The system of biological classification used today derives from the hierarchical scheme devised by Carolus Linnaeus or Carl von Linné (1707-1778). In the 10th edition of his Systema naturae (1758), he listed every type of animal known to him, organizing them into groups based on overall similarity. The Linnaean system consists of seven major categories called taxa. Arranged from the broadest to the narrowest category, these taxa are: kingdom, phylum, class, order, family, genus and species. According to this system, each species is given a unique, two-part name called a binomen, with each one consisting of a genus and species name. The species is the basic unit of classification and the only one considered to be natural. All other taxa are arbitrary, and thus subject to changes due to new data or interpretations. It is surprising, therefore, that taxonomists differ in their definition of what exactly is a species. Put simply, a species is a population or series of populations of closely related and similar organisms. In theory, members of a given species are supposed to resemble one another more than they do members of any other species. In reality, variation within species is sometimes greater than variation between species. In sexually reproducing organisms, species are often more narrowly defined by the species concept, namely, a population or series of populations that freely interbreed with one another under natural conditions to produce viable offspring (i.e., offspring that are themselves capable of breeding). Evolutionary systematics, or gradistic taxonomy, based on a combination of branching and divergence so as to reconstruct phylogenetic trees, continued to modify the Linnaean system. However, due to certain shortcomings (e.g., the arbitrary inclusion of characters), it has given way to cladistics. The main difference between cladistics and evolutionary systematics is that the former includes all descendant species together with the ancestral species within taxonomic groups, while the latter contends that the classification should account for different rates of adaptation within the descendant groups of a single ancestor. Still another approach to classification includes the use of techniques from molecular biology such as direct amino acid sequencing, DNA hybridization, electrophoresis, and immunology. As a final comment, it should be mentioned that the first to classify animals into invertebrates and vertebrates was Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (1744-1829) who was also the first to use taxa as Arachnida and Crustacea.
See Cladistics, Evolutionary biology, Lamarckism, Lumping (versus splitting), Palaeontology, Phylogeny, Population (biology and ecology), Species, Race-ethnicity