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Organizing the Animal World
Naturalists like von Linne first formalized taxonomy (the classification of plants and animals) in an attempt to avoid confusion when referring to different organisms. It soon became clear that animals could be efficiently divided into a nested hiererchy of groups within groups. Animals with many shared characteristics were placed in the same group by virtue of these traits.
Contemporary scientists have fine-tuned the system to allow for rapid sorting, identification and referencing of animal information. Any given organism is placed within eight groups of increasing specificity: domain, kingdom, phylum, class, order, family, genus and species. Numerous sub-categories exist between these levels -- subspecies, superorder, subphylum and so on. This system has changed considerably since Systema's publication, but the basic scheme of hierarchical categorization and binomial nomenclature (the proper two-name term for each species) remains intact.
The Commonality of Life
One hundred years after von Linne's formalized taxonomy, Charles Darwin and his contemporaries developed the theory of evolution. According to this theory, all life on Earth is derived from a single ancestral organism. This neatly explains the convenient ordering of organisms that von Linne observed: related animals share similar traits through a common ancestor.
Scientists categorize animals according to both shared traits and shared ancestors. For instance, cetaceans (whales and dolphins) had been considered mammals since the time of von Linne, but there was no clear-cut reason on why they should be mammals rather than fish since they share some features of both. The theory of evolution gave a new justification for classifying animals: common descent. Cousins on the tree of life are assigned to the same clade (branch).
Reporting New Animals
The careful categorization of animals makes it very unlikely that scientists will confuse two species. When a naturalist suspects an animal is a new species, he assesses the traits of a species and assigns it a genus and species name based on a set of rules governed by the International Code of Zoological Nomenclature. If the species is sufficiently unique, it is given a new genus name. If it is similar to existing organisms, the animal species is placed within an existing genus.
These rules determine what names are available to naturalists. For instance, genus names must be unique among animals, and thus cannot be reused. Priority is granted to older naming descriptions. If it turns out that the new species was previously described, the old nomenclature is always given precedence. These rules exist to avoid confusion in describing animals and to keep species names consistent even if that animal's position on the tree of life shifts a bit.
Modern Phylogenetics
For decades, zoologists categorized animals based on various measurable traits such as skeletal structure, diet and behavior. Molecular biology -- in particular, the ability to sequence the genomes of entire organisms -- has refined this process. When scientists categorize animals, they frequently do so through a method called cladistics, which reflects the strict evolutionary relationship between organisms. In this system, organisms are arranged on a best-guess tree of life and classified based upon that placement.
This method of classification has been invaluable in tracking the evolution and prehistory of the human species. The classification of various diseases and parasites, both microbial and animal, has proven similarly effective in predicting the spread of diseases and suggesting possible cures. In a very real sense, this method of classification has shaped how scientists think about organisms.