Classification
What this domain tests
Classification asks you to organize biodiversity and infer evolutionary relationships. Exam items cover the hierarchy of taxonomy, binomial nomenclature, the three domains, dichotomous keys, and cladograms. You should be able to read a key or cladogram and defend a classification using shared traits or molecular evidence.
Core rules and concepts
- Taxonomic hierarchy. From broadest to narrowest: domain, kingdom, phylum, class, order, family, genus, species. A species name is binomial: genus capitalized, species lowercase, both italicized or underlined (e.g., Homo sapiens).
- Domains and kingdoms. The three domains are Bacteria, Archaea, and Eukarya. Eukarya includes protists, fungi, plants, and animals. Viruses are not placed in the three domains; they lack cellular organization and most biologists do not consider them living.
- Phylogeny. Cladograms show shared derived characters. A monophyletic group contains an ancestor and all of its descendants. Homologous structures reflect shared ancestry; analogous structures reflect similar function but not close ancestry.
- Dichotomous keys. Each step has two mutually exclusive choices. Start at step 1, follow the choice that matches the specimen, and record the endpoint.
Worked example
A cladogram shows species W, X, Y, and Z. W branches first; X branches next; Y and Z share the most recent common ancestor. Which two species are most closely related? Reasoning: Y and Z share the most recent common ancestor, so they are sister taxa and should have the most recent shared derived traits. W is the outgroup relative to the others.
Traps that cost marks
- Writing a species name with a capitalized species epithet or without italics.
- Reversing the hierarchy (for example, saying family is broader than order).
- Treating analogous structures as evidence of close relatedness.
- Placing viruses in Bacteria or Archaea.
- Assuming a key’s first couplet always identifies the organism; it only splits the choices.