Scientific classification

Scientific classification refers to how biologists group and categorize extinct and living species of organisms. Modern classification has its roots in the system of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have been revised since Linnaeus to improve consistency with the Darwinian principle of common descent. Molecular systematics, which uses Genomic DNA analysis has driven many recent revisions and is likely to continue to do so. Scientific classification belongs to the science of taxonomy or biological systematics.

Early Systems

The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle, who classified animals based on their means of transportation (air, land, or water).

The next major advance in developing scientific classification was by the Swiss professor, Conrad Gessner (1516 - 1565). Gessner's work was a critical compilation of life known at the time.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bears a debt to the research of medical anatomists, such as Fabricius (1537 - 1619), Petrus Severinus (1580 - 1656), William Harvey (1578 - 1657), and Tyson (1649 - 1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628 - 1694), Jan Swammerdam (1637 - 1680), and Robert Hooke (1635 - 1702).

John Ray (1627 - 1705) was an English naturalist who published important works on plants, animals, and natural theology. His classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.

Linnaean taxonomy

Two years after John Ray's death Carolus Linnaeus (1707 - 1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). He is best known for his introduction of a method of modern classification; he created systematic zoology and botany in their present form.

Linnaeus adopted Ray's conception of species, but he made the concept a practical reality by insisting that every species must have a unique Latin binomen, that is, a double name — the first half to be the name of the genus, common to several species, and the second half to be a single word, which is called the specific epithet. This convention is now referred to as binomial nomenclature, and the name formed from the two parts is known as the scientific name or "systematic name" of a species. When a species in further subdivided, the trinomial nomenclature is used. For a name to be scientifically complete, an author label and publication details have to be added.

Before Linnaeus, long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus' system made it easy to identify unambiguously any given species of plant or animal. He proceeded further to introduce into his system a series of groups: genus, order, class.

The Linnaeus System works by placing each organism into a layered hierarchy of groups. Each group at a given layer is composed of a set of groups from the layer directly below. Simply knowing the two-part scientific name makes it possible to determine the other six layers.

The groupings (taxa) of taxonomy from most general to most specific are:

• Kingdom
• Phylum (animals) or Division (plants)
• Class
• Order
• Family
• Genus
• Species

Several acronym mnemonics have been made for these, for instance King Phillip came over for good soup, or Kings Play Chess On Funny Green Squares.

Intermediate ranks may be created by adding prefixes, for instance:

• Superorder
• Order
• Suborder
• Infraorder
• Microorder
• Nanoorder
• etc.

In addition, species are often subdivided into subspecies and other infraspecific categories (see subspecies, variety, subvariety and form). In certain circumstances prefixes beyond sub- need be used, normally micro-, nano-, etc., may be used. Some other ranks are also sometimes added. For instance, domains or empires may be given above the level of kingdom, tribes between the levels of family and genus, and sections and series between the genus and species.

Modern developments

The approach Linnaeus took to classifying species and the majority of his taxonomic groupings remained the standard in biology for at least two centuries. Since the 1960s, however, a trend called cladism or cladistic taxonomy, has emerged and may eventually supplant Linnaean classification. In classifying species, cladists place a priority on defining taxa with explicit reference with the Darwinian principle of common descent.

Meanwhile, at the top of the hierarchy of classification, there has been movement towards a three domain system. The domains originally were replacements for the different kingdoms, but many scientists regard them as groupings above the formerly paramount kingdom level.

Cladistics

In grouping species, cladists look for "derived similarities," meaning those aspects that species can be expected to share by virtue of a common evolutionary ancestry. This approach differs from that of phenetics, which does not address ancestry and associates species based on overall similarity. It also differs also from classification based on ad hoc "key characters." Cladists avail themselves of all the types of evidence available, including DNA sequences and hybridization studies, biochemistry, and traditional morphology. They often make use of computerized algorithms to identify the most likely phylogeny or "family tree" that relates the species they are considering.

Cladistics requires taxa (groups of species) to be clades. A formal code of phylogenetic nomenclature, the PhyloCode [1] , is currently under development for a cladistic taxonomy that abandons the Linnaean structure. The Phylocode is controversial among taxonomists.

More at: cladistics.

Examples

The usual classifications of three species follow: the Fruit Fly so familiar in genetics laboratories (Drosophila melanogaster), Humans, and the Sweetbay Magnolia.

Fruit Fly (Drosophila melanogaster)

Human (Homo sapiens)

Sweetbay Magnolia (Magnolia virginiana)

Note in this last example, that most of the taxa are named after the type genus, Magnolia. Sweetbay magnolia is the first species of the genus named, and is the type species of the genus Magnolia, and thus also of all flowering plants.

Group Suffixes

Taxa above the genus level are often given names derived from the type genus. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as follows:

Related topics

• Binomial nomenclature
• Trinomial nomenclature
• Taxonomy
• List of biological orders
• List of Latin and Greek words commonly used in systematic names
• Phylogenetic tree

External links

• Phylocode - replacing older system with Cladistic system
• Classification of all livings with lots of pictures