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7.1. Classification systems of living beings

Classification systems of living beings

Almost 3 million different species of living beings are currently known, although it is believed that there may be between 5 and 50 million species, not counting those that are already extinct. This variety of living things is called biodiversity.

Scientists, in order to study such a variety of living beings, have tried to classify them into groups, ordering them according to their characteristics. The science that deals with the classification of living things is taxonomy.

In our daily lives, classifications are also necessary. If I need window cleaner and I walk into a supermarket I don't know, I'll quickly locate it when I see the cleaning products aisle. This is so because the products are well classified. The same would happen to me if I want to find a book in the library. It won't take me long to find it if they're sorted properly, but if they're sorted by color or size, it'll be hard to find.

The Greek philosopher and scientist Aristotle (384 BC - 322 BC) was the first to classify living beings into two groups based on their external appearance:

A disciple of Aristotle, Theophrastus (371 BC - 287 BC) classified plants into trees, shrubs, and herbs.

These classification systems were maintained for many centuries, but when explorers in the 16th and 17th centuries discovered many new species from other distant lands, other systems had to be found to classify them.

Types of classification of living beings

There are several ways to classify living things:

  • Utilitarian classification. It consists of ordering plants and animals by their usefulness to humans. It has the problem that a living being can have several functions or not have any, so it is not useful. It is a practical but not a scientific order.
  • Contrived classification. It is based on ordering living things according to their easily observable characteristics. It looks at the external similarities and differences of living things, such as size, color, shape, where it lives, type of food, etc. This type of classification can include a pigeon and a wasp in the same group because they both fly, or an octopus and a sardine because they are both aquatic. Nor would it be a useful classification.
  • Natural classification. This classification is based on the evolutionary history of living beings, grouping living beings by their cellular, genetic, biochemical, anatomical, physiological characteristics, etc.

The criteria used to make any classification must be:

  • Objectives, which do not depend on the interpretation of the person who is classifying. For example, the number of limb limbs of an animal would be an objective criterion.
  • Discriminatory, that a certain characteristic is common to some elements, but not to all.

For example, to classify the students of a class of 1st ESO, the long or short hair criterion would not be correct, since there may be cases in which their classification would not be objective, depending on what the classifier thought. Having a nose would not be discriminatory either, since all students have one, and this criterion would not serve to classify them.

dichotomous key is a tool that allows us to identify living things. It is based on descriptions of the characteristics of the organisms grouped in pairs, objective and discriminatory, depending on whether or not they have a certain character, repeating this process until the organism in question is classified.

Interactive activity: Classification of living beings.

Taxonomy and nomenclature

In addition to the problem of how to classify living beings, there is the problem of what to call them. Some species of plants and animals receive several names, not only in different languages, but in different localities with the same language. For example, we refer to the same bird when we talk about a magpie, picaraza, picaza, fag or pega, and this can lead to confusion, so there is a common nomenclature that avoids these problems. If we call that bird Pica pica , all scientists will know exactly what living being we are referring to.

Linnaeus (1707 - 1778) was the creator of the classification of living beings or taxonomy. He developed a system of binomial nomenclature that is still used today as it groups living things into ever broader categories.


Taxonomy is the science that deals with the classification of living beings, grouping them according to their characteristics, from the most general to the most specific. The lowest level is the species level and they are grouped into broader taxonomic categories or taxa.

  • Species. species is the set of organisms with similar characteristics, capable of interbreeding with each other and having fertile offspring. For example, the house fly (Musca domestica) is a species.
  • Gender. Species are grouped into genera. The species Musca domestica is included together with other species with similar characters in a higher taxon called the Genus, in this case, the genus Musca.
  • Family. Genera are grouped into Families. The stable fly (Stomoxys calcitrans) and the house fly (Musca domestica) belong to the same Family, Muscids.
  • Order. Families with similar characteristics are grouped into Orders. The different families of flies and mosquitoes belong to the same Order, Diptera.
  • Class. Orders are grouped into Classes. Flies and butterflies belong to the same Class, Insects.
  • Phylum or Type and Division. Animal classes are grouped into distinct types or phyla (phylum, plural phyla). Flies and crabs belong to the Phylum Arthropods.

In vegetables, we do not speak of Phylum, but of Division.

  • Kingdom. The Kinds or Row (phylum, plural phyla) are grouped into Kingdoms. Flies and seals belong to the same Kingdom, Animal.
  • Domain. Category with three taxa: Archaea (Archaea), Bacteria in the narrower sense (Bacteria) and Eukaryotes (Eukarya) . The fly belongs to the domain of eukaryotes (Eukarya).

Niveles de clasificación de los seres vivos

Pengo [Public domain], from Wikimedia Commons


Order the taxa from largest to smallest:

  • Dominio
  • Reino
  • Phylum, tipo y división
  • Clase
  • Orden
  • Familia
  • Género
  • Especie



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Linnaeusbinomial nomenclature system is based on the use of two terms to determine the species. Species names are italicized (or underlined).

  • The first term, with the first letter capitalized, indicates the gender to which the individual belongs.
  • The second term, in lowercase, indicates the species described.

Let's see an example to clarify it. Within the genus Canis there are several different species:

  • Canis familiaris (dog).
  • Canis lupus (wolf).
  • Canis mesomelas (jackal).
  • Canis latrans (coyote).

This binomial system has two great advantages:

  • It is much more precise than colloquial language. Dogs, pigeons, daisies, etc., are not spoken of, but with precise names.
  • The name is valid throughout the world and in all languages, so the language problem is avoided.

Let's see as an example, the classification of the lion:

  • Species: There are some varieties of lions, such as the Asiatic lion (Panthera leo persica) and the African lion (Panthera leo leo). All of these varieties belong to the Panthera Leo species.
  • Gender: In addition to the lions, the tiger, the jaguar, the leopard, ..., belong to the genus Panthera.
  • Family: In addition to the genus Panthera, the genus felis (cats), leopardus (leopards), lynx (lynxes), puma, etc., belong to the family Felidae (Felidae).
  • Order: The families of the felids, canids (dogs), ursids (bears),..., belong to the carnivorous order (Carnivora).
  • Class: The order carnivores, in addition to rodents (squirrels), primates (monkeys), etc. belong to the class mammalia (Mammalia)
  • Type or phylum: The classes mammals, reptiles, amphibians, etc., belong to the chordate type or phylum.
  • Kingdom: The chordate phylum and the rest of the animals belong to the Metazoan kingdom.
  • Domain: Animals and other organisms made up of eukaryotic cells belong to the Eukarya domain.