Kingdoms of Life in Biology (2024)

In biology, a kingdom of life is a taxonomy rank that is below domain and above phylum. In other words, it is a broad classification of organisms according to their characteristics. Here is a look at how many kingdoms there are, their main properties, and examples of organisms from each kingdom.

History

Carl Linnaeus proposed his biology nomenclature in 1735, placing “kingdom” as the top rank, followed by class, order, genus, and species. Nomenclature changes over time, so that as of 1990 the system is domain, kingdom, phylum or division, class, order, family, genus, and species. The increasing use of molecular biology in establishing relationships between organisms means we’re moving away from the classic taxonomy. However, kingdoms still offer a convenient classification method that uses observable characteristics (phenotypes) for identifying organisms.

How Many Kingdoms Are There?

There are different ways of organizing life into kingdoms. Which model you use largely depends on where you live, with one model not necessarily being better than the other. The United States and Canada often use a system of six kingdoms: Animalia, Plantae, Fungi, Protista, Archaea or Archaebacteria, and Bacteria or Eubacteria. Biology texts in Great Britain, India, Brazil, Greece, and several other countries use a system with five kingdoms: Animalia, Plantae, Fungi, Protista, and Monera. Some American and Canadian texts also classify organisms into five kingdoms.

5 Kingdoms of Life

Here are the 5 kingdoms of life, with examples of organisms they contain:

• Animalia
• Plantae
• Fungi
• Protista
• Monera

6 Kingdoms of Life

The system of 6 kingdoms breaks Monera into Archaea or Archaebacteria and Bacteria or Eubacteria, but it is otherwise the same as the 5 kingdom classification:

• Animalia
• Plantae
• Fungi
• Protista
• Archaea or Archaebacteria
• Bacteria or Eubacteria

A Closer Look at the Kingdoms

Animalia, Plantae, Fungi, and Protista are all eukaryotes. Monera (Archaea and Bacteria) are prokaryotes.

Animalia

Animals are multicellular creatures that eat other organisms for nutrition. Animals vary widely in size and usually use sexual reproduction

• Domain: Eukarya
• Examples: Humans, birds, crustaceans, sponges
• Nutrition: Heterotrophs
• Metabolism: Require oxygen
• Reproduction: Usually sexual, but asexual in some species

Plantae

Plants are multicellular organisms that made their own food via photosynthesis. The primary producers. Plants are classified according to whether they are vascular or nonvascular, flowering or nonflowering, and other characteristics.

Domain: Eukarya
Examples: Flowers, grasses, conifers, multicellular algae, ferns, mosses
Nutrition: Autotrophs
Metabolism: Require oxygen and carbon dioxide
Reproduction: Both sexual and asexual

Fungi

Fungi include both unicellular and multicellular forms. Unlike plants, fungi do not perform photosynthesis. Instead, they decompose organic material and absorb nutrients.

Domain: Eukarya
Examples: Mushrooms, yeast, molds
Nutrition: Saprotrophs
Metabolism: Require oxygen
Reproduction: Both sexual and asexual

Protista

Protists or protozoa are single-celled eukaryotes. However, some species aggregate into masses. Unlike the cells of plants or fungi, they lack cell walls. Many are capable of movement. Some perform photosynthesis.

Domain: Eukarya
Examples: Amoebas, diatoms, dinoflagellates, ciliates, slime molds, single-celled algae
Nutrition: Photoautotrophs or chemoheterotrophs
Metabolism: Require oxygen
Reproduction: Both sexual and asexual

Archaea or Archaebacteria

The Archaea are single-celled prokaryotic bacteria that contain ribosomal RNA. Some species live in extreme environments, such as hydrothermal vents or within the guts of animals.

Domain: Prokarya
Examples: Halophilic bacteria, methanogenic bacteria, thermophiles, psychrophiles
Nutrition: Varies: non-photosynthetic autotrophs, chemoheterotrophs
Metabolism: Varies: oxygen, hydrogen, carbon dioxide, or sulfur
Reproduction: Asexual

Bacteria or Eubacteria

The eubacteria or true bacteria are microscopic unicellular prokaryotes.

Domain: Prokarya
Examples: Gram-positive and Gram-negative bacteria, cyanobacteria, actinobacteria
Nutrition: Varies: photoautotrophs, chemoautotrophs, chemoheterotrophs
Metabolism: Varies: some require oxygen, while it is toxic to others
Reproduction: Asexual

Other Numbers of Kingdoms

The two kingdoms of life classifies organisms as plants or animals. This system dates back at least to Aristotle (384-322 BC) and is not in use today.

In 1860, British naturalist John Hogg proposed a third kingdom, Protoctista. Ernst Haeckel’s 1866 proposal names these organisms as the Protista. Originally, scientists considered protists as more primitive species. Modern science simply identifies them as unicellular.

The addition of the kingdom Monera occurred in the 1960s. The Monera fell under the Empire of Prokaryota, while Animalia, Plantae, and either Protista or Protoctista came under Empire Eukaryota.

During all this time, fungi were grouped together with plants. Robert Whittaker proposed the five-kingdom system in 1969. Whittaker’s system addressed nutrition and energy sources. Animals are multicellular heterotrophs. Plants are mainly multicellular autotrophs. Fungi are mostly multicellular saprotrophs.

Carl Woese and his colleagues proposed dividing prokaryotes into Eubacteria and Archaebacteria in 1977. The distinction arises from ribosomal RNA structure. This model leads to six kingdoms.

Thomas Cavalier-Smith and his colleagues propose seven kingdoms: Bacteria, Archaea, Protozoa, Chromista, Plantae, Fungi, and Animalia.

One eight kingdom system divides the Eubacteria into Negibacteria (Gram negative bacteria) and Posibacteria (Gram positive bacteria). Another system of eight kingdoms is Eubacteria, Archaebacteria, Archezoa, Protozoa, Chromista, Plantae, Fungi, and Animalia. In this system, the Archezoa are protozoa that lack mitochondia.

Which Kingdom Are Viruses?

There is debate regarding whether or not viruses are alive and warrant inclusion in biology taxonomy. On the one hand, some viruses are complex and large, like cells. On the other hand, they obligate intracellular parasites that cannot reproduce without a host.

Usually, viruses are not listed as a kingdom. However, some classification systems include viruses and viroids as a separate kingdom called Virusbiota. This raises additional issues in classification, because viruses contain genetic material from their hosts and are not necessarily related to one another because not all viruses trace back to a common ancestor.

Cladistics

Linnaean taxonomy classifies organisms according to their observable characteristics or phenotypes. But, genetic data shows the relationships between groups are a bit different than their appearance suggests. For example, the eukaryotes (plants, animals, and fungi) are more closely related to archaebacteria than they are to eubacteria. Some plants trace their origins to protists and eubacteria. Meanwhile, animals and fungi have protist origins. It’s very complex and a bit confusing.

At present, scientists do not agree on a new classification that uses cladistics. For now, kingdoms are the accepted taxonomy method, even though they are not monophyletic. In other words, all organisms within a kingdom do not trace back to a common ancestor.

References

• Case, Emily (2008-10-01). “Teaching Taxonomy: How Many Kingdoms?”. American Biology Teacher. 70 (8): 472–477. doi:10.2307/30163328
• Kelly Reese, J. B.; Taylor, M. R.; Simon, E. J.; et al. (2020) Campbell Biology (12th ed.). Pearson. ISBN: 978-0135188743.
• Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
• Margulis, L.; Chapman, M.J. (2009). Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. Academic Press. ISBN 9780080920146.
• Woese, C.; Kandler, O.; Wheelis, M. (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): 4576–4579. doi:10.1073/pnas.87.12.4576

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