5.1 Taxonomy and Classification

Why Classification Matters on the Praxis

Classification gives biologists a shared vocabulary for the roughly 1.5 million described species and the millions more yet to be named. On the Praxis Biology test, expect questions that ask you to (1) order the taxonomic ranks, (2) interpret a scientific name, (3) place an organism in the correct domain or kingdom, or (4) explain why molecular data reshaped 20th-century taxonomy.

The Linnaean Hierarchy

Carl Linnaeus (1707-1778) introduced the hierarchical system biologists still use. Each rank is a set nested inside the next-larger set: every species belongs to exactly one genus, every genus to one family, and so on.

Mnemonics that survive every biology classroom:

• Did King Phillip Come Over For Good Soup
• Dear King Philip Came Over For Good Spinach

The ranks move from most general (Domain) to most specific (Species). Members of the same genus share more recent ancestry than members of the same family.

Binomial Nomenclature

Linnaeus's two-name system gives every species a universal, language-independent label:

• Format: Genus species (italicized; genus capitalized; specific epithet lowercase)
• Handwritten: underline both parts when you cannot italicize
• Abbreviation: after first use, the genus may be shortened (e.g., H. sapiens)
• Authority: the original describer's surname sometimes follows the name (e.g., Homo sapiens Linnaeus, 1758)

A scientific name is unique worldwide, while common names ("robin" means different birds in Europe and North America) are ambiguous.

The Three-Domain System

In 1977, Carl Woese compared small-subunit ribosomal RNA (16S rRNA in prokaryotes, 18S rRNA in eukaryotes) sequences and showed that the kingdom Monera was not a natural group. He proposed three domains:

Key molecular finding: Archaea share more derived features with Eukarya (similar transcription machinery, RNA polymerases with multiple subunits, histone-like proteins) than they do with Bacteria. Bacteria branched off the tree of life first.

From Five Kingdoms to Three Domains

The older five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia) collapsed because:

1. Monera was polyphyletic - Bacteria and Archaea are not each other's closest relatives.
2. Protista is paraphyletic - it is a grab-bag of unrelated eukaryotic lineages.
3. Molecular data trump morphology when ancestry is the question.

Modern textbooks may show six kingdoms (splitting Monera into Bacteria and Archaea while keeping the rest) or shift fully to domain-based phylogeny with multiple eukaryotic supergroups (Excavata, SAR, Archaeplastida, Amoebozoa, Opisthokonta).

Cladistics and Phylogenetic Trees

Modern systematics emphasizes monophyletic clades - a clade is an ancestor plus all its descendants. Three classification styles:

• Monophyletic (clade): includes the common ancestor and every descendant - the only group type cladistics accepts.
• Paraphyletic: includes the ancestor and some descendants (e.g., "reptiles" excluding birds).
• Polyphyletic: groups based on convergent traits, excluding the common ancestor (e.g., "warm-blooded animals" treating mammals + birds as one group).

Shared derived characters (synapomorphies) - like feathers in birds or hair in mammals - define clades. Shared ancestral characters (plesiomorphies) do not.

High-Yield Praxis Pitfalls

• Italicization rules: "Escherichia coli" is correct; "Escherichia Coli" is wrong (lowercase species epithet).
• Order of ranks: confusing Class and Order is a classic distractor; remember the C of Class comes before the O of Order in the mnemonic.
• Viruses are not in any domain. They are acellular and excluded from cellular taxonomy.
• Archaea ≠ Bacteria. Even though both are prokaryotic, they are as distinct from each other as either is from you.

The Three-Domain System and Binomial Rules

Modern classification uses three domains: Bacteria and Archaea (both prokaryotic, no nucleus) and Eukarya (nucleus and organelles). Archaea, though prokaryotic, are more closely related to eukaryotes in key molecular features (RNA polymerase, membrane lipids) - a frequently tested surprise.

The Linnaean hierarchy, broadest to narrowest, is Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species (mnemonic: 'Dear King Philip Came Over For Good Soup'). The lower the rank, the more shared characteristics and fewer organisms it contains.

Binomial nomenclature rules ETS checks: the genus is capitalized, the species epithet is lowercase, and both are italicized (or underlined), e.g., Homo sapiens. The genus name may be abbreviated after first use (H. sapiens).

A modern point: classification aims to be phylogenetic - groups should be monophyletic (an ancestor and all its descendants). 'Reptiles' excluding birds is paraphyletic and disfavored. When a stem asks which two organisms are most related, count shared derived traits, not superficial resemblance.

Dichotomous Keys and Practical Identification

ETS includes practical classification skills, especially the dichotomous key - a tool that identifies an organism through a series of paired, either/or choices (e.g., '1a. leaves needle-like -> go to 2 / 1b. leaves broad -> go to 3'). Each choice narrows the possibilities until a single species remains. Items give a short key and an organism's traits and ask which name results; work the couplets in order and do not skip steps. Good keys use observable, contrasting characters, not subjective ones.

Cladistics vs. Traditional Classification

Two organisms can look alike yet be distantly related (analogy/convergence), so modern taxonomy weights shared derived characters and increasingly molecular data (DNA and protein sequences) over gross morphology. The more similar two species' DNA sequences, the more recently they shared a common ancestor. When a stem pits appearance against molecular evidence, the molecular and shared-derived-character evidence wins - it reflects actual evolutionary history, the goal of a phylogenetic classification.