We explain what biological kingdoms are and the history of these groups of species. Also, the characteristics of each one.
What are the biological kingdoms?
In biology, and especially in taxonomy, it is called kingdom to each of the large groups into which the known species of living beings are classified. The five biological kingdoms are: the animal kingdom (Animalia), the plant kingdom (Plantae), the fungal kingdom (Fungi), the protoctist or protist kingdom (Protozoa) and the monera kingdom (Monera).
This is the second level of classification of living beings, below the domains and above the phyla (or phylum).
Throughout the history of science, humans have made efforts to understand the evolutionary relationships of different species.
As scientists understand more and more detail about the characteristics of living things, new classification possibilities appear. Sometimes it happens that old assumptions end up becoming obsolete. For that reason, There are different classification systems in biological kingdoms not always coincident with each other.
The most recent system is the one proposed by the Anglo-Canadian Thomas Cavalier-Smith in 2015, although there is still debate about it in the specialized scientific community.
- See also: Branches of biology
Animal kingdom
the animal kingdom (Animals) It includes all known types of animals.
Animals are heterotrophic eukaryotic multicellular organisms. That is, they are composed of more than one cell and all of them are cells with a true nucleus and internal structures. In addition, they need to obtain nutrients from the ingestion and digestion of other organisms (such as plants or other animals).
Animals have their own mobility and most reproduce sexually (although there are some species, such as sponges, that can do it asexually). The animal kingdom is enormously diverse, spread across aquatic, terrestrial and even airborne habitats. It can be divided into two groups: vertebrates and invertebrates, depending on whether or not they have a backbone.
Some examples of animals are:
- invertebrate animals. For example, sponges, jellyfish, worms, octopuses, earthworms, ants and hedgehogs.
- Vertebrate animals. For example, fish, toads, crocodiles, birds and monkeys.
- Animal kingdom
Plant kingdom
The plant kingdom (Plantae) includes all known types of plants.
Plants are eukaryotic, multicellular and autotrophic organisms. That is, they are made up of many eukaryotic cells, and are capable of making their food from light, atmospheric carbon dioxide and water.
In addition, plant cells have a cellulose wall. Plants do not move, they are immobile. Depending on the species and environmental conditions, they can reproduce sexually or asexually.
The plant kingdom is essential for supporting life as we know it, especially terrestrial plants.
Some examples of plants are:
- avascular plants (which have no internal structures). For example, mosses, liverworts and hornworts.
- vascular plants (which have internal ducts). For example, ferns, pines, wheat and fruit trees.
- Plantae Kingdom
mushroom kingdom
The mushroom kingdom (Fungi) It is composed of heterotrophic multicellular eukaryotic organisms, unable to make their own food. They can have a saprophytic existence (they secrete digestive enzymes and then absorb nutrients) or parasitic (they infect the bodies of other living organisms and obtain nutrients from them).
Fungi have a cell wall, like plants, but it is made of chitin instead of cellulose. They can reproduce both sexually and asexually.
The organisms of the kingdom Fungi have great ecological importance, because are the decomposers of the food chains. Its metabolic activity makes many nutrients available in the environment for other organisms.
Some examples of mushrooms are:
- mushrooms. For example, mushrooms, truffles, shiitake mushrooms and other edible mushrooms.
- Molds and yeasts. For example, the fungi in Roquefort cheese, the mold that appears on the ceilings and walls of houses, the yeasts used to make bread.
- Kingdom Fungi
protist kingdom
The protoctist or protista kingdom (Protozoa) It is considered the basal group of eukaryotic organisms, that is, the first to emerge from which the others would have later gotten rid of. It is a paraphyletic group, that is, it includes the first common ancestor but not all its descendants.
In this group there are eukaryotic organisms that cannot be classified as animals, plants, or fungi. For this reason, protists are very diverse. They can have different shapes and sizes, and with different types of nutrition and reproduction.
Most protists are single-celled organisms. There are some protists that are multicellular, but do not form tissues. Some protists are autotrophs, others are heterotrophs, and there are even protists with both types of nutrition, called mixotrophs. Almost all of them reproduce asexually.
Protists live in water or humid environments. Some lead a parasitic life, that is, they develop inside the body of other organisms.
Some examples of protists are:
- The algae. For example, sea lettuce, algae that are used to produce vegetable gelatin (agar-agar) and unicellular algae that live in plankton (a community of microorganisms that live suspended in water and play an important role in the balance of ecosystems).
- The protozoa. For example, amoebas, paramecia, and Plasmodium (causing agent of malaria disease).
- Kingdom protista
Monera Kingdom
The kingdom of the monkeys (Monera) It is composed of prokaryotic unicellular organisms (bacteria and archaea). This kingdom includes microscopic organisms made up of a single, very simple cell which has no nucleus and contains simple cellular structures. These microorganisms are very ancient forms of life and are found in almost all habitats in the world, even in regions of extreme temperatures.
Members of the Monera kingdom are the most abundant and diverse organisms on the planet. They can be autotrophs or heterotrophs and reproduce asexually.
Archaea are characterized by being adapted to living in extreme environments (for example, environments without oxygen, with high temperatures or high salinity). They have a type of metabolism called chemosynthetic (they can manufacture organic matter from inorganic matter, without using light).
Some examples of moneras are:
- The archaea. For example, methanogenic archaea (which live in swamps and produce methane gas), halophytic archaea (which live in soils with a large amount of salt, such as salt flats) and thermophilic archaea (which live in high temperature environments, like the inside of geysers).
- The bacteria. For example, lactobacilli (which are used to produce yogurt), photosynthetic bacteria (which live in water) and enterobacteria (which are part of our digestive system).
- Monera Kingdom
What is systematics?
Systematics is the scientific discipline that studies living beings with the intention of classifying them and giving order to diversity. The first areas of biology that dealt with systematics were botany and zoology.
There are different systematic schools that have given rise to the different classifications throughout the history of science. Thus, it is possible to have different views on what are the best criteria to make a system for organizing living beings.
- Systematics
Debates about microscopic organisms
Microscopic organisms have always been difficult to categorize. To study their structure and life forms, special instruments and techniques are needed to facilitate the observation and description of such small beings.
During the 20th century, there were enormous advances in the study of genetics and cell biology, which prompted some taxonomy experts to rethink the classifications of some microorganisms.
Carl Woese and G. Fox studied the kingdom Monera in depth and discovered that bacteria and archaea were more different than previously thought. For this reason, proposed separating them into two groups, which they called domains.
Later, the Canadian scientist Cavalier-Smith dedicated himself to studying the boundaries between the plant kingdom and the protist kingdom. He postulated that a new kingdom should be incorporated to classify green algae, called the chromist kingdom. (Chromista). Until today, the boundaries between the chromist kingdom and the protist kingdom are not entirely clear and continue to be a point of scientific controversy.
History of biological kingdoms
The first life classification systems date back to ancient times, when ancient philosophers proposed approaches to life by distinguishing between its basic observable properties.
two kingdom system
Attributed to the Greek philosopher Aristotle (IV BC), he divided living beings into two large categories, based on what he called “vegetative soul” (which included the ability to grow, nourish and reproduce) and “sensitive soul” (which implied desire, movement and perception).
This system was taken by the naturalist Carlos Linnaeus, who in 1735 proposed a classification system of two kingdoms properly speaking: Vegetabilia and Animals.
three kingdom system
Based on Linnaeus' two kingdoms, Richard Owen proposed the kingdom Protozoa to resolve the difficulty in classifying some microorganisms made up of cells with a nucleus. All forms of microscopic life with animal, plant and mixed characters were included in this group. For the first time, a distinction was made between unicellular and multicellular organisms. This new kingdom was renamed Protist in 1860 by the Englishman John Hogg.
Four kingdom system
In 1938 Herbert Copeland proposed a four kingdom system: Animals, Plantae, Protist and a new group for anucleate bacteria: Monera. This new category arose because advances in microbiology made the distinction between prokaryotic organisms (without a cell nucleus) and eukaryotes (with a cell nucleus) increasingly evident.
five kingdom system
The fifth kingdom emerged in 1959, when Robert Whittaker confirmed that fungi constituted a group totally different from plants, and in 1969 he proposed a system of five kingdoms that included the kingdom Fungi. This was one of the most popular systems in history, it included the kingdoms Animals, Plantae, Protista, Monera and Fungi.
Three domain system
Carl Woese and G. Fox reinvented the classification system, after the important advances that occurred in the techniques for studying and exploring DNA and RNA during the second half of the 20th century. They proposed a higher category, in which three domains are distinguished: Bacteria, Archaea and Eukarya.
The domain Eucarya It groups all organisms made up of eukaryotic cells, and includes the animal kingdom, the plant kingdom, the protist kingdom and the fungal kingdom. Under this classification system, the members of the kingdom Monera They were divided into the domains Bacterium and Archaeabecause new technologies revealed that these organisms were, in fact, very far apart from each other in evolutionary terms. Currently this system is the most popular.
Seven Kingdoms System
In 2015, a collaborative work led by Canadian scientist Cavalier-Smith was published. This project aimed to review all available information on the species and propose a more up-to-date taxonomic classification.
The “catalog of life system” states that there are two superkingdoms (Prokaryota and Eukaryota). Inside the superkingdom Prokaryota kingdoms exist Archaea and Bacterium. Inside the superkingdom Eukaryota kingdoms exist Protozoa, Chromista, Plantae, Fungi, Animalia.
the kingdom Chromist It was introduced by Cavalier-Smith in 1981 to make a distinction between some organisms that were grouped in the kingdom Protozoa. the kingdom Chromist includes diatom algae, oomycetes and other related microorganisms.
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References
- Curtis H., Barnes N., Massarini A., Schnerck A., BIOLOGY. 7th Edition. Panamericana Medical Editorial (2008).
- Audesirk T., Audesirk G., Bruce E., BIOLOGY 3: Evolution and ecology. 6th edition. Pearson education (2003).
- Ruggiero MA, Gordon DP, Orrell TM, Bailly N, Bourgoin T, Brusca RC, Cavalier-Smith T, Guiry MD, Kirk PM. A higher level classification of all living organisms. PLoS One. 2015.
