OCR A Level Biology A: Classification and taxonomy (H420)

This guide covers 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. It sits in Module 4, Biodiversity, evolution and disease. It teaches the taxonomic hierarchy, the binomial system, the five kingdoms, the three domains, the evidence behind new classification systems, and the link between classification and phylogeny. The rest of 4.2.2, from (e) onwards (evolution, variation, adaptation and natural selection), is outside this unit.

Use it with the revision notes and the practice questions. The OCR A Level Biology hub and the printable checklist list the rest of the course, and the free diagnostics show where to start. Cell structure from Cell structure and prokaryotes is assumed.

What this unit covers

Spec reference	What you must be able to do
4.2.2(a)	Use the taxonomic hierarchy: domain, kingdom, phylum, class, order, family, genus, species
4.2.2(b)	Use the binomial system of naming species and state its advantage
4.2.2(c)(i)	Describe the features used to place organisms in the five kingdoms: Prokaryotae, Protoctista, Fungi, Plantae, Animalia, including the use of similarities in observable features in original classification
4.2.2(c)(ii)	Explain the evidence that led to new systems such as the three domains, including similarities in biological molecules and other genetic evidence; describe the three domains and compare the kingdom and domain systems
4.2.2(d)	Explain the relationship between classification and phylogeny. Cladistics is not required

4.2.2(a) The taxonomic hierarchy

Evolution has produced a huge variety of organisms. Because all organisms share a common ancestry, they can be sorted into groups. Classification imposes a hierarchy on this variety. Taxonomy is the study and practice of naming and grouping organisms.

The hierarchy has eight ranks, from largest to smallest:

domain, kingdom, phylum, class, order, family, genus, species

Each group at a rank is a taxon (plural taxa). The groups are nested: each taxon sits wholly inside one taxon of the rank above, and no taxon overlaps another at the same rank. As you go down, the organisms in a group have more features in common and the groups get smaller. A species is a group of similar organisms that can interbreed to produce fertile offspring.

Rank	Domestic cat	Lion	Brown bear	Common frog
Domain	Eukarya	Eukarya	Eukarya	Eukarya
Kingdom	Animalia	Animalia	Animalia	Animalia
Phylum	Chordata	Chordata	Chordata	Chordata
Class	Mammalia	Mammalia	Mammalia	Amphibia
Order	Carnivora	Carnivora	Carnivora	Anura
Family	Felidae	Felidae	Ursidae	Ranidae
Genus	Felis	Panthera	Ursus	Rana
Species	catus	leo	arctos	temporaria

Worked example 1

For each of the other three animals, find the lowest taxon it shares with the domestic cat.

1. Cat and lion share everything down to family (Felidae) and split at genus. The lowest shared taxon is the family Canidae.
2. Cat and bear share down to order (Carnivora) but have different families. The lowest shared taxon is the order Carnivora.
3. Cat and frog share down to phylum (Chordata) but are in different classes. The lowest shared taxon is the phylum Chordata.

The lower the rank at which two organisms first share a taxon, the more features they have in common, and (as section 4.2.2(d) explains) the more recently they share a common ancestor. The cat is therefore most like the lion and least like the frog.

4.2.2(b) The binomial system

Many organisms have several common names, and one common name can mean different organisms. In Britain a buzzard is a bird of prey, Buteo buteo. In North America the same word is often used for vultures. Common names also differ between languages.

The binomial system gives every species a two-part Latin name: the genus first, then the species. The genus name starts with a capital letter and the species name with a lower-case letter. Print them in italics, or underline each word if you handwrite them. Rana temporaria is the common frog.

After you have given the full name once, you can shorten the genus to its initial: R. temporaria.

The advantage of the system is that it is universal. Scientists in any country use the same name for the same species, so there is no confusion between common names, and a name stays the same whatever language is used. The name also shows relationships: Quercus robur and Quercus petraea are both oaks because they share a genus.

Worked example 2

A student writes “Rana Temporaria” in a report. Correct it and explain why the name is useful.

1. Genus capital, species lower case: Rana temporaria (underlined if handwritten).
2. A second mention can be R. temporaria.
3. The name identifies one species worldwide. A common name such as “frog” could mean many species.

4.2.2(c)(i) The five kingdoms

The specification asks you to know the features used to place organisms into five kingdoms. In the original classification the groupings were built mainly on similarities in observable features: what cells look like under a microscope, how the organism gets its food and how the body is organised.

Kingdom	Cell type and size	Body form	Cell wall	Nutrition
Prokaryotae	Prokaryotic: no nucleus, DNA free in cytoplasm, no membrane-bound organelles, 70S ribosomes; smaller than eukaryotic cells	Unicellular	Peptidoglycan	Autotrophic or heterotrophic
Protoctista	Eukaryotic, with a nucleus and organelles	Mostly unicellular; some multicellular (for example seaweeds)	Absent or cellulose, depending on the group	Autotrophic (with chloroplasts) or heterotrophic
Fungi	Eukaryotic, no chloroplasts	Hyphae forming a mycelium, or single cells (yeasts)	Chitin	Heterotrophic: secrete enzymes and absorb the products
Plantae	Eukaryotic, with chloroplasts	Multicellular, with tissues and organs	Cellulose	Autotrophic (photosynthesis); starch store
Animalia	Eukaryotic, no chloroplasts	Multicellular, with tissues and organs	None	Heterotrophic: ingest food; glycogen store

Examples: Escherichia coli (Prokaryotae), Amoeba and Chlamydomonas (Protoctista), Saccharomyces cerevisiae and Penicillium (Fungi), ferns and flowering plants (Plantae), insects and mammals (Animalia).

Protoctista works as a catch-all for eukaryotes that are not plants, animals or fungi. Its members are not all closely related, which is one reason the system was later questioned.

Worked example 3

An organism from a pond is unicellular, has a nucleus, 80S ribosomes and chloroplasts, and has a cellulose wall. Place it in a kingdom.

1. The nucleus and 80S ribosomes show it is eukaryotic, so not Prokaryotae.
2. Chloroplasts and a cellulose wall might suggest Plantae, but Plantae are multicellular with tissues.
3. A unicellular eukaryote that photosynthesises belongs in Protoctista (a green alga).

4.2.2(c)(ii) New evidence and the three domains

Classification based on appearance has limits. Distantly related organisms can look alike, and microbes have few features to compare. From the 1970s, scientists could compare biological molecules and other genetic evidence. Two kinds of evidence are used:

• DNA base sequences (and sequences of RNA made from them), including whole genomes.
• Amino acid sequences of proteins, which reflect the DNA that codes for them.

The more recently two species shared a common ancestor, the fewer differences have built up in their sequences through mutation. Sequence data are objective and can be counted.

Carl Woese and colleagues compared the base sequences of ribosomal RNA from many prokaryotes. They found that the prokaryotes fell into two groups as different from each other as either is from eukaryotes. In 1990 this led to the proposal of three domains: Archaea, Bacteria and Eukarya.

Feature	Bacteria	Archaea	Eukarya
Nucleus	Absent	Absent	Present
Ribosomes	70S	70S	80S
Cell wall	Peptidoglycan	No peptidoglycan	Cellulose, chitin or none
Membrane lipids	Ester-linked, unbranched	Ether-linked, branched	Ester-linked, unbranched
DNA	Circular, no histones	Circular, with histones	Linear, with histones
First amino acid in a protein	Formylmethionine	Methionine	Methionine
Kingdoms within	Prokaryotae (part)	Prokaryotae (part)	Protoctista, Fungi, Plantae, Animalia

All four eukaryotic kingdoms sit inside the single domain Eukarya, while the old kingdom Prokaryotae is split across two domains.

Comparing the two systems

	Five kingdoms	Three domains
Rank	Kingdom	Domain (a rank above kingdom)
Main original basis	Observable features such as cell structure and nutrition	Differences in rRNA and other molecular evidence
Prokaryotes	One kingdom, Prokaryotae	Two domains, Bacteria and Archaea
Eukaryotes	Four kingdoms	One domain, Eukarya (still divided into the four kingdoms)

The domain system does not replace the kingdoms. It adds a higher rank and splits the prokaryotes in two, because the molecular evidence showed Archaea are not just another kind of bacteria. Classification is open to change as knowledge grows.

Worked example 4

The table shows differences in a 250-base section of one gene, aligned for four species P, Q, R and S.

Pair	Bases different	Percentage different
P and Q	5	2.0
P and R	20	8.0
P and S	45	18.0
Q and R	21	8.4
Q and S	44	17.6
R and S	43	17.2

For P and S: 45 ÷ 250 × 100 = 18.0 % different, so 82.0 % similar.

Interpretation: P and Q differ least, so they share the most recent common ancestor. R is next closest to both. S differs most from all three, so it branched off earliest.

4.2.2(d) Classification and phylogeny

Phylogeny is the evolutionary history of a group of organisms: how species are related through descent from common ancestors. Because all organisms share common ancestry, classification should reflect phylogeny. Organisms in the same genus share a more recent common ancestor than organisms in the same family but different genera. The nested hierarchy is the result of branching descent.

A phylogenetic tree is a branching diagram of these relationships. Tips are species. Each branch point is a common ancestor. Two species whose lines join closest to the tips are the most closely related.

For Worked example 4, the data give this tree:

            +------ P
        +---+
        |   +------ Q
    +---+
    |   +---------- R
----+
    +-------------- S

Reading the tree:

• P and Q share the most recent common ancestor.
• R shares a more distant ancestor with P and Q.
• S is the least closely related to the other three.

Evidence for relationships includes fossils and similarities in anatomy, but molecular evidence is more reliable for distant relatives. Species can look alike because they live in similar ways, not because of recent common ancestry. Dolphins and sharks both have streamlined bodies and fins, but a dolphin’s DNA is far more like a cow’s than a shark’s. You are not required to know cladistics.

Common errors

• Writing the ranks in the wrong order, or leaving out domain. Domain is the largest rank.
• Writing “Rana Temporaria” or leaving the name unitalicised and not underlined.
• Saying the binomial system is useful “because it is in Latin”. The advantage is that it is universal and unambiguous.
• Saying Archaea are a type of bacteria. In the three-domain system they are a separate domain.
• Claiming the five-kingdom system was based on DNA. It was based on observable features; molecular evidence led to the domains.
• Stating that more similar sequences mean more similar appearance. Similar sequences show a more recent common ancestor.

Next steps

Test yourself with the practice questions, then use the revision notes for quick recall. Related content on sampling and species diversity is in Measuring biodiversity.

Official syllabus

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