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9.3: Classification

Difficulty Level: At Grade Created by: CK-12
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Key Concept

The Linnaean classification system is based on similarities in obvious physical traits. It consists of a hierarchy of taxa, from the species to the kingdom, and gives each species a unique genus and species name. The recently added domain is a larger and more inclusive taxon than the kingdom. Phylogeny is the evolutionary history of group of related organisms. It is represented by a phylogenetic tree. A clade is used to classify organisms based on evolutionary relationships.


  • CA.9–12.LS.8.f
  • NSES.9–12.C.3.5
  • AAAS.9–12.5.A.2; AAAS.9–12.5.F.2

Lesson Objectives

  • Outline the Linnaean classification, and define binomial nomenclature.
  • Describe phylogenetic classification, and explain how it differs from Linnaean classification.

Lesson Vocabulary

  • binomial nomenclature: method of naming species with two names, consisting of the genus name and species name
  • clade: group of related organisms that includes an ancestor and all of its descendants
  • domain: taxon in the revised Linnaean system that is larger and more inclusive than the kingdom
  • genus: taxon above the species in the Linnaean classification system; group of closely related species
  • kingdom: largest and most inclusive taxon in the original Linnaean classification system
  • Linnaean classification system: system of classifying organisms based on observable physical traits; consists of a hierarchy of taxa, from the kingdom to the species
  • phylogenetic tree: diagram that shows how species are related to each other through common ancestors
  • phylogeny: evolutionary history of a group of related organisms
  • species: group of organisms that are similar enough to mate together and produce fertile offspring
  • taxa: (singular, taxon) grouping of organisms in a classification system, such as the Linnaean system; for example, species or genus
  • taxonomy: science of classifying organisms

Teaching Strategies

Introducing the Lesson

On the board, write the scientific name of a common species such as the domestic cat (Felis catus). Call on volunteers to explain (or explain yourself if necessary) what the two parts of the name represent (the genus and species names). Tell the class they will learn more about how organisms are named and classified in this lesson.


Students can apply principles of Linnaean and phylogenetic classification by doing the activities at the URLs below. In the first activity, they will develop a classification system for different kinds of “organisms” in the “pasta kingdom.” In the second activity, they will apply cladistics to a group of organisms represented by a collection of nails, screws, and bolts.

Differentiated Instruction

Help students focus on the main ideas in the lesson. Pair less proficient readers with more proficient readers, and ask partners to complete cloze sentences, such as those below, while they read the lesson. LPR

  1. Taxonomy is ... (a method of organizing living things into groups.)
  2. Biologists classify organisms in order to ... (make sense of the incredible diversity of life on Earth.)
  3. Binomial nomenclature is ... (Linnaeus’ method of naming species.)
  4. The scientific name of a species consists of ... (its genus and species names.)


Ask a small group of students who need extra challenges to create a taxonomy board game. The object of the game should be to identify correctly an organism’s species based on a series of increasingly specific clues about the organism’s traits. Encourage other students to play the game.

Science Inquiry

Have groups of students create a cladogram for the “evolution” of a familiar technology, such as audio devices. Students can do online research to identify the sequence in which major innovations occurred in their technology. (For example, audio devices evolved through stages of record players, 8-track tape players, cassette players, Walkmans, CD players, and iPods). Discuss their completed cladograms, and relate them to cladograms in the evolution of life.

Common Misconceptions

Students may have the misconception that evolutionary relationships are linear, when in fact phylogenies are branched like bushes. At the URL below, you can find an interactive cartoon series that cleverly makes this point and clarifies the misconception.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.

Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.

Points to Consider

This chapter gives you a glimpse of 4 billion years of evolution on Earth. In the next chapter, you will read about the forces that bring about evolution. Natural selection is one of these forces. It generally results in a population or species becoming better adapted to its environment over time.

  • How does natural selection work? How does it bring about evolutionary change?
    • (Fitter members of a species live longer and produce more offspring than other members of the species. Over time, their traits become more common in the species.)
  • What might be the other forces of evolution?
    • (Students might mention mutation, but they may not be able to identify gene flow or genetic drift.)

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