Taxonomy
Taxonomy is the practice of classifying and naming biological organisms and groups of biological organisms. Modern taxonomy originated in the eighteenth century with the work of a Swedish botanist, Carolus Linnaeus, who developed an organized hierarchical classification system that could be applied to all biological organisms. This system is still referred to as Linnaean taxonomy. Under the Linnaean system, all organisms are classified hierarchically into a series of groups from most inclusive to least inclusive. Each species belongs successively to a kingdom, a phylum, a class, an order, a family, a genus, and a species. Sometimes additional categories are used, such as subphylum, superorder, superfamily, subfamily, etc.
One important aspect of taxonomy is the description and naming of biological species. Since the time of Linnaeus, species have been named using what is called binomial nomenclature, in which each species has a two-word name that includes a genus name and a species name. Species namesare universal, that is, the same names are used by scientists worldwide. In addition, no two species can have the same name. If more than one name has been applied to a given species, the oldest name generally takes precedence. There are a few rules that govern the choice of species names. The entire binomial name must be in Latin or must be Latinized, with the genus name capitalized and the species name in lowercase. The binomial species name is always italicized or underlined in print.
As an example of taxonomic classification, the domesticated dog belongs to the kingdom Animalia (multicellular animals), the phylum Chordata (multicellular animals possessing a nerve cord), the subphylum Vertebrata (having a backbone), the class Mammalia (mammals), the order Carnivora (which includes carnivores such as dogs, cats, weasels, and hyenas), the family Canidae (including dogs, wolves, foxes), the genus Canis, and the species familiaris. The binomial species name for the domesticated dog is therefore Canis familiaris, where Canis designates the genus, and familiaris designates the species.
In Linnaeus's time, classification was based primarily on similarities between species, particularly anatomical similarities. Modern classification systems remain hierarchical, but use evolutionary relationships, rather than similarity among organisms, as the basis of classification. In order to achieve this, taxonomists try to describe supraspecific taxa that are monophyletic —that is, they attempt to create supraspecific taxa that include all the species that are descended from a common ancestor. Monophyletic groups are also known as clades. Many currently recognized groups are not monophyletic. For example, the group Reptilia, as it is traditionally conceived, is not monophyletic. Traditionally, Reptilia has included only turtles, lizards, snakes, and crocodiles. However, an analysis of the evolutionary relationships among different vertebrate groups reveals that the common ancestor of the traditionally recognized reptiles also gave rise to birds. Therefore, to make the group Reptilia monophyletic, birds should be included as reptiles.
The study of evolutionary relationships among organisms, which makes up the field of systematics, is a necessary first step in the creation of a modern taxonomy. As scientists study evolutionary relationships among organisms, they make changes in taxonomy in order to eliminate nonmonophyletic groups. Consequently, taxonomy is constantly changing as new knowledge is gained.
Classification Systems; Carolus Linnaeus; Phylogenetic Relationships of Major Groups.
Bibliography
Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1998.
Ridley, Mark. Evolution. Boston: Blackwell Scientific, 1993.
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