18.1 Classification
- Syllabus
- 9700–2028–2029
- Topic
- 18.1
- Level
- A2
Learning objectives
18.1.1Species concepts• Species concepts: biological, morphological, ecological18.1.2Three domains• Three domains: Archaea, Bacteria, Eukarya18.1.3Archaea and Bacteria as prokaryotes• Archaea and Bacteria as prokaryotes: differences in membrane lipids, ribosomal RNA, cell wall composition18.1.4Eukarya domain taxonomic hierarchy• Eukarya domain taxonomic hierarchy: kingdom, phylum, class, order, family, genus, species18.1.5Characteristic features of kingdoms• Characteristic features of kingdoms: Protoctista, Fungi, Plantae, Animalia18.1.6Virus classification• Virus classification: nucleic acid type (RNA/DNA), single/double stranded
Species concepts use different evidence
A species concept is a criterion for deciding whether organisms belong to the same species. Biological, morphological and ecological concepts use different evidence, so they can lead to different conclusions about the same organisms.
| Concept | Same species when… | Important limit |
|---|---|---|
| Biological | they can interbreed and produce fertile offspring | difficult or impossible to apply to fossils, asexual organisms or populations that cannot be tested together |
| Morphological | they share the observable structural features used to define the group | variation within a species and very similar-looking separate species can mislead |
| Ecological | they occupy the same ecological niche: they use resources and interact with the environment in the same way | sharing a habitat alone is not enough; different niches can exist in the same place |
For example, two bear populations may look different and use different habitats and foods, supporting separate morphological and ecological classifications. If they interbreed and produce fertile offspring, the biological concept instead supports treating them as the same species. The answer depends on the stated concept and evidence.
The ecological species concept is based on niche, not simply on organisms living in the same area at the same time. No one concept works perfectly in every case.
The three domains reflect deep evolutionary divisions
The three domains are Bacteria, Archaea and Eukarya. Domain is the highest taxonomic rank, and grouping organisms into domains helps represent deep evolutionary divisions.
Bacteria and Archaea contain prokaryotic cells: they lack a nucleus. Eukarya contains eukaryotic cells: genetic material is enclosed in a nucleus and the cell has compartmentalised structures, including membrane-bound organelles.
Cell type is an important first boundary, but it is not enough to classify all prokaryotes. Molecular evidence, especially comparisons of RNA genes, supports separating the two prokaryotic domains and recognising their deeper relationships.
Use the classification in this order:
- Ask whether the cell is prokaryotic or eukaryotic.
- If prokaryotic, keep Bacteria and Archaea as separate possible domains rather than treating “prokaryote” as a domain.
- Use molecular evidence alongside cell organisation before assigning the domain.
- State the result as Bacteria, Archaea or Eukarya; detailed Archaea–Bacteria differences and lower Eukaryotic kingdoms are separate questions.
A simple cell shape cannot by itself identify the domain. “Prokaryote” describes cell organisation, whereas Bacteria and Archaea are separate domains; Eukarya is the eukaryotic domain. Viruses are outside this cellular three-domain classification.
Three features separate Archaea from Bacteria
Archaea and Bacteria are both prokaryotes: their cells lack a nucleus and membrane-bound organelles. They are separate domains because they differ in membrane lipids, ribosomal RNA and cell-wall composition.
| Feature | Bacteria | Archaea |
|---|---|---|
| Membrane lipids | fatty acids are ester-linked to glycerol | branched hydrocarbon chains are ether-linked to glycerol |
| Ribosomal RNA | one characteristic prokaryotic type | distinct rRNA types; sequences and ribosomal subunits show similarities to Eukarya |
| Cell wall | contains peptidoglycan | does not contain peptidoglycan; composition is different |
First establish that the organism is prokaryotic, then compare evidence from all three features. Lack of a nucleus narrows the classification to Archaea or Bacteria but does not decide between them.
Living in an extreme environment is not the definition of Archaea, and not all Archaea are extremophiles. Domain classification depends on cellular and molecular evidence.
Eukaryotic taxonomy is nested
Taxonomy places similar organisms into nested ranks. Within Eukarya, the broad domain contains kingdoms, and each lower rank contains a smaller, more similar group.
Read the hierarchy from broad to specific:
Domain: Eukarya
→ Kingdom
→ Phylum
→ Class
→ Order
→ Family
→ Genus
→ Species
At a higher rank, the group contains more organisms with less similarity between them. Moving down the nested hierarchy applies more shared classification evidence, so the group contains fewer organisms with greater similarity. Species is the lowest rank in this sequence.
Use the hierarchy in this order:
- Confirm the organism belongs to Eukarya.
- Place it in a kingdom, then progressively narrower phylum, class, order, family, genus and species groups.
- Treat each lower group as nested within the preceding one, not as an unrelated label.
- Use the evidence appropriate to the rank; do not substitute a kingdom name for a species identification.
Domain is the highest rank, while species is the lowest in this hierarchy. The card establishes nested classification; detailed kingdom features and virus classification are separate cards.
Four kingdoms have distinct feature patterns
Protoctista, Fungi, Plantae and Animalia are kingdoms within Eukarya. Classify them from a pattern of cell structure, body organisation and nutrition rather than from one feature alone.
| Kingdom | Characteristic feature pattern |
|---|---|
| Protoctista | mostly unicellular, though some are multicellular; highly varied; some are photosynthetic with chloroplasts, while others are heterotrophic; cell walls occur in some groups |
| Fungi | unicellular or multicellular; no chloroplasts; cell walls contain chitin; heterotrophic by extracellular digestion and absorption; multicellular forms have hyphae forming a mycelium and commonly reproduce by spores |
| Plantae | multicellular; cellulose cell walls and chloroplasts; autotrophic by photosynthesis; cells form differentiated tissues and organs |
| Animalia | multicellular; no cell walls or chloroplasts; heterotrophic by ingestion; specialised cells form tissues and organs |
Compare several features together. A cell wall does not by itself mean Plantae: fungal walls contain chitin, plant walls contain cellulose, and some protoctists also have walls. Likewise, both fungi and animals are heterotrophic, but fungi digest externally and absorb products whereas animals ingest food.
Fungi are not plants: they lack chloroplasts, do not photosynthesise and have chitin rather than cellulose in their cell walls. Protoctista is especially diverse, so not every member has every listed feature.
Viruses can be classified by nucleic acid
Viruses are acellular and have no metabolism of their own, so they depend on host cells for replication. They are therefore outside the cellular three-domain and Eukaryotic-kingdom classifications.
Classify a virus by two genome features:
Nucleic-acid type — DNA or RNA.
Strand form — single-stranded or double-stranded.
These give four broad combinations: single-stranded DNA, double-stranded DNA, single-stranded RNA and double-stranded RNA.
Use the classification in this order:
- Identify whether the viral genome is DNA or RNA.
- Determine whether that genome is single-stranded or double-stranded.
- Report the combination as the virus classification; do not use the host organism to replace the genome evidence.
- Keep the boundary clear: this is a nucleic-acid classification of an acellular, host-dependent entity, not a domain or kingdom assignment.
A virus is not placed in Bacteria, Archaea, Eukarya or one of the four Eukarya kingdoms simply because of the cell it infects. This card stops at genome-based classification and does not add named virus examples or replication-cycle detail.