sexually (and many asexually); life cycle alternates between gametophyte and sporophyte.
• Includes mosses, liverworts, ferns, gymnosperms (conifers) and angiosperms (flowering plants).

Kingdom Animalia. Multicellular eukaryotic heterotrophs. Distinguishing features:

• No cell walls (cells held together by extracellular matrix and junctions).
• Usually motile at some life stage.
• Store glucose as glycogen.
• Mostly sexual reproduction; complex life cycles with embryonic development.
• Invertebrates (no backbone — sponges, cnidarians, molluscs, arthropods, etc.) and vertebrates (fish, amphibians, reptiles, birds, mammals).

A quick comparison table:

Kingdom	Cells	Wall	Nutrition	Storage
Protoctista	Eukaryotic, often unicellular	Variable	Autotrophic OR heterotrophic	Variable
Fungi	Eukaryotic, multicellular	Chitin	Heterotrophic (saprotrophic)	Glycogen
Plantae	Eukaryotic, multicellular	Cellulose	Photoautotrophic	Starch
Animalia	Eukaryotic, multicellular	None	Heterotrophic	Glycogen

What about viruses? Viruses are not in any kingdom. They are acellular and cannot reproduce independently — see the dedicated section below.

The taxonomic hierarchy. Modern classification uses eight ranks, ordered from broadest to most specific:

Rank	Description	Example for humans	Example for E. coli
Domain	Highest rank	Eukarya	Bacteria
Kingdom	Major life group	Animalia	(Prokaryotae)
Phylum	Major body-plan group	Chordata	Proteobacteria
Class	Major body-plan subdivision	Mammalia	Gammaproteobacteria
Order	Closely related families	Primates	Enterobacterales
Family	Closely related genera	Hominidae	Enterobacteriaceae
Genus	Closely related species	Homo	Escherichia
Species	Reproductively isolated group	sapiens	coli

Mnemonic. Remember the order using "Did King Phillip Come Over For Good Soup" — D K P C O F G S.

Binomial nomenclature. Introduced by Carl Linnaeus in Systema Naturae (1758). Every species has a unique two-part Latin name:

• Genus (capitalised, italicised): the broader group of closely related species.
• Specific epithet (lower-case, italicised): identifies the species within the genus.

Examples:

• Homo sapiens — modern humans
• Panthera leo (lion) and Panthera tigris (tiger) — same genus
• Escherichia coli — common gut bacterium
• Quercus robur — English oak

Formatting rules:

• Print: italicised with capital genus and lower-case species. After first mention, the genus may be abbreviated: H. sapiens.
• Handwriting: underline instead of italicise.
• The full name with author and year may include the discoverer: Homo sapiens Linnaeus, 1758.

Why a universal system? Common names vary by language, region and historical convention:

• Puma concolor is called puma, cougar, mountain lion, panther, or catamount depending on region.
• "Robin" refers to American Robin (Turdus migratorius) in the USA but to European Robin (Erithacus rubecula) in the UK.

The binomial system gives every species a unique, universal name recognised globally — essential for cataloguing biodiversity (~2 million species named, perhaps 10 million in total), communicating research and avoiding confusion.

Higher ranks also encode evolutionary information: species sharing a genus are closely related; species in the same family are slightly less so; and so on up the hierarchy.

The biological species concept (BSC) — developed by Ernst Mayr (1942) — defines a species as:

A group of organisms that can interbreed in the wild to produce fertile offspring, and that is reproductively isolated from other such groups.

Two key requirements:

• Interbreeding produces offspring — successful mating + viable embryo + healthy young.
• The offspring themselves are fertile — they can reproduce.

This is more demanding than just "they can mate". Horse × donkey produces a mule, but mules are sterile (their odd-numbered chromosome sets cannot pair correctly in meiosis). Therefore horse and donkey are two separate species, not one.

Strengths of the BSC:

• Provides an objective biological criterion (reproductive compatibility).
• Explains why species look distinct — reproductive isolation maintains the gene pool.
• Aligns with the goal of speciation (formation of reproductively isolated groups).

Limitations of the BSC:

(1) Asexual species. Many organisms (bacteria, archaea, asexual fungi, some plants) reproduce only asexually. They cannot "interbreed" at all, so the BSC cannot be applied. Bacteriologists use molecular criteria (rRNA sequence similarity, biochemical tests) instead.

(2) Ring species. A chain of populations around a geographical barrier where each can interbreed with its neighbours, but the populations at the ends of the chain cannot interbreed where they meet. Example: the greenish warbler (Phylloscopus trochiloides) around the Tibetan Plateau. Is it one species or two?

(3) Hybrid zones. Some species occasionally hybridise where their ranges overlap. Polar bears and brown bears can produce fertile hybrids ("pizzly bears"), as can wolves and coyotes. Strict application of the BSC would lump them, but they are clearly distinct in most respects.

(4) Extinct organisms / fossils. We cannot test whether fossil organisms could interbreed. Palaeontologists use the morphospecies concept (define species by skeletal morphology) instead.

(5) Geographically separated populations. Two allopatric populations may never get the chance to attempt interbreeding. They might still produce fertile offspring if brought together — or might have diverged so much that they cannot. The BSC alone can't decide.

Alternative species concepts. Biologists also use:

• Morphospecies concept — defined by morphological similarity (useful for fossils, asexual species).
• Phylogenetic species concept — smallest monophyletic group on a phylogenetic tree.
• Ecological species concept — defined by ecological niche.

For Cambridge 9700, the biological species concept is the one to know — but be ready to discuss its limitations in extended-response questions.

Classification was originally based on morphology — shape, size, anatomy. From the late 20th century onwards, molecular evidence has overtaken morphology as the primary tool. The 9700 syllabus expects you to know both and to explain why molecular is more reliable.

Morphological evidence (traditional).

• Anatomy: skeleton, vasculature, organ structure.
• External features: number of limbs, fur/feathers/scales, flower parts.
• Microscopic structure: leaf cross-section, sperm morphology.
• Embryology: similar embryos suggest shared ancestry (e.g. all vertebrate embryos have pharyngeal arches).

Problems with morphology alone:

• Convergent evolution. Unrelated lineages exposed to similar selection pressures evolve similar features independently. Dolphins (mammals) and ichthyosaurs (reptiles) both became streamlined torpedo-shaped swimmers — yet they are very distantly related. Marsupial mammals (Australia) and placental mammals (rest of world) independently evolved many similar forms.
• Plasticity. Morphology can vary with environment (e.g. shade vs sun leaves on the same tree). Morphological "species" may not reflect genetic groupings.
• Microbes have few features. Bacteria are almost all small rods, cocci or spirals — morphology cannot distinguish thousands of bacterial species.
• Subjective. Different taxonomists weight different features differently.

Molecular evidence (modern).

• DNA sequences — whole genome or specific genes.
• RNA sequences — especially 16S / 18S ribosomal RNA, present in all cellular life. This is the gene Woese used to discover Archaea.
• Protein sequences — especially well-conserved proteins like cytochrome c (component of the electron transport chain). The number of amino acid differences between species' cytochrome c proteins correlates beautifully with the time since their divergence.
• Immunological comparisons — antibody cross-reactivity gives a quick measure of protein similarity.

Advantages of molecular evidence:

(1) Direct measure of inherited information. DNA is the actual heritable genetic material. Two species sharing a recent common ancestor will share more DNA.

(2) Quantitative. Sequence similarity is a precise number (e.g. humans and chimps share 98.8% DNA).

(3) Resistant to convergence. Convergent evolution rarely produces convergent DNA sequences (because there are billions of possible sequences for the same protein), so molecular data avoids the dolphin-vs-ichthyosaur trap.

(4) Molecular clocks. Mutations in neutral DNA accumulate at roughly constant rates, allowing divergence time estimation calibrated against fossils.

(5) Universally applicable. Works for microbes, fossils (DNA from preserved tissue), plants, animals — anything containing DNA or recoverable proteins.

Combined approach. Modern classification uses molecular data as the primary evidence, supported by morphological, ecological and behavioural data. Disagreements between molecular and morphological classifications usually favour the molecular interpretation — this is how the three-domain system became established and how many traditional groupings have been revised (e.g. birds are now classed as living dinosaurs based on molecular + fossil data).

Viruses are biological entities of profound importance — they cause many diseases, influence evolution, and even outnumber cellular organisms. Yet they are not classified into any kingdom or domain of cellular life.

What is a virus? A virus is a tiny infectious particle (virion) consisting of:

• A core of nucleic acid — DNA or RNA, single- or double-stranded.
• A protein capsid enclosing the nucleic acid, made of repeating protein subunits (capsomeres).
• Sometimes a lipid envelope acquired from a host cell membrane, studded with viral glycoproteins (e.g. influenza, HIV, coronavirus).

Why viruses are not "alive" by the cellular standard:

(1) Acellular. Viruses are not cells. They have:

• No cytoplasm or cytosol.
• No membrane-bound organelles.
• No own ribosomes.
• No cell wall or plasma membrane (in non-enveloped viruses).

All life on Earth is built of cells (one or many). The five kingdoms and three domains are defined by cellular features (cell wall composition, organelles, nuclear membrane). Viruses simply don't fit this framework.

(2) Cannot reproduce independently. Viruses lack the metabolic and synthetic machinery needed for self-replication:

• No DNA polymerase or RNA polymerase capable of full self-copying (some bring viral polymerases with them, but still need the host cell).
• No ribosomes — cannot synthesise their own proteins.
• No metabolism — cannot generate ATP or build nucleotides from precursors.

A virus particle on a doorknob is metabolically inert — no more "alive" than a chemical crystal. Only when it invades a host cell and hijacks the host's enzymes and ribosomes can it replicate. They are therefore obligate intracellular parasites.

(3) Lack other features of life. The textbook seven characteristics of life — movement, respiration, sensitivity, growth, reproduction, excretion, nutrition (MRS GREN) — apply to cellular life. Viruses display none of these outside a host cell.

So how are viruses classified? Viruses have their own separate classification system. The most-used scheme is the Baltimore classification (David Baltimore, 1971), which divides viruses by their genome type and replication strategy:

• Class I: dsDNA (e.g. herpesviruses, adenoviruses)
• Class II: ssDNA (e.g. parvoviruses)
• Class III: dsRNA (e.g. rotaviruses)
• Class IV: positive-sense ssRNA (e.g. SARS-CoV-2, polio, hepatitis C)
• Class V: negative-sense ssRNA (e.g. influenza, rabies)
• Class VI: ssRNA-RT (retroviruses, e.g. HIV — replicate via reverse transcription)
• Class VII: dsDNA-RT (e.g. hepatitis B)

Within each class, viruses are grouped by family, genus and species — but always separately from the cellular tree of life.

A philosophical aside. Some biologists argue that giant viruses (mimivirus, pandoravirus) have so many genes — sometimes more than small bacteria — that they should be considered "alive" and added to a fourth domain. The mainstream view remains that the acellular and host-dependent nature of viruses keeps them outside the cellular classification.