OCR A Level Biology A: Classification and taxonomy (H420) – Revision Notes

These notes condense section 4.2.2(a) to (d) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. They cover the taxonomic hierarchy, binomial names, the five kingdoms, the three domains, new evidence for classification and phylogeny. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions. The OCR A Level Biology hub, the printable checklist and the free diagnostics show what else to revise.

Key terms

Term	Meaning
Classification	Sorting organisms into groups in a hierarchy, based on shared features and common ancestry
Taxonomy	The study and practice of naming and classifying organisms
Taxon	Any named group in the hierarchy (for example the family Felidae)
Species	A group of similar organisms that can interbreed to produce fertile offspring
Binomial	A two-part Latin name: genus, then species
Domain	The highest rank, above kingdom
Phylogeny	The evolutionary history and relationships of organisms
Phylogenetic tree	A branching diagram showing relationships; branch points are common ancestors

4.2.2(a) The hierarchy

Largest to smallest:

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

A memory aid: Dear King Philip Came Over For Good Soup.

• Groups are nested: each taxon sits inside one taxon of the next rank up. No overlap at the same rank.
• Lower ranks hold fewer species with more features in common.
• Two organisms that first share a taxon at a low rank are more closely related than two that first share one only at a high rank.

Method: finding how closely related two organisms are

1. Write both classifications side by side.
2. Work down from domain until they first differ.
3. The lowest rank where they still match is the lowest shared taxon.
4. The lower that rank, the more closely related they are.

4.2.2(b) Binomial system

• Genus first, with a capital letter. Species second, lower case.
• Italics in print, underlined when handwritten.
• Shorten to an initial after the first full use: R. temporaria.
• Advantage: universal, so the same name is used worldwide and there is no confusion over common names (one animal with many names, or one name for different animals).
• A shared genus shows a close relationship.

4.2.2(c)(i) Five kingdoms

Kingdom	Key features
Prokaryotae	No nucleus; DNA free in cytoplasm; no membrane-bound organelles; 70S ribosomes; peptidoglycan wall; unicellular and small
Protoctista	Eukaryotic; mostly unicellular but some multicellular; may have chloroplasts or not; wall absent or cellulose; autotrophic or heterotrophic; a mixed group
Fungi	Eukaryotic; chitin wall; hyphae and mycelium, or single cells (yeast); no chloroplasts; absorb food after external digestion; glycogen store
Plantae	Multicellular; eukaryotic; cellulose wall; chloroplasts; photosynthesis; starch store
Animalia	Multicellular; eukaryotic; no wall; no chloroplasts; ingest food; glycogen store; nervous coordination

The original system used similarities in observable features (cell structure, nutrition and body organisation).

4.2.2(c)(ii) Three domains

Evidence behind new systems:

• Comparison of DNA base sequences and RNA sequences, including rRNA.
• Comparison of amino acid sequences in proteins.
• Whole-genome data as technology improved.

More recent common ancestor means fewer sequence differences. Sequence differences can be counted.

Woese compared rRNA sequences of prokaryotes. They split into two very different groups, which led to the three-domain system (proposed in 1990).

Feature	Bacteria	Archaea	Eukarya
Nucleus	No	No	Yes
Ribosomes	70S	70S	80S
Wall	Peptidoglycan	No peptidoglycan	Cellulose, chitin or none
Membrane lipids	Ester-linked	Ether-linked, branched	Ester-linked
DNA	Circular, no histones	Circular, with histones	Linear, with histones
First amino acid	Formylmethionine	Methionine	Methionine

Kingdoms versus domains

	Kingdoms	Domains
Number of groups	Five	Three
Prokaryotes	One kingdom	Two domains
Eukaryotes	Four kingdoms	One domain, still divided into four kingdoms
Main basis	Observable features	Molecular evidence (rRNA and other sequences)

The domain system adds a rank above kingdom and splits Prokaryotae. It does not discard the eukaryotic kingdoms.

4.2.2(d) Classification and phylogeny

• Classification should reflect phylogeny, because the hierarchy arises from branching descent.
• Species in the same genus share a more recent common ancestor than species in the same family but different genera.
• On a tree, the closer the branch point is to the tips, the more recent the common ancestor and the closer the relationship.
• Molecular evidence supports and sometimes corrects classification based on appearance. Unrelated organisms can look alike because they live in similar ways.
• Cladistics is not required.
• The order of branch points on a tree shows the order in which lineages split, not how long ago in years unless the tree is drawn to a time scale.
• Fossils and anatomy help, but molecular data usually settle close calls.

Method: turning sequence differences into a tree

1. Convert each count to a percentage: (differences ÷ bases compared) × 100.
2. Find the pair with the smallest percentage. This pair joins first.
3. Add the next closest species to that pair.
4. The species with the largest differences from all the others branches off earliest.

Quick check: 16 differences in 160 bases gives 16 ÷ 160 × 100 = 10.0 % different, so 90.0 % similar.

Must-know distinctions

• Classification versus phylogeny. Classification is the grouping; phylogeny is the evolutionary history it should reflect.
• Kingdom versus domain. Domain is the higher rank. Eukaryotic kingdoms sit inside Eukarya.
• Bacteria versus Archaea. Both lack a nucleus. Archaea have ether-linked lipids, histones with their DNA and no peptidoglycan.
• Observable features versus molecular evidence. The first is how the kingdoms were set up; the second led to the domains.
• Genus versus species. The genus is shared by close relatives; the species name alone is not a full binomial.

Quick self-test

1. Put these ranks in order, largest first: family, class, domain, species, phylum.
2. Write the binomial name of the common frog correctly.
3. State the advantage of the binomial system.
4. Which kingdom has a chitin cell wall and absorbs nutrients from outside the body?
5. Give two features that separate Prokaryotae from the other four kingdoms.
6. Which type of evidence led to the three-domain system?
7. Name the three domains.
8. Give two differences between Bacteria and Archaea.
9. A gene fragment has 18 differences between two species in 240 bases. What percentage is different?
10. Two species differ in 3 bases out of 200. Another pair differs in 24 bases out of 200. Which pair is more closely related, and by what percentage points do the differences differ?
11. Why is the domain Eukarya described as containing four kingdoms?
12. Why can two species look alike but not be closely related?

Answers

1. Domain, phylum, class, family, species.
2. Rana temporaria (genus capital, species lower case, italics or underlined).
3. It is universal, so the same species has the same name worldwide and confusion between common names is avoided.
4. Fungi.
5. Any two of: no nucleus (DNA free in the cytoplasm); no membrane-bound organelles; 70S ribosomes; peptidoglycan wall; smaller cells.
6. Comparison of molecular evidence, especially ribosomal RNA base sequences (with other DNA and protein sequence data).
7. Archaea, Bacteria and Eukarya.
8. Any two of: Archaea have ether-linked branched membrane lipids, Bacteria ester-linked; Archaea have histones with their DNA, Bacteria do not; Bacteria have a peptidoglycan wall, Archaea do not; the first amino acid is formylmethionine in Bacteria and methionine in Archaea.
9. 18 ÷ 240 × 100 = 7.5 % different.
10. 3 ÷ 200 × 100 = 1.5 % and 24 ÷ 200 × 100 = 12.0 %. The first pair (1.5 %) is more closely related, with a difference of 10.5 percentage points between the two pairs.
11. Protoctista, Fungi, Plantae and Animalia are all eukaryotes, so all four sit inside the one domain Eukarya.
12. Similar appearance can come from living in similar ways, not from a recent common ancestor. Sequence data show the true relationship.

Where marks are usually lost

• Writing the ranks in the wrong order or forgetting domain.
• Writing a binomial with a capital on the species name, or without italics or underlining.
• Saying the binomial system is useful because it is “in Latin” rather than because it is universal.
• Giving a feature of the kingdom as “has cells” or “alive”, which every kingdom shares.
• Describing Prokaryotae as “having no organelles” without saying no membrane-bound organelles. Ribosomes are still present.
• Treating Protoctista as a single closely related group.
• Saying the three-domain system came from new microscopes, when the key evidence was molecular.
• Saying Archaea are bacteria, or placing Archaea in the Eukarya because they have histones.
• Misreading a tree: closeness depends on the branch point, not on how near the tips sit on the page.
• Failing to state which evidence supports a relationship when asked to “suggest” one.
• Quoting a percentage difference when the question asks for percentage similarity, or the other way round.
• Writing “Archaea have no organelles” when the point is that they lack a nucleus and other membrane-bound organelles.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025, section 4.2.2(a) to (d).