Classification

· Classification = organising organisms into groups based on shared characteristics and evolutionary relationships.
· Modern classification uses evidence such as cell structure, biochemistry, DNA / RNA sequence data, ribosomal RNA, and evolutionary ancestry.
· Classification helps scientists identify organisms, name species consistently, and show relationships between organisms.
· Exam focus: know the three domains, the Eukarya taxonomic hierarchy, species concepts, kingdom features, and virus classification.

Species concepts

· Species = a group of organisms defined using a particular species concept; no single definition works perfectly for all organisms.
· Biological species concept: organisms are the same species if they can interbreed naturally and produce fertile offspring.
· Limitation: not useful for asexual organisms, fossils, or organisms that do not reproduce in observed conditions.
· Morphological species concept: organisms are grouped by physical structure / appearance.
· Limitation: can be misleading when species look similar but are genetically different, or when individuals in one species show variation.
· Ecological species concept: organisms are grouped by their ecological niche, including their role in the ecosystem and interactions with the environment.
· Limitation: niche data may be incomplete, and related organisms may have overlapping niches.

Species can be defined in different ways depending on whether biologists focus on breeding, appearance, or ecological role. Each species concept is useful in some situations but has limitations. Source

Three-domain classification

· Organisms are classified into three domains: Archaea, Bacteria, and Eukarya.
· Archaea and Bacteria are both prokaryotes.
· Prokaryotes have cells without a nucleus or membrane-bound organelles.
· Eukarya includes organisms with eukaryotic cells, which contain a nucleus and membrane-bound organelles.
· The three-domain system reflects major differences in cell structure, biochemistry, and evolutionary relationships.

The three-domain system separates cellular life into Bacteria, Archaea and Eukarya. It shows that Archaea and Bacteria are both prokaryotic but are not the same group. Source

Archaea vs Bacteria

· Archaea and Bacteria are both prokaryotic, but they differ in key molecular features.
· Membrane lipids: Archaea have different membrane lipid chemistry from Bacteria.
· Ribosomal RNA: Archaea have rRNA sequences that differ from Bacteria.
· Cell walls: Bacteria usually have peptidoglycan cell walls, while Archaea do not have bacterial peptidoglycan.
· Exam tip: do not describe Archaea as “just bacteria”; state they are distinct prokaryotic domains.

Eukarya taxonomic hierarchy

· In Eukarya, organisms are classified using the hierarchy: kingdom → phylum → class → order → family → genus → species.
· The hierarchy becomes more specific from kingdom to species.
· Organisms in the same species are more closely related than organisms only in the same genus, family, or kingdom.
· Scientific names use binomial nomenclature: Genus species, e.g. Homo sapiens.
· The genus name starts with a capital letter; the species name starts with a lower-case letter.
· In typed work, scientific names should be italicised; when handwritten, they should be underlined.

This diagram shows the order of taxonomic ranks from the broadest group to the most specific. It helps students remember the Eukarya hierarchy used in classification questions. Source

Kingdoms within Eukarya

· Protoctista: mostly unicellular eukaryotes, but some are multicellular; may be autotrophic or heterotrophic; examples include protozoa and algae.
· Fungi: eukaryotic, usually multicellular except yeasts; have chitin cell walls; feed by saprotrophic nutrition using extracellular digestion; store carbohydrate as glycogen.
· Plantae: multicellular eukaryotes; have cellulose cell walls; contain chloroplasts for photosynthesis; store carbohydrate as starch.
· Animalia: multicellular eukaryotes; no cell walls; cells are organised into tissues and organs; nutrition is heterotrophic by ingestion.
· Exam tip: compare kingdoms using cell wall, nutrition, chloroplasts, unicellular/multicellular structure, and storage carbohydrate.

Virus classification

· Viruses are not classified into the three cellular domains because they are non-cellular.
· Viruses are classified by their type of nucleic acid: DNA or RNA.
· Viral nucleic acid may be single-stranded or double-stranded.
· CIE 18.1 requires virus classification only by: DNA or RNA, and single-stranded or double-stranded.
· Avoid adding unnecessary detail unless asked; do not overcomplicate with full viral life cycles.

This diagram classifies viruses according to their nucleic acid type. For CIE 18.1, focus only on whether the genome is DNA or RNA and whether it is single-stranded or double-stranded. Source

Exam comparison points

· Archaea vs Bacteria: both are prokaryotes, but differ in membrane lipids, rRNA, and cell wall composition.
· Bacteria vs Eukarya: Bacteria are prokaryotic; Eukarya are eukaryotic with a nucleus and membrane-bound organelles.
· Protoctista vs Fungi: Protoctista are very diverse and often unicellular; Fungi have chitin cell walls and saprotrophic nutrition.
· Plantae vs Animalia: Plantae have cellulose cell walls and chloroplasts; Animalia lack cell walls and feed by ingestion.
· Species concepts: biological = fertile offspring, morphological = appearance, ecological = niche.

Common exam mistakes

· Calling Archaea “bacteria” without stating they are a separate domain.
· Forgetting that Archaea and Bacteria are both prokaryotes.
· Mixing up the hierarchy order: kingdom → phylum → class → order → family → genus → species.
· Saying all Protoctista are “animal-like”; many are photosynthetic or algae-like.
· Classifying viruses as living organisms in one of the three domains; viruses are non-cellular.
· Giving too much virus detail instead of the syllabus requirement: DNA/RNA and single/double stranded.