Kingdoms of Life in Biology

In biology, a kingdom is a broad taxonomic group traditionally used to classify living organisms. In the familiar taxonomic hierarchy, kingdom ranks below domain and above phylum. The five-kingdom and six-kingdom systems are still widely taught because they provide a convenient introduction to biological diversity, although modern classification increasingly relies on evolutionary relationships revealed by genetic and molecular evidence.

The five kingdoms of life are Animalia, Plantae, Fungi, Protista, and Monera. In the six-kingdom system, Monera is divided into two kingdoms, Bacteria and Archaea. Modern phylogenetic classification is more complex, particularly for microorganisms and protists, so there is no single kingdom system accepted for all purposes.

Key Takeaways: Kingdoms of Life

• A kingdom is a major taxonomic rank below domain and above phylum.
• The traditional five kingdoms are Animalia, Plantae, Fungi, Protista, and Monera.
• The six-kingdom system separates Monera into Bacteria and Archaea.
• Animals, plants, fungi, and protists are eukaryotes, while bacteria and archaea are prokaryotes.
• The five- and six-kingdom systems remain useful teaching models, but they do not fully represent modern evolutionary relationships.
• Modern classification uses genetic and molecular evidence and recognizes three domains: Bacteria, Archaea, and Eukarya.
• Viruses have their own taxonomic system and are not included among the kingdoms of cellular life.

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 is no single universally accepted number of kingdoms of life. The answer depends on the classification system being used.

The five-kingdom system recognizes Animalia, Plantae, Fungi, Protista, and Monera. The commonly taught six-kingdom system divides Monera into Bacteria and Archaea. Other classification systems recognize additional kingdoms, particularly among eukaryotic microorganisms.

These differences exist because biological classification has changed as scientists have learned more about evolutionary relationships. DNA and RNA sequence data revealed, for example, that archaea differ fundamentally from bacteria and that organisms traditionally grouped together as “protists” do not form a single natural evolutionary group.

For introductory biology, the five- and six-kingdom systems remain useful models. For modern evolutionary biology, the three-domain system and phylogenetic groups provide a more accurate picture of the relationships among organisms.

5 Kingdoms of Life

Robert Whittaker’s influential five-kingdom system recognizes:

• Animalia
• Plantae
• Fungi
• Protista
• Monera

In this system, Monera contains all prokaryotic organisms.

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

This division reflects the fundamental differences between bacteria and archaea revealed by molecular studies.

6 Kingdoms of Life at a Glance

A Closer Look at the Kingdoms

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

Animalia

Animals are multicellular eukaryotes that obtain nutrients by consuming other organisms or organic matter. Animal cells lack cell walls. Most animals are capable of movement during at least one stage of their life cycle, and most reproduce sexually, although asexual reproduction occurs in many groups.

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

Plantae

Plants are primarily multicellular eukaryotes that use photosynthesis to capture energy. Their cells typically contain chloroplasts and cellulose cell walls. Plants include nonvascular groups such as mosses and vascular groups such as ferns, conifers, and flowering plants.

Domain: Eukarya
Examples: Flowers, grasses, conifers, ferns, mosses
Nutrition: Autotrophs
Cell walls: Present; mainly cellulose
Metabolism: Require oxygen and carbon dioxide
Reproduction: Sexual, asexual, or both

Fungi

Fungi are eukaryotes that obtain nutrients by secreting digestive enzymes and absorbing dissolved organic compounds. Unlike plants, fungi do not perform photosynthesis. Most fungi are multicellular, although yeasts and some other fungi are unicellular. Fungal cell walls contain chitin.

Domain: Eukarya
Examples: Mushrooms, yeast, molds, rusts
Nutrition: Heterotrophic by absorption
Cell walls: Present; mainly chitin
Metabolism: Require oxygen
Reproduction: Sexual, asexual, or both

Protista

Protista is a traditional kingdom containing diverse eukaryotes that are not classified as animals, plants, or fungi. Most protists are unicellular, but some are colonial or multicellular. Some perform photosynthesis, some consume or absorb organic matter, and some combine different nutritional strategies. Depending on the group, protists may have cell walls, coverings, or no cell wall.

Protista is useful as a teaching category, but it is not a single natural evolutionary group. Organisms traditionally called protists occur on many different branches of the eukaryotic tree of life.

Domain: Eukarya
Examples: Amoebas, diatoms, dinoflagellates, ciliates, slime molds, single-celled algae
Nutrition: Autotrophic, heterotrophic, or mixotrophic
Cell walls: Present in some groups; absent in others
Metabolism: Require oxygen
Reproduction: Both sexual and asexual

Archaea or Archaebacteria

Archaea are unicellular prokaryotes that form one of the three domains of life. Although archaea resemble bacteria in size and general cell structure, they differ significantly in their cell membranes, molecular machinery, and genetics. Some archaea live in extreme environments, but many inhabit ordinary environments, including oceans, soils, sediments, and the digestive tracts of animals.

Domain: Archaea
Examples: Methanogens, extreme halophiles, thermophiles, ammonia-oxidizing archaea
Nutrition: Autotrophic or heterotrophic, depending on the species
Cell walls: Usually present, but lacking peptidoglycan
Metabolism: Varies: oxygen, hydrogen, carbon dioxide, or sulfur
Reproduction: Asexual

Bacteria or Eubacteria

Bacteria are unicellular prokaryotes that form the domain Bacteria. They occur in nearly every environment on Earth and display enormous metabolic diversity. Some bacteria perform photosynthesis, some obtain energy from inorganic chemicals, and others consume organic compounds. Bacterial cell walls typically contain peptidoglycan.

Domain: Bacteria
Examples: Gram-positive and Gram-negative bacteria, cyanobacteria, actinobacteria
Nutrition: Varies: photoautotrophs, chemoautotrophs, chemoheterotrophs
Cell walls: Usually present and containing peptidoglycan
Metabolism: Varies: some require oxygen, while it is toxic to others
Reproduction: Primarily asexual by binary fission

Kingdoms vs Domains

Domain is a taxonomic rank above kingdom. The three-domain system proposed by Carl Woese, Otto Kandler, and Mark Wheelis recognizes three domains:

• Bacteria – prokaryotic organisms with characteristic bacterial cell structure and molecular biology
• Archaea – prokaryotic organisms genetically and biochemically distinct from bacteria
• Eukarya – organisms whose cells contain nuclei and other membrane-bound organelles

In the traditional six-kingdom model, Bacteria and Archaea each correspond to a domain and kingdom, while Animalia, Plantae, Fungi, and Protista all fall within Eukarya.

Domains and kingdoms therefore are not competing ranks. Domain is the broader taxonomic level.

History of the Kingdom Classification System

Biological classification has changed repeatedly as scientists discovered new organisms and learned more about their evolutionary relationships.

Two kingdoms: Early systems divided organisms into plants and animals. Carl Linnaeus formalized the kingdoms Animalia and Vegetabilia (later Plantae) in his 18th-century classification.

Three kingdoms: During the 19th century, microscopic organisms proved difficult to fit into either the plant or animal kingdom. Ernst Haeckel proposed Protista as a third kingdom in 1866.

Five kingdoms: In 1969, Robert Whittaker proposed the influential five-kingdom system of Monera, Protista, Fungi, Plantae, and Animalia. His system emphasized differences in cell organization and modes of nutrition.

Six kingdoms and three domains: Studies of ribosomal RNA by Carl Woese and colleagues revealed a fundamental division between bacteria and archaea. The three-domain system recognizes Bacteria, Archaea, and Eukarya, while a commonly taught six-kingdom model separates Bacteria and Archaea into kingdoms.

Modern classification: Genetic sequencing has revealed that several traditional kingdoms, especially Protista, do not represent single evolutionary lineages. As a result, modern systematics increasingly describes organisms using phylogenetic clades rather than relying on a fixed number of kingdoms.

Other systems divide eukaryotes into additional kingdoms, such as Protozoa and Chromista. These systems illustrate why the question “How many kingdoms are there?” does not have one universally accepted scientific answer.

5 Kingdoms vs 6 Kingdoms

The main difference between the five- and six-kingdom systems is how they classify prokaryotes.

The five-kingdom system places all prokaryotes in the kingdom Monera. The six-kingdom system separates them into Bacteria and Archaea.

Both systems recognize Animalia, Plantae, Fungi, and Protista. However, the six-kingdom system better reflects the profound genetic and biochemical differences between bacteria and archaea.

Which Kingdom Are Viruses?

Viruses are not included in the five- or six-kingdom systems of cellular life. They are acellular infectious agents that depend on host cells for replication.

Viruses do, however, have their own formal taxonomy maintained by the International Committee on Taxonomy of Viruses (ICTV). Virus taxonomy uses ranks including realm, kingdom, phylum, class, order, family, genus, and species. These viral kingdoms are part of a separate classification system and should not be confused with kingdoms such as Animalia, Plantae, or Bacteria.

Whether viruses should be considered “alive” depends partly on how life is defined, but this debate does not place viruses within the traditional kingdoms of cellular organisms.

Kingdoms and Modern Phylogeny

Traditional kingdom systems classify organisms using broad characteristics such as cell type, body organization, and mode of nutrition. Modern systematics also uses DNA and RNA sequences, molecular characteristics, and other evidence to reconstruct evolutionary relationships.

This evidence shows that some traditional kingdoms do not correspond neatly to evolutionary clades. Protista is the clearest example: organisms traditionally called protists belong to several distinct evolutionary lineages rather than one branch containing a single common ancestor and all of its descendants.

Kingdoms therefore remain useful for introducing biological diversity, but modern taxonomy does not depend on a single five- or six-kingdom model. Phylogenetic trees and clades provide a more detailed picture of how organisms are related.

Frequently Asked Questions

What are the 5 kingdoms of life?

The five kingdoms are Animalia, Plantae, Fungi, Protista, and Monera.

What are the 6 kingdoms of life?

The six kingdoms are Animalia, Plantae, Fungi, Protista, Bacteria, and Archaea.

What is the difference between the 5 and 6 kingdoms?

The five-kingdom system places all prokaryotes in Monera. The six-kingdom system separates prokaryotes into Bacteria and Archaea.

What are the 3 domains of life?

The three domains are Bacteria, Archaea, and Eukarya. Domain ranks above kingdom.

Are viruses a kingdom of life?

No. Viruses are not included in the traditional kingdoms of cellular life. They have a separate taxonomy maintained by the ICTV.

Is Protista still a kingdom?

Protista remains common in introductory classification systems, but modern phylogenetic evidence shows that traditional protists do not form a single natural evolutionary group.

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