Taxonomy is the science of collecting, identifying, describing, classifying and naming organisms, the various groupings referred as taxa (Singl., taxon). An enormous number of distinguishable groups of organisms exist, and studying them requires organization, just as books in a public or academic library requires organization for purposes of study. Historically, biologists named and organized organisms according to similarities in structure (morphology). Those with certain characteristics were put into one group, and those with different characteristics placed into another. These groups were divided and subdivided, producing a hierarchy of taxa, each with common structural characteristics according to one or another specified principle of classification. However, recent efforts have been made to reorganize some parts of the system based on the phylogeny of an organism — that is, its relation to other organisms based on its evolutionary history. Some biologists have expressed concern that traditional taxonomy is a fading discipline, and have lamented the implications for biology.
Biologists group all organisms, living or dead, into one of three domains — Bacteria, Archaea, and Eukarya. In Eukarya, domains subdivide into Kingdoms. For example, the animal kingdom (Animalia, animals) is divided into subgroups called phyla (e.g., Chordata), each of which (a phylum) is divided into classes (e.g., Mammalia, mammals), each class into orders (e.g., Carnivora, carnivorous animals), and each order into families (e.g., Felidae, cats), and each family into genera (e.g., Panthera, big cats). Each genus (singular of genera) is divided into species (e.g., Panthera tigris, tigers). To identify and name a particular species, biologists use two Latin words (Latin binomial), consisting first of the genus name (e.g. Panthera) and second of the so-called species epithet (e.g., tigris). Thus, the species we recognize as tigers biologists name Panthera tigris, written italicized, with an initial capital for the genus name.
The category of subspecies is sometimes used, often where the species does not satisfy the biological definition of species. That is, subspecies are usually organisms that have mostly identical characteristics and can interbreed, but live in different geographic locations, or demes.
For humans: Domain, Eukaya; Kingdom, Animalia; Phylum, Chordata; Subphylum, Vertebrata;....
In groups of many distinguishable groups of organisms, the classification scheme may add other levels of classification: subclasses, suborders, superfamilies, subfamilies, and tribes.
Taxonomy or systematics: systematic taxonomy
Although biologists often use the terms ´taxonomy´ and ´systematics´ synonymously, they refer to separate, though related, disciplines, and more recently also merged disciplines.
The Systematics Home Page of the National Biological Information Infrastructure distinguishes taxonomy and systematics as follows:
Taxonomy (from Greek taxis meaning arrangement or division and nomos meaning law) is the science of classification according to a pre-determined system, with the resulting catalog used to provide a conceptual framework for discussion, analysis, or information retrieval. In theory, the development of a good taxonomy takes into account the importance of separating elements of a group (taxon) into subgroups (taxa) that are mutually exclusive, unambiguous, and taken together, include all possibilities. In practice, a good taxonomy should be simple, easy to remember, and easy to use.
Biological systematics is considered to be the study of the diversity of life on the planet earth, both past and present, and the relationships among living things through time. Systematics, is used to understand the evolutionary history of life on earth. Systematics is often used synonymously with "taxonomy" and "scientific classification."[] However, taxonomy is the describing, identifying, classifying, and naming of organisms. Classification is focused on categorizing organisms within specific groups that show their relationships to other organisms. Systematics uses taxonomy as a means to understand organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly. Scientific classifications are aids in recording and reporting information to other scientists and to laymen. A scientist who specializes in systematics is called a systematist. A systematist must be able to use existing classification systems to insure proper identification and classification of species. This Systematics section of the NBII provides users with access to sites that provide biological names of organisms and their classification that shows the evolutionary relationship among species.
The National Center for Biotechnology Information states:
Carolus Linnaeus was also credited with pioneering systematics, the field of science dealing with the diversity of life and the relationship between life's components. Systematics reaches beyond taxonomy to elucidate new methods and theories that can be used to classify species based on similarity of traits and possible mechanisms of evolution, a change in the gene pool of a population over time.
Phylogenetic systematics is that field of biology that does deal with identifying and understanding the evolutionary relationships among the many different kinds of life on earth, both living (extant) and dead (extinct). Evolutionary theory states that similarity among individuals or species is attributable to common descent, or inheritance from a common ancestor. Thus, the relationships established by phylogenetic systematics often describe a species' evolutionary history and, hence, its phylogeny, the historical relationships among lineages or organisms or their parts, such as their genes. 
When evolutionary biology emerged, the older discipline of taxonomy merged with it as the discipline of ´systematic taxonomy´, or simply ´systematics´. The evolutionary history of organisms — phylogeny — emerged as the focus of systematic taxonomy, wherein evolutionary context informed the classification and organization of the diversity of organisms. Systematic taxonomists develop phylogenetic trees that hypothetically elucidate the evolutionary ties of groups of species, using many tools together, including evidence from morphology, the fossil record, patterns of embryological development, and genetics.
Non-systematic taxonomy continues to thrive, as biologists identify new species, and as recognized organisms, such as asexual fungi, resist systematic classification — though many express concerns about its survival.
References and notes cited in text as superscripts
- Grant B. (2009) A Fading Field
- Systematics Home Page of the National Biological Information Infrastructure.
- Taxonomy and Systematics. London´s Natural History Museum.
- Systematics and Molecular Phylogenetics. National Center for Biotechnology Information.