Use classification systems (domains, kingdoms, and the taxonomic hierarchy) and phylogenetic trees to organize organisms by evolutionary relationship (Tennessee Academic Standards for Science, Biology I, BIO1.LS4).

What this topic is asking

The Tennessee LS4 standards ask you to use classification systems and phylogenetic trees to organize organisms by evolutionary relationship. For the Biology I EOC that means knowing the three domains, the taxonomic hierarchy from domain down to species, how organisms are named (binomial nomenclature), how modern classification relies on molecular and structural evidence, and how to read a phylogenetic tree (cladogram) to judge relatedness. Tree-reading items are common and often technology-enhanced.

The taxonomic hierarchy

The order is worth memorizing in both directions, because EOC items test "broadest to most specific" and the reverse. A common mnemonic is Dear King Philip Came Over For Good Soup (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species). The more levels two organisms share, the more closely related they are: sharing the same genus means a closer relationship than only sharing the same kingdom.

The three domains

At the broadest level, life is divided into three domains:

• Bacteria. Single-celled prokaryotes (no nucleus), found almost everywhere.
• Archaea. Single-celled prokaryotes that are biochemically distinct from bacteria and often live in extreme environments.
• Eukarya. All eukaryotes (cells with a nucleus): the protists, fungi, plants, and animals.

So the first split (prokaryote versus eukaryote, and bacteria versus archaea) connects to the cell-types standard: domains reflect deep differences in cell structure and biochemistry.

Naming species: binomial nomenclature

Each species has a two-part scientific name, a system called binomial nomenclature. The first part is the genus (capitalized), and the second is the species (lowercase); the whole name is italicized, for example Homo sapiens or Canis lupus. This universal naming avoids the confusion of common names, which differ by language and region, and it places each organism in the hierarchy (the genus is the next level up from species).

Modern classification and evolutionary relationship

Reading a phylogenetic tree

A phylogenetic tree (or cladogram) is a branching diagram that shows how species are related by descent. The key to reading one:

• Each branch point (node) represents a common ancestor of the species above it.
• Species that share a more recent branch point are more closely related to each other.
• Species that branch off earlier (lower) in the tree are more distantly related to the rest.

So to judge which two of several species are most closely related, find the pair that shares the most recent common ancestor (the branch point closest to the tips). EOC items may give a tree and ask which species are most or least closely related, or which share a particular ancestor.

Try this

Q1. List the levels of classification from broadest to most specific. [2]

• Cue. Domain, kingdom, phylum, class, order, family, genus, species.

Q2. Explain why two organisms with very similar DNA are classified close together. [2]

• Cue. Similar DNA indicates a recent common ancestor, and modern classification groups organisms by evolutionary relationship, so greater molecular similarity means a closer classification.