462

The biological classification system in widespread use today is derived from a system developed by the Swedish biologist Carolus Linnaeus in the mid-1700s. Linnaeus developed a naming system called binomial nomenclature that has allowed scientists throughout the world to refer unambiguously to the same organisms by the same names.

Linnaeus gave each species a two-part name, one part identifying the species itself and the other the genus to which it belongs. A genus (plural genera) is a group of closely related species. Optionally, the name of the taxonomist who first proposed the species name may be added at the end. Thus Homo sapiens Linnaeus is the name of the modern human species. Homo is the genus to which the species belongs, and sapiens identifies the particular species in the genus Homo; Linnaeus proposed the species name Homo sapiens.

You can think of Homo as equivalent to your surname and sapiens as equivalent to your first name. The first letter of the genus name is capitalized, and the specific name is lowercase. Both of these formal designations are italicized. Rather than repeating the name of a genus when it is used several times in the same discussion, biologists often spell it out only once and abbreviate it to the initial letter thereafter (e.g., D. melanogaster rather than Drosophila melanogaster).

As we noted earlier, any group of organisms that is treated as a unit in a biological classification system, such as all species in the genus Drosophila, or all insects, or all arthropods, is called a taxon. In the Linnaean system, species and genera are further grouped into a hierarchical system of higher taxonomic categories. The taxon above the genus in the Linnaean system is the family. The names of animal families end in the suffix “-idae.” Thus Formicidae is the family that contains all ant species, and the family Hominidae contains humans and our recent fossil relatives, as well as our closest living relatives, the chimpanzees and gorillas. Family names are based on the name of a member genus; Formicidae is based on the genus Formica, and Hominidae is based on Homo. The same rules are used in classifying plants, except that the suffix “-aceae” is used for plant family names instead of “-idae.” Thus Rosaceae is the family that includes the genus Rosa (roses) and its relatives.

In the Linnaean system, families are grouped into orders, orders into classes, classes into phyla (singular phylum), and phyla into kingdoms. However, the ranking of taxa within Linnaean classification is subjective. Whether a particular taxon is considered, say, an order or a class is informative only with respect to the relative ranking of other related taxa. Although families are always grouped within orders, orders within classes, and so forth, there is nothing that makes a “family” in one group equivalent (in number of genera or in evolutionary age, for instance) to a “family” in another group.

Linnaeus recognized the overarching hierarchy of life, but he developed his system before evolutionary thought had become widespread. Biologists today recognize the tree of life as the basis for biological classification and often name taxa without placing them into the various Linnaean ranks.