BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.1. Classification

2.1.3. Taxonomic Hierarchy

Linnaeus ultimately expanded the binomial system by incorporating more groups beyond just genera and species. He established a hierarchy of groups, placing the largest group—the kingdom—at the top. The hierarchical Structure he developed is still in use today. It comprises the following taxonomic ranks (in descending order of hierarchical significance):

Kingdom*

Phylum (Division in plants) — introduced by Haeckel in the late 19th century

Class*

Order* (referred to as order in plants)

Family — introduced during Linnaeus's lifetime Genus*

Species*

* Introduced by Linnaeus

A specific example of the Classification of the animal kingdom is shown in Fig. 2.1. As can be seen from the figure, each group, or taxon, may in turn include A number of other groups (taxa) of lower rank. For example, the subphylum Vertebrata includes six classes, and the genus Homo consists of three species, two of which are extinct. Each group possesses characteristics unique to its members. Such features are referred to as diagnostic characters. Hair, for instance, is found only in mammals (Class Mammalia). Consequently, hair is a diagnostic feature of mammals. However, mammals—like birds, reptiles, amphibians, and fish—possess all the diagnostic Features of the preceding group in the hierarchy, namely the vertebrates.

Hierarchical groups can, in turn, be subdivided into subgroups, such as the subphylum Vertebrata (vertebrates; Fig. 2.1), or conversely, combined into supergroups, such as a superclass, whenever it proves convenient. Because hierarchies must be structured for ease of use, they are frequently modified.

Fig. 2.1. Example of a taxonomic group hierarchy. Not all animal groups are shown. The diagram provides both Latin and standard English names.