18.1.4—Eukarya domain taxonomic hierarchy

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.