4.2.2(a)-(b) - Taxonomic hierarchy and binomial naming

Classification As A Hierarchy

Biological classification groups organisms into a hierarchy of named groups. A group at any level in the hierarchy is called a taxon; the plural is taxa. The hierarchy is nested, so a species belongs to a genus, that genus belongs to a family, that family belongs to an order, and so on.

The order required here is:

As you move from domain towards species, the group usually contains fewer organisms and those organisms usually share more features. This is why the hierarchy is useful: it does not just list organisms; it places them into nested groups that can be compared.

Be precise about the difference between a rank and a named taxon. “Family” is a rank. Suidae is a named family. “Genus” is a rank. Sus is a named genus.

The hierarchy is a model of classification. Like all scientific models, it is useful because it helps organise evidence, but it can be revised when better evidence becomes available.

Nested taxa in mammals

A named taxon contains all the lower taxa assigned to it; the nesting lets us infer memberships that a table leaves unstated. The wild boar can be classified through the ranks like this:

[DIAGRAM: taxonomy_hierarchy_wild_boar: Lesson 085: Taxonomic hierarchy and binomial naming - diagram 01; asset_slug: 085_m04_2_2_taxonomic_hierarchy_and_binomial_naming__diagram_01; recommended_method: drawn_biology; description: A clean 16:9 hierarchy diagram showing nested taxonomic ranks from domain to species using the wild boar example, with rank labels on the left, example taxa in boxes, arrows showing increasing specificity, and a small note that the full species name is the binomial Sus scrofa.]

Notice two things in the example.

First, each lower rank sits inside the rank above it. The genus Sus is inside the family Suidae, which is inside the order Artiodactyla.

Second, the species entry is best written as the full binomial name, Sus scrofa. In some classification tables, the genus may already be written in a separate column, so the species column may contain only the species epithet, scrofa. That does not mean scrofa alone is normally a full species name.

When you answer hierarchy questions, work in order rather than trying to remember isolated names. If you know that family sits below order and above genus, you can often place a missing rank even when the organism is unfamiliar.