What is the broadest category of classification in taxonomy?

Learning Outcomes

• Explain how relationships are indicated by the binomial naming system

Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. Think about how a grocery store is organized. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. This organization from larger to smaller, more specific categories is called a hierarchical system.

In the eighteenth century, a scientist named Carl Linnaeus first proposed organizing the known species of organisms into a hierarchical taxonomy. In this system, species that are most similar to each other are put together within a grouping known as a genus. Furthermore, similar genera (the plural of genus) are put together within a family. This grouping continues until all organisms are collected together into groups at the highest level. The current taxonomic system now has eight levels in its hierarchy, from lowest to highest, they are: species, genus, family, order, class, phylum, kingdom, domain. Thus species are grouped within genera, genera are grouped within families, families are grouped within orders, and so on (Figure 1).

Figure 1. This diagram shows the levels of taxonomic hierarchy for a dog, from the broadest category—domain—to the most specific—species. Click for a larger image.

The kingdom Animalia stems from the Eukarya domain. For the common dog, the classification levels would be as shown in Figure 1. Therefore, the full name of an organism technically has eight terms. For the dog, it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is its two-word scientific name, in what is called binomial nomenclature. Each species has a unique binomial to allow for proper identification.

Therefore, the scientific name of the dog is Canis lupus. It is important that the correct formatting (capitalization and italics) is used when calling an organism by its specific binomial.

The name at each level is also called a taxon. In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are considered separate subspecies due to geographic or behavioral isolation or other factors.

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A taxonomy is a hierarchical scheme for classifying and identifying organisms. It was developed by Swedish scientist Carl Linnaeus in the 18th century. In addition to being a valuable tool for biological classification, Linnaeus's system is also useful for scientific naming. The two main features of this taxonomy system, binomial nomenclature and categorical classification, make it convenient and effective.

Binomial Nomenclature

The first feature of Linnaeus's taxonomy, which makes naming organisms uncomplicated, is the use of binomial nomenclature. This naming system devises a scientific name for an organism based on two terms: The name of the organism's genus and the name of its species. Both of these terms are italicized and the genus name is capitalized when writing.

Example: The bionomical nomenclature for humans is Homo sapiens. The genus name is Homo and the species name is sapiens. These terms are unique and ensure that no two organisms have the same scientific name.

The foolproof method of naming organisms ensures consistency and clarity across the field of biology and makes Linnaeus's system simple.

Classification Categories

The second feature of Linnaeus's taxonomy, which simplifies organism ordering, is categorical classification. This means narrowing organism types into categories but this approach has undergone significant changes since its inception. The broadest of these categories within Linnaeus's original system is known as kingdom and he divided all of the world's living organisms into only an animal kingdom and plant kingdom.

Linnaeus further divided organisms by shared physical characteristics into classes, orders, genera, and species. These categories were revised to include kingdom, phylum, class, order, family, genus, and species over time. As more scientific advancements and discoveries were made, domain was added to the taxonomic hierarchy and is now the broadest category. The kingdom system of classification was all but replaced by the current domain system of classification.

Domain System

Organisms are now grouped primarily according to differences in ribosomal ​RNA structures, not physical properties. The domain system of classification was developed by Carl Woese and places organisms under the following three domains: 

• Archaea:This domain includes prokaryotic organisms (which lack a nucleus) that differ from bacteria in membrane composition and RNA. They are extremophiles capable of living in some of the most inhospitable conditions on earth, such as hydrothermal vents.
• Bacteria: This domain includes prokaryotic organisms with unique cell wall compositions and RNA types. As part of the human microbiota, bacteria are vital to life. However, some bacteria are pathogenic and cause disease.
• Eukarya: This domain includes eukaryotes or organisms with a true nucleus. Eukaryotic organisms include plants, animals, protists, and fungi.

Under the domain system, organisms are grouped into six kingdoms which include Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia. The process of classifying organisms by categories was conceived by Linnaeus and has been adapted since.

Taxonomy Example

The table below includes a list of organisms and their classification within this taxonomy system using the eight major categories. Notice how closely dogs and wolves are related. They are similar in every aspect except species name.

Intermediate Categories

Taxonomic categories can be even more precisely divided into intermediate categories such as subphyla, suborders, superfamilies, and superclasses. A table of this taxonomy scheme appears below. Each main category of classification has its own subcategory and supercategory.

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