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9.3: Classification

Created by: CK-12

Lesson Objectives

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

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, fro 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
a grouping of organisms in a classification system such as the Linnaean system; for example, species or genus
taxonomy
science of classifying organisms

Introduction

The evolution of life on Earth over the past 4 billion years has resulted in a huge variety of species. For more than 2,000 years, humans have been trying to classify the great diversity of life. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth.

Linnaean Classification

All modern classification systems have their roots in the Linnaean classification system. It was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things that were known at his time. He grouped together organisms that shared obvious physical traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the “father of taxonomy.” You can learn more about Linnaeus and his system of classification by watching the video at this link: http://teachertube.com/viewVideo.php?video_id=169889. The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon). Taxa range from the kingdom to the species (see Figure below). The kingdom is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. Examples are the plant and animal kingdoms. The species is the smallest and most exclusive grouping. It consists of organisms that are similar enough to produce fertile offspring together. Closely related species are grouped together in a genus.

Linnaean Classification System: Classification of the Human Species. This chart shows the taxa of the Linnaean classification system. Each taxon is a subdivision of the taxon below it in the chart. For example, a species is a subdivision of a genus. The classification of humans is given in the chart as an example.

Binomial Nomenclature

Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called binomial nomenclature, gives each species a unique, two-word Latin name consisting of the genus name and the species name. An example is Homo sapiens, the two-word Latin name for humans. It literally means “wise human.” This is a reference to our big brains. Why is having two names so important? It is similar to people having a first and a last name. You may know several people with the first name Michael, but adding Michael’s last name usually pins down exactly whom you mean. In the same way, having two names uniquely identifies a species.

Revisions in Linnaean Classification

Linnaeus published his classification system in the 1700s. Since then, many new species have been discovered. The biochemistry of organisms has also become known. Eventually, scientists realized that Linnaeus’s system of classification needed revision. A major change to the Linnaean system was the addition of a new taxon called the domain. A domain is a taxon that is larger and more inclusive than the kingdom. Most biologists agree there are three domains of life on Earth: Bacteria, Archaea, and Eukaryota (see Figure below). Both Bacteria and Archaea consist of single-celled prokaryotes. Eukaryota consists of all eukaryotes, from single-celled protists to humans. This domain includes the Animalia (animals), Plantae (plants), Fungi (fungi), and Protista (protists) kingdoms.

Three-Domain Classification. This diagram shows the three domains of organisms that currently live on Earth.

Phylogenetic Classification

Linnaeus classified organisms based on obvious physical traits. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s (discussed in the following chapter), scientists looked for a way to classify organisms that showed phylogeny. Phylogeny is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, like the one in Figure below.

Phylogenetic Tree. This phylogenetic tree shows how three hypothetical species are related to each other through common ancestors. Do you see why Species 1 and 2 are more closely related to each other than either is to Species 3?

One way of classifying organisms that shows phylogeny is by using the clade. A clade is a group of organisms that includes an ancestor and all of its descendants. Clades are based on cladistics. This is a method of comparing traits in related species to determine ancestor-descendant relationships. Clades are represented by cladograms, like the one in Figure below. This cladogram represents the mammal and reptile clades. The reptile clade includes birds. It shows that birds evolved from reptiles. Linnaeus classified mammals, reptiles, and birds in separate classes. This masks their evolutionary relationships.

Mammal and Reptile Clades. This cladogram classifies mammals, reptiles, and birds in clades based on their evolutionary relationships.

Lesson Summary

  • Classification is an important step in understanding life on Earth. All modern classification systems have their roots in the Linnaean classification system. The Linnaean system is based on similarities in obvious physical traits. It consists of a hierarchy of taxa, from the kingdom to the species. Each species is given a unique two-word Latin 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 that shows how species are related to each other through common ancestors. A clade is a group of organisms that includes an ancestor and all of its descendants. It is a phylogenetic classification, based on evolutionary relationships.

Lesson Review Questions

Recall

1. What is taxonomy?

2. Define taxon and give an example.

3. What is binomial nomenclature? Why is it important?

4. What is a domain? What are the three domains of life on Earth?

5. What is cladistics, and what is it used for?

Apply Concepts

6. Create a taxonomy, modeled on the Linnaean classification system, for a set of common objects, such as motor vehicles, tools, or office supplies. Identify the groupings that correspond to the different taxa in the Linnaean system.

7. Dogs and wolves are more closely related to each other than either is to cats. Draw a phylogenetic tree to show these relationships.

Think Critically

8. Compare and contrast a Linnaean taxon, such as the family or genus, with the clade.

9. Explain why reptiles and birds are placed in the same clade.

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?
  • What might be the other forces of evolution?

Opening image copyright Dariush M., 2010. Used under license from Shutterstock.com.

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