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.

An eight-level structure illustrating the taxonomic hierarchy of the domestic dog. Each level is followed examples of organisms included within the level—note that with the exception of the species, all organisms listed are examples, and the levels include multiple organisms that are not listed as well. The first level is the species: canis lupus, commonly called the domestic dog. The second level is the genus canis, which includes all domestic and wild dogs, wolves, and jackals. The third level is the family candidate, which includes all dogs and dog-like animals (like foxes). Level four is the order carnivora, which includes all animals that only eat flesh (carnivores, like lions). The fifth level is the class mammal, which includes all animals that produce milk to feed their young; this level includes all animals from previous levels, as well as whales and human beings. The sixth level is the phylum chordata, which includes all animals that have a backbone. This level includes animals from previous levels, as well as snakes. The seventh level is the kingdom animalia, which includes all animals. This level includes animals from previous levels, as well as earthworms. The eighth level is the domain eukarya, which includes all organisms whose cells have a nucleus and organelles. This level includes animals from previous levels, as well as trees, mushrooms, and some single-celled organisms.

Figure 1. This diagram shows the levels of taxonomic hierarchy for a dog, from the broadest category—domain—to the most specific—species. In order to see the full image, right click on the image and open in a new tab or window.

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).

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. Therefore, the scientific name of the dog is Canis lupus. 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.