18.1 Classification

A species concept is a rule for deciding which organisms should be grouped as one species. Different concepts answer different questions, so classification must match the evidence available.

Biological species concept — asks whether organisms with similar morphological and physiological features can interbreed and produce fertile offspring. Its boundary is reproductive evidence, but testing fertility can be difficult and time-consuming; it is also limited when organisms cannot be tested directly.

Morphological species concept — asks whether organisms share physical features that distinguish them from other groups. It is practical when morphology is observable, but appearance alone may not reveal reproductive or ecological differences.

Ecological species concept — groups similar organisms living in the same area at the same time. Its boundary is ecological context, so it should not be treated as a fertility test or as a purely visual definition.

Use the evidence in this order:

1. Identify whether the question gives reproductive, morphological or ecological evidence.
2. Match that evidence to the corresponding concept.
3. State the conclusion with its boundary: a concept supports a classification decision, but no single concept works universally.
4. Do not infer a new species from one visible difference alone when the relevant reproductive or ecological evidence is missing.

Phylogenetic definitions are not added here because the matched SME page supports biological, morphological and ecological concepts, not a separate phylogenetic treatment. Do not move from this card into domains, kingdoms or viruses.

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:

1. Ask whether the cell is prokaryotic or eukaryotic.
2. If prokaryotic, keep Bacteria and Archaea as separate possible domains rather than treating “prokaryote” as a domain.
3. Use molecular evidence alongside cell organisation before assigning the domain.
4. 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.

Archaea and Bacteria are both domains of unicellular prokaryotes, so both lack a nucleus. They are nevertheless separate domains because molecular and structural evidence shows important differences.

Shared boundary — both have prokaryotic cells and no nucleus; a bacterial-looking shape or a similar size does not identify the domain.
Membrane evidence — archaeal membrane lipids are distinctive, whereas bacterial membrane lipids have a different structure.
Cell-wall evidence — bacterial cell walls contain peptidoglycan; archaeal cell walls do not contain peptidoglycan.
Genetic evidence — both have 70S ribosomes, but archaeal ribosomal RNA and ribosomal structure show greater similarity to Eukarya than to Bacteria.

Use the evidence in this order:

1. Confirm the organism is prokaryotic; this places it in the Bacteria-or-Archaea branch, not automatically in Bacteria.
2. Compare membrane-lipid, cell-wall and ribosomal-RNA evidence.
3. Assign the domain from the combined evidence rather than from habitat, shape or the fact that the cell lacks a nucleus.
4. Treat extreme-environment survival as a possible observation, not as the definition of Archaea; not all Archaea are extremophiles.

“Both lack a nucleus” does not mean Archaea and Bacteria are the same domain. This card stops at their comparison; Eukarya nested taxonomy, kingdoms and viruses belong to later cards.

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:

1. Confirm the organism belongs to Eukarya.
2. Place it in a kingdom, then progressively narrower phylum, class, order, family, genus and species groups.
3. Treat each lower group as nested within the preceding one, not as an unrelated label.
4. 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.

The main Eukarya kingdoms are Protoctista, Fungi, Plantae and Animalia. Kingdom classification combines cell structure, body organisation and nutrition; one visible feature is rarely enough.

Protoctista — a broad group of eukaryotic organisms outside the other three kingdoms; members vary widely in cell structure, feeding and body form.
Fungi — eukaryotic heterotrophs with non-cellulose cell walls; they obtain nutrients by extracellular digestion and absorption, including from decaying material or living hosts.
Plantae — multicellular eukaryotes with cellulose cell walls, large vacuoles and usually chloroplasts; they are autotrophs and form complex body systems.
Animalia — multicellular eukaryotes with no cell walls; they are heterotrophs with varied feeding mechanisms and specialised cells that can form tissues and organs.

Classify from combined evidence:

1. Confirm the organism is eukaryotic.
2. Record cell-wall and organelle features, body organisation and the way nutrition is obtained.
3. Compare the whole feature pattern with the four kingdom profiles.
4. State the best-supported kingdom while keeping exceptions and the broad Protoctista boundary in view; do not infer a kingdom from multicellularity or one cell feature alone.

Plants and fungi are not defined by being multicellular, and fungi are not autotrophs. The kingdom profiles are evidence-based teaching groupings; this card does not extend into virus nucleic-acid classification.

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:

1. Identify whether the viral genome is DNA or RNA.
2. Determine whether that genome is single-stranded or double-stranded.
3. Report the combination as the virus classification; do not use the host organism to replace the genome evidence.
4. 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.