The scientific classification of organisms

From the time of the first humans, classification of organisms has been a useful, even necessary tool. Primitive classification schemes sorted things into simple, utilitarian categories:  edible and inedible, dinner for you and you are dinner for it, dangerous and safe.

Modern western science continues to use the same basic method for sorting objects and organisms. It is based on a series of choices. Items are classified through two choices: this or that. This system is called a dichotomous key. Although it uses highly technical terms and some of the differences in organisms are microscopic, the key uses the basic principle of two choices, this or that. When published, the dichotomous key begins to resemble a family tree. To identify an organism, the scientist follows the limb as it branches. When the organism is finally identified, it occupies a unique space surrounded by its relatives.

The first pair of questions a western scientist will ask about an unidentified object is:  “Living or non-living.” It is a variation on the 20 questions “Animal, vegetable or mineral.”

Living organisms are customarily sorted under a hierarchy based on taxonomy developed by Carolus Linnaeus. If the organism is living, it must be sorted into plant or animal. When Van Leeuwenhoek invented the microscope he opened a world of single-celled organisms which are motile, usually a characteristic of animals, but containing chlorophyll and able to synthesize nutrients from sunlight, normally a characteristic of plants. Instead of inventing a new classification scheme, new categories were added to the existing scheme. One current scheme adds Domain as the initial category and adds four kingdoms to Linnaeus’ scheme for a total of six Kingdoms.

In order to better understand how this works, let us take the case of an unknown organism which has just alit on a scientist’s arm.


  • 1a Organism cells have a true nucleus—Eucaryote, move to level 2.
  • 1b Organism is a single cell and has no true nucleus—Eubacteria, Archaebacteria

Our unknown organism is multicellular and is, therefore, a Eucaryote.


  • 2a Organism is microscopic—Protista
  • 2b Organism is multicellular—Fungi, Plantae and Animalia, move to level 3.
  • 3a Organism manufactures its own food (autotrophic)—Plantae
  • 3b Organism cannot manufacture its own food (heterotrophic)—Fungi and Animalia move to level 4.
  • 4a Organism is mobile—Animalia, move to level 5.
  • 4b Organism is non-mobile—Fungi.


  • 5a Organism has an exoskeleton and jointed appendages—Arthropoda, move to level 6.
  • 5b Organism does not have an exoskeleton and jointed appendages—move to a different branch on the key.


  • 6a Organism has six legs, three body divisions and two pairs of wings. Our unknown organism has only one pair of actual wings, the second pair has evolved into a highly sophisticated gyroscope for flight control.—Insecta, move to level 7.
  • 6b Organism has more legs, two body divisions and lacks wings—move to a different branch on the key.


  • 7a Second pair of wings has been modified into a structure called a “haltere” -Diptera, move to level 8.
  • 7b Second pair of wings unmodified—move to a different branch on the key.


  • 8a Wings present and well developed, branched antennae with six or more segments, eyes not meeting above the antennae, proboscis long, scales present on wing veins, wing margins and body—Culicidae,  move  to a mosquito key.
  • 8b Body form of the organism other than above—move to a different branch on the key to determine the family.

Now that we have determined that the organism is a mosquito, we will probably have to move into a lab in order to determine the remaining levels of classification, as many important species traits are distinguished by the male terminalia. Under a microscope we can see the traits necessary to determine Genus and Species. In some mosquitoes, taxonomists have identified two further categories, Sub-Species and Variety. 

By convention, the genus name is capitalized and it is followed by the species name, which is lower case. The genus, species and sub-species names are also by convention either underlined or italicized. Thus, Culex tarsalis tarsalis, the blood-thirsty mosquito of the irrigated west, is Genus:  Culex, Species: tarsalis, Sub-species: tarsalis.

This classification scheme has endured because it is flexible enough to encompass new discoveries without changing the entire structure. Individual organisms can be identified to a unique position on the scheme, but that identification is based on relationship to other, similar individuals. By looking at the relative position in the classification of two individual organisms, one can rapidly determine how closely any two organisms are related.