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5.1: Classification

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    In biology, what is classification?

    There are millions and millions of species, so classifying organisms into proper categories can be a difficult task. To make it easier for all scientists to do, a classification system had to be developed.

    Linnaean Classification

    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.

    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.”

    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.

    201412291419892666645407_f37bdd26d477c40f654a911555451994-201412291419895577098142.pngLinnaean 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 many 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 Eukarya (see Figure below). Both Bacteria and Archaea consist of single-celled prokaryotes. Eukarya consists of all eukaryotes, from single-celled protists to humans. This domain includes the Animalia (animals), Plantae (plants), Fungi (fungi), and Protista (protists) kingdoms.

    201412291419892666675793_adebd92de33082d3d315a6d31261a827-201412291419895577341567.jpgThis phylogenetic tree is based on comparisons of ribosomal RNA base sequences among living organisms. The tree divides all organisms into three domains: Bacteria, Archaea, and Eukarya. Humans and other animals belong to the Eukarya domain. From this tree, organisms that make up the domain Eukarya appear to have shared a more recent common ancestor with Archaea than Bacteria.

    Hemimastigotes: A New Kingdom?

    In late 2018, Canadian researchers reported on what could be a novel kingdom of life. Hemimastigotes, an eukaryotic microscopic organism, does not fit into any of the known kingdoms of life.

    These eukaryotic organisms have numerous flagella, but seem to move in a random fashion, unlike the coordinated fashion of other microscopic organisms with many flagella. Genetic analysis confirmed a greater difference to other organisms than that seen between animals and fungi. The authors estimate the common ancestor between these organisms and other known living organisms lived over one billion years ago, about 500 million years before the first animals. See the link below for additional information.

    Rare microbes lead scientists to discover new branch on the tree of life

    Science Friday: Isn't this Octopus Adorable?

    What do you call a tiny octopus with big eyes, gelatinous skin and is cute as a button? Nobody knows quite yet! In this video by Science Friday, Stephanie Bush aims to classify and name this presently undescribed deep-sea cephalopod.


    • 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.


    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. 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.
    Image Reference Attributions
    201412291419892666612075_48de3e74433ef6b8872fa207c7879132-201412291419895576909004.jpg [Figure 1] Credit: Christopher Auyeung (based on image by Peter Halasz)
    Source: CK-12 Foundation (original image from
    License: CC BY-NC
    201412291419892666645407_f37bdd26d477c40f654a911555451994-201412291419895577098142.png [Figure 2] Credit: Christopher Auyeung (based on image by Peter Halasz);Barbara Akre and Jin Yu, based on data from and
    Source: CK-12 Foundation (original image from ; CK-12 Foundation
    License: CC BY-NC 3.0; CC BY-NC 3.0; (Original image) Public Domain
    201412291419892666675793_adebd92de33082d3d315a6d31261a827-201412291419895577341567.jpg [Figure 3] Credit: Barbara Akre and Jin Yu, based on data from and
    Source: ; CK-12 Foundation
    License: Public Domain; CC BY-NC 3.0

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