Zoology: organisms in the biosphere

It is estimated that there are on Earth millions of different types of living organisms sharing the biosphere. The recognition of these species is closely related to human history.

Source: View to Bzerpy cornice and Pseashkho pass from peak Tabunnaya. The territory of Caucasian Biosphere Reserve in vicinities of Sochi, Russian Federation. SKas, 2021.

At a certain point in evolutionary history, man began to use animals and plants for food, curing diseases, manufacturing weapons, agricultural objects and shelter. The need to transmit the acquired experiences to the descendants forced him to name plants and animals. The oldest known zoological document is a Greek work of medicine, from the 5th century BC, which contained a simple classification of edible animals, mainly fish.

Thus, the classification of living beings came about with man’s own need to recognize them. The large number of living species led him to organize them in order to facilitate their identification and, consequently, their use.

The classification of living beings

Initially, based on the way of life, direction of evolution and type of organization of their body, living beings were divided into two great kingdoms: Animal and Plant. Later, as the development of the microscope, among other techniques, it became obvious that many organisms did not fit into any of these kingdoms, for example a bacterium, which is a unicellular organism, devoid of nuclear envelope and intracellular membranous structures such as reticulum. endoplasmic, mitochondria, chloroplasts and Golgi complex. These differences that distinguish it from other organisms are much more fundamental than the differences between animals and plants.

Another proposed system, and also no longer used, was that of three kingdoms: Protista, Plantae and Animalia. In this system, organisms with plant and animal characteristics gathered in the Protist kingdom.

Source: This cladogram shows the upper systematics of the Opisthokonta, based mainly on Shalchian-Tabrizi (2008). It also shows two paraphyletic groups. Dreisam, 2021

Later, a new classification system emerged, grouping organisms into four kingdoms: Monera (bacteria and cyanophyceae), Protista (other algae, protozoa and fungi), Plantae or Metaphyta (from bryophytes to angiosperms) and Animalia or Metazoa (from spongy to mammals).

A more recent classification system, comprising five kingdoms, was proposed by Whittaker (1969). It comprises a prokaryotic kingdom, Monera, and four other eukaryotic kingdoms. Of the eukaryotic groups, it is believed that Protista gave rise to the other three remaining groups (Plantae, Animalia and Fungi). Such groups, mostly multicellular, fundamentally differ in their nutritional mode.

The Monera kingdom is made up of prokaryotic unicellular organisms, colonial or not, autotrophs or heterotrophs. It encompasses bacteria and cyanobacteria (cyanobacteria).
Single-celled eukaryotic organisms, colonial or not, constitute the Protist kingdom. In it, there are several nutritional methods, including photosynthesis, absorption and ingestion. Comprises single-celled algae and protozoa.

The kingdom Fungi is composed of heterotrophic eukaryotic organisms, usually multinucleated (cenocytic), and their nutrition is carried out by absorption. Multicellular photosynthetic (autotrophic) eukaryotic organisms make up the kingdom Plantae, which ranges from multicellular algae to higher plants.

Source: Boletus growing in a Finnish forest at Heinäsaari island of Lake Saimaa in Lappeenranta, Finland. Petritap, 2021.

The kingdom Animalia is composed of heterotrophic multicellular eukaryotic organisms, which have ingestion, or absorption in some cases, as their primary form of nutrition. It encompasses everything from sponges to man.

It is clear that any classification system presents many difficulties, as living beings change and evolve over time and, with the advancement of science, new discoveries about the existing relationships between organisms emerge. However, the classification system proposed by Whittaker is, even today, the most accepted and used.

The systematics

The descriptive study of all species of living beings and their classification within a true hierarchy of groups constitutes systematics or taxonomy. Let’s begin to interpret the role of taxonomy by reviewing the species concept.

The Species Concept

Species in Latin simply means “type”. Species are, in the simplest sense, the different types of organisms. A more technical definition of a species is: “a group of organisms that interbreed with each other, without normally interbreeding with representatives of other groups”.

Source: Imagen de un ejemplar de Geodia cydonium en su entorno natural. Piero Mescalchin, 2021.

Organisms belonging to a species must have structural and functional similarities, biochemical similarities and even karyotype, in addition to the ability to reproduce with each other. The above definition, while useful for animals, is, however, not useful in plant taxonomy, because fertile crosses can occur between plants of quite different types. Nor does this distinction apply to organisms that do not reproduce sexually.

In the light of evolutionary theory, it is observed that a species is constantly changing, in space and time, instead of being an immutable, ideal form, as conceived by Linnaeus. Thus, the word “species” has different meanings for different types of organisms, which is not surprising considering that evolution in the various groups of organisms followed different paths. However, the term remains useful and provides an adequate way of referring to organisms and cataloging them.

Other Taxonomic Groups

The kingdom is the largest unit used in biological classification. Between the kingdom and genus level, however, Linnaeus and later taxonomists added several categories (or taxa). We then have genera grouped into families, families into orders, orders into classes and classes into phyla (or division, for botanists), following a hierarchical pattern. These categories can be subdivided or aggregated into several less important ones, such as subgenres and superfamilies.


Mayr, Ernst. “Principles of systematic zoology.” Principles of systematic zoology. (1969).

Simpson, G. G., Roe, A., & Lewontin, R. C. (2003). Quantitative zoology. Courier Corporation.

Barnes, Robert D. Invertebrate zoology. No. Ed. 5. WB Saunders company, 1987.

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