Unit Four. The Evolution and Diversity of Life
15. How We Name Living Things
15.3. Higher Categories
Like the mayor of New York, a biologist needs more than two categories to classify all the world’s living things. Taxonomists group the genera with similar properties into a cluster called a family. For example, the eastern gray squirrel at the bottom of figure 15.3 is placed in a family with other squirrellike animals including prairie dogs, marmots, and chipmunks. Similarly, families that share major characteristics are placed into the same order (for example, squirrels placed in with other rodents). Orders with common properties are placed into the same class (squirrels in the class Mammalia), and classes with similar characteristics into the same phylum (plural, phyla) such as the Chordata. Botanists (that is, those who study plants) also call plant phyla “divisions.” Finally, the phyla are assigned to one of several gigantic groups, the kingdoms. Biologists currently recognize six kingdoms: two kinds of prokaryotes (Archaea and Bacteria), a largely unicellular group of eukaryotes (Protista), and three multicellular groups (Fungi, Plantae, and Animalia). To remember the seven categories in their proper order, it may prove useful to memorize a phrase such as “kindly pay cash or furnish good security” or “King Philip came over for green spaghetti” (kingdom- phylum-class-order-family-genus-species).
Figure 15.3. The hierarchical system used to classify an organism.
In this example, the organism is first recognized as a eukaryote (domain: Eukarya). Second, within this domain, it is an animal (kingdom: Animalia). Among the different phyla of animals, it is a vertebrate (phylum: Chordata, subphylum: Vertebrata). The organism's fur characterizes it as a mammal (class: Mammalia). Within this class, it is distinguished by its gnawing teeth (order: Rodentia). Next, because it has four front toes and five back toes, it is a squirrel (family: Sciuridae). Within this family, it is a tree squirrel (genus: Sciurus), with gray fur and white-tipped hairs on the tail (species: Sciurus carolinensis, the eastern gray squirrel).
In addition, an eighth level of classification, called domains, is sometimes used. Domains are the broadest and most inclusive taxa, and biologists recognize three of them, Bacteria, Archaea, and Eukarya—which are discussed later in this chapter.
Each of the categories in this Linnaean system of classification is loaded with information. For example, consider a honeybee:
Level 1: Its species name, Apis mellifera, identifies the particular species of honey bee.
Level 2: Its genus name, Apis, tells you it is a honey bee.
Level 3: Its family, Apidae, are all bees, some solitary, others living in hives as A. mellifera does.
Level 4: Its order, Hymenoptera, tells you that it is likely able to sting and may live in colonies.
Level 5: Its class, Insecta, says that A. mellifera has three major body segments, with wings and three pairs of legs attached to the middle segment.
Level 6: Its phylum, Arthropoda, tells us that it has a hard cuticle of chitin and jointed appendages.
Level 7: Its kingdom, Animalia, says that it is a multicellular heterotroph whose cells lack cell walls.
Level 8: An addition to the Linnaean system, its domain, Eukarya, says that its cells contain membrane- bounded organelles.
Key Learning Outcome 15.3. A hierarchical system is used to classify organisms in which higher categories convey more general information about the group.