The Linnaean system of classification

by Rosario Douglas

Have you ever wondered why scientific names are used (especially in writing) when referring to animals or plants?  Or, why we even need to have scientific names when there are so many common names we already use for plants and animals?

In the 18th century a Swiss naturalist, Carolus Linnaeus, created the binomial system for classifying living things.  For this reason Linnaeus is considered the father of taxonomy – the classification of organisms.

Latin  being a “dead” language – minimizing any possible changes overtime, was Linnaeus’s language of choice for naming organisms.

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A 1775 painting of Carl von Linné 1707-1778.  Painted by Alexander Roslin (1718-1793). Public Domain.  Wikipedia

The binomial system of classification was needed to avoid the confusion created by the existence of many common names for the same organism (plant or animal).  A major disadvantage of common names is that they are not universal since they are inherent to the native language of the person using them.  Moreover, most plants and animals have many common names yet uncommon organisms may not have a common name at all.

The binomial system of classification assigns every organism a unique name consisting of a genus and a species. The scientific name for an organism remains the same unless new evidence is discovered (e.g. genetic information) that requires a major change to the name or group placement of the organism.  Regardless of the language you speak or what country you are in, an organism has the same scientific name.  Thus the binomial system of classification is a kind of  “universal” ID for every organism that has been discovered and named.   Linnaeus started using the binomial system of classification after his 1753 “Species Plantarum” was published.

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Title page of Species Plantarum (1753), by Carolus Linnaeus.  Special collections, National Agricultural Library.

There are many rules associated with naming a new species that we will not discuss here, but we will mention some basic rules to keep in mind when writing scientific names.  These include  1) scientific names have two parts (a genus and a species), 2) both genus and species are italicized, 3) the genus is always capitalized but the species is not and the plural for genus is genera.

The basic Linnaean binomial system of classification looks like this:

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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.  Source: CK-12 foundation.org

Taxonomy is a changing science and as new knowledge is acquired changes to the original system of classification are needed.  Linnaeus  originally recognized three kingdoms: Plants, Animals and minerals.  In 1990 the addition of the term “domain” to the original classification system was proposed (Woese et.al.,1990).  The three-domain system divides cellular life forms into archaea which are single-celled microorganisms with no cell nucleus (prokaryotes), bacteria (a type of  biological cell with a nucleus (Eukaryot)  and eukarya (organisms whose cells have a nucleus enclosed within membranes) (Wikipedia).

A more “current” classification scheme (using the added concept of domain) is shown below.  The division Life has also been added as the largest unit to which all organisms belong.   A further division of species into subspecies and variety is also in use today.

 

Variety   Is a naturally occurring variation of plant populations that are not genetically

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Carl R. Woese, Otto Kandler, Mark L. Wheelis, 1990: “Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya”. Proceedings of the National Academy of Sciences of the United States of America, 87(12):4576-9. doi:10.1073/pnas.87.12.4576.  Wikipedia Creative Commons

isolated from each other.  The abbreviation var is used and it is not capitalized or italized.  Variety is used for plants but it is not used when referring to animals.

Subspecies   This further division of species is applied to populations of plants or animals that are geographically isolated and thus are not able to mix or interbreed, although genetically they could produce fertile offspring if they were not geographically isolated.  The abbreviation used for subspecies is subsp and it is not capitalized or italized.

Species   One of the basic units of biological classification and a taxonomic rank.  A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring.

Genus   A collection of related species that share some features but are distinct from one another in some other features.

Family   All closely related genera. The name of families always end in eae

Order   All related families

Class   All related orders

Phylum   All related classes

Kingdom    All related phyla

Domain   Consists of three groups Archaea, Eubacteria, Eukarya

Life   Encompasses all of life

 

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Saguaro (Carnegia gigantea).  Photo by Michael Douglas

Here is an example of the Linnaean system of classification as we apply it to the Saguaro cactus.  Saguaro is the common name and as is often the case with other plants or animals there could be other common names for the same plant.

Using the binomial system of classification the saguaro cactus has a scientific name, Carnegia gigantea and this cactus can be classified as follows.

Domain     Eukarya

Kingdom   Plantae

Phylum      Tracheophyta

Class           Magnoliopsida

Order         Caryophyllales

Family        Cactaceae

Genus         Carnegia

Species       gigantea

As far as we know there are no subspecies or varieties of Carnegia gigantea

It is our hope that the next time you come across a scientific name for a plant or animal  you will be able to better understand why such a system, albeit devised long ago, is still very much in use today.

 

References:

University of British Columbia  Botanical Garden https://forums.botanicalgarden.ubc.ca/threads/use-of-the-terms-subspecies-and-variety.91693/

CK-12 Foundation https://www.ck12.org/biology/classification/lesson/Linnaean-Classification-BIO/

Encyclopedia Britannica https://www.britannica.com/biography/Carolus-Linnaeus

Science Prof Online https://www.scienceprofonline.com/biology-general/what-are-biological-systematics-taxonomy-phylogeny.html

Biology the Unity and Diversity of Life, Third edition by Cecie Starr and Ralph Taggart. 1978

The IUCN Red List  https://www.iucnredlist.org/species/152495/121476885

Woese CR, Kandler O, Wheelis ML (1990) Towards a natural system of organisms. Proposal for the domains Archaea, Bacteria and Eucarya. Proc Natl Acad Sci U S A 87:4576–4579

Wikipedia