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Scientific classification

2007 Schools Wikipedia Selection. Related subjects: Evolution and
reproduction; General Biology

   Scientific classification or biological classification is a method by
   which biologists group and categorize species of organisms. Scientific
   classification can also be called scientific taxonomy, but should be
   distinguished from folk taxonomy, which lacks scientific basis. Modern
   classification has its root in the work of Carolus Linnaeus, who
   grouped species according to shared physical characteristics. These
   groupings have since been revised to improve consistency with the
   Darwinian principle of common descent. Molecular systematics, which
   uses DNA sequences as data, 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 known system of classifying forms of life comes from the Greek
   philosopher Aristotle, who classified all living organisms known at
   that time as either a plant or an animal. He further classified animals
   based on their means of transportation (air, land, or water).

   In 1172 Ibn Rushd ( Averroes), who was a judge ( Qadi) in Seville,
   translated and abridged Aristotle's book de Anima ( On the Soul) into
   Arabic. His original commentary is now lost, but its translation into
   Latin by Michael Scot survives.

   An important advance was made by the Swiss professor, Conrad von Gesner
   (1516–1565). Gesner's work was a critical compilation of life known at
   the time.

   The exploration of parts of the New World that produced large numbers
   of new plants and animals that needed descriptions and classification.
   The old systems made it difficult to study and locate all these new
   specimens within a collection and often the same plants or animals were
   given different names because the number of specimens were too large to
   memorize. A system was needed that could group these specimens together
   so they could be found, the binomial system was developed based on
   morphology with groups having similar appearances. 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 bear a debt to the research of
   medical anatomists, such as Fabricius (1537–1619), Petrus Severinus
   (1580–1656), William Harvey (1578–1657), and Edward 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). Lord Monboddo (1714-1799) was one of the early abstract
   thinkers whose works illustrate knowledge of species relationships and
   who foreshadowed the theory of evolution. Successive developments in
   the history of insect classification may be followed on the website by
   clicking on succeeding works in chronological order.

Early methodists

   Since late in the 15th century, a number of authors had become
   concerned with what they called methodus, or method. By method they
   meant an arrangement of minerals, plants, and animals according to the
   principles of logical division. The term methodists was coined by
   Carolus Linnaeus in his Bibliotheca Botanica to denote the authors who
   care about the principles of classification (in contrast to the mere
   collectors who are concerned primarily with the description of plants
   paying little or no attention to their arrangement into genera, etc).
   Important early methodists were an Italian philosopher, physician, and
   botanist Andrea Caesalpino, an English naturalist John Ray, a German
   physician and botanist Augustus Quirinus Rivinus, and a French
   physician, botanist, and traveller Joseph Pitton de Tournefort.

   Andrea Caesalpino ( 1519– 1603) in his De plantis libri XVI ( 1583)
   proposed the first methodical arrangement of plants. On the basis of
   the structure of trunk and fructification he divided plants into
   fifteen "higher genera".

   John Ray ( 1627– 1705) was an English naturalist who published
   important works on plants, animals, and natural theology. The approach
   he took to the 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.

   Both Caesalpino and Ray used traditional plant names and thus, the name
   of a plant did not reflect its taxonomic position (e.g. even though the
   apple and the peach belonged to different "higher genera" of John Ray's
   methodus, both retained their traditional names Malus and Malus Persica
   respectively). A further step was taken by Rivinus and Pitton de
   Tournefort who made genus a distinct rank within taxonomic hierarchy
   and introduced the practice of naming the plants according to their
   genera.

   Augustus Quirinus Rivinus ( 1652– 1723), in his classification of
   plants based on the characters of the flower, introduced the category
   of order (corresponding to the "higher" genera of John Ray and Andrea
   Caesalpino). He was the first to abolish the ancient division of plants
   into herbs and trees and insisted that the true method of division
   should be based on the parts of the fructification alone. Rivinus
   extensively used dichotomous keys to define both orders and genera. His
   method of naming plant species resembled that of Joseph Pitton de
   Tournefort. The names of all plants belonging to the same genus should
   begin with the same word (generic name). In the genera containing more
   than one species the first species was named with generic name only,
   while the second, etc were named with a combination of the generic name
   and a modifier (differentia specifica).

   Joseph Pitton de Tournefort ( 1656– 1708) introduced an even more
   sophisticated hierarchy of class, section, genus, and species. He was
   the first to use consistently the uniformly composed species names
   which consisted of a generic name and a many-worded diagnostic phrase
   differentia specifica. Unlike Rivinus, he used differentiae with all
   species of polytypic genera.

Linnaeus

   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). In this work, nature was divided
   into three kingdoms: mineral, vegetable and animal. Linnaeus used five
   ranks: class, order, genus, species, and variety.

   He abandoned long descriptive names of classes and orders and two-word
   generic names (e. g. Bursa pastoris) still used by his immediate
   predecessors (Rivinus and Pitton de Tournefort) and replaced them with
   single-word names, provided genera with detailed diagnoses (characteres
   naturales), and reduced numerous varieties to their species, thus
   saving botany from the chaos of new forms produced by
   horticulturalists.

   Linnaeus is best known for his introduction of the method still used to
   formulate the scientific name of every species. Before Linnaeus, long
   many-worded names (composed of a generic name and a differentia
   specifica) had been used, but as these names gave a description of the
   species, they were not fixed. In his Philosophia Botanica (1751)
   Linnaeus took every effort to improve the composition and reduce the
   length of the many-worded names by abolishing unnecessary rhetorics,
   introducing new descriptive terms and defining their meaning with an
   unprecedented precision. In the late 1740s Linnaeus began to use a
   parallel system of naming species with nomina trivialia. Nomen
   triviale, a trivial name, was a single- or two-word epithet placed on
   the margin of the page next to the many-worded "scientific" name. The
   only rules Linnaeus applied to them was that the trivial names should
   be short, unique within a given genus, and that they should not be
   changed. Linnaeus consistently applied nomina trivialia to the species
   of plants in Species Plantarum (1st edn. 1753) and to the species of
   animals in the 10th edition of Systema Naturae (1758).

   By consistently using these specific epithets, Linnaeus separated
   nomenclature from taxonomy. Even though the parallel use of nomina
   trivialia and many-worded descriptive names continued until late in the
   eighteenth century, it was gradually replaced by the practice of using
   shorter proper names combined of the generic name and the trivial name
   of the species. In the nineteenth century, this new practice was
   codified in the first Rules and Laws of Nomenclature, and the 1st edn.
   of Species Plantarum and the 10th edn. of Systema Naturae were chosen
   as starting points for the Botanical and Zoological Nomenclature
   respectively. This convention for naming species is referred to as
   binomial nomenclature.

   Today, nomenclature is regulated by Nomenclature Codes, which allows
   names divided into ranks; see rank (botany) and rank (zoology).

Modern developments

   Whereas Linnaeus classified for ease of identification, it is now
   generally accepted that classification should reflect the Darwinian
   principle of common descent.

   Since the 1960s a trend called cladistic taxonomy (or cladistics or
   cladism) has emerged, arranging taxa in an evolutionary tree. If a
   taxon includes all the descendants of some ancestral form, it is called
   monophyletic, as opposed to paraphyletic. Other groups are called
   polyphyletic.

   A new formal code of nomenclature, the PhyloCode, is currently under
   development, intended to deal with clades rather than taxa. It is
   unclear, should this be implemented, how the different codes will
   coexist.

   Domains are a relatively new grouping. The three-domain system was
   first invented in 1990, but not generally accepted until later. Now,
   the majority of biologists accept the domain system, but a large
   minority use the five-kingdom method. One main characteristic of the
   three-domain method is the separation of Archaea and Bacteria,
   previously grouped into the single kingdom Bacteria (sometimes Monera).
   A small minority of scientists add Archaea as a sixth kingdom but do
   not accept the domain method.

Examples

   The usual classifications of five species follow: the fruit fly so
   familiar in genetics laboratories (Drosophila melanogaster), humans
   (Homo sapiens), the peas used by Gregor Mendel in his discovery of
   genetics (Pisum sativum), the "fly agaric" mushroom Amanita muscaria,
   and the bacterium Escherichia coli. The eight major ranks are given in
   bold; a selection of minor ranks are given as well.
   Rank Fruit fly Human Pea Fly Agaric E. coli
   Domain Eukaryota Eukaryota Eukaryota Eukaryota Bacteria
   Kingdom Animalia Animalia Plantae Fungi
   Phylum or Division Arthropoda Chordata Magnoliophyta Basidiomycota
   Proteobacteria
   Subphylum or subdivision Hexapoda Vertebrata Magnoliophytina
   Hymenomycotina
   Class Insecta Mammalia Magnoliopsida Homobasidiomycetae Proteobacteria
   Subclass Pterygota Eutheria Magnoliidae Hymenomycetes
   Gammaproteobacteria
   Order Diptera Primates Fabales Agaricales Enterobacteriales
   Suborder Brachycera Haplorrhini Fabineae Agaricineae
   Family Drosophilidae Hominidae Fabaceae Amanitaceae Enterobacteriaceae
   Subfamily Drosophilinae Homininae Faboideae Amanitoideae
   Genus Drosophila Homo Pisum Amanita Escherichia
   Species D. melanogaster H. sapiens P. sativum A. muscaria E. coli

   Notes:
     * Higher taxa and especially intermediate taxa are prone to revision
       as new information about relationships is discovered. For example,
       the traditional classification of primates (class Mammalia —
       subclass Theria — infraclass Eutheria — order Primates) is
       challenged by new classifications such as McKenna and Bell (class
       Mammalia — subclass Theriformes — infraclass Holotheria — order
       Primates). See mammal classification for a discussion. These
       differences arise because there are only a small number of ranks
       available and a large number of branching points in the fossil
       record.

     * Within species further units may be recognised. Animals may be
       classified into subspecies (for example, Homo sapiens sapiens,
       modern humans) or morphs (for example Corvus corax varius morpha
       leucophaeus, the Pied Raven). Plants may be classified into
       subspecies (for example, Pisum sativum subsp. sativum, the garden
       pea) or varieties (for example, Pisum sativum var. macrocarpon,
       snow pea), with cultivated plants getting a cultivar name (for
       example, Pisum sativum var. macrocarpon 'Snowbird'). Bacteria may
       be classified by strains (for example Escherichia coli O157:H7, a
       strain that can cause food poisoning).

     * A mnemonic for remembering the order of the taxa is: Do Koalas
       Prefer Chocolate Or Fruit, Generally Speaking? Other mnemonics are
       available at and .

Terminations of names

   Taxa above the genus level are often given names based on the type
   genus, with a standard termination. The terminations used in forming
   these names depend on the kingdom, and sometimes the phylum and class,
   as set out in the table below.
           Rank          Plants    Algae     Fungi    Animals Bacteria
   Division/Phylum            -phyta       -mycota
   Subdivision/Subphylum     -phytina      -mycotina
   Class                 -opsida -phyceae  -mycetes           -ia
   Subclass              -idae   -phycidae -mycetidae         -idae
   Superorder                       -anae
   Order                            -ales                     -ales
   Suborder                         -ineae                    -ineae
   Infraorder                       -aria
   Superfamily                      -acea             -oidea
   Epifamily                                          -oidae
   Family                           -aceae            -idae   -aceae
   Subfamily                       -oideae            -inae   -oideae
   Infrafamily                                        -odd
   Tribe                             -eae             -ini    -eae
   Subtribe                         -inae             -ina    -inae
   Infratribe                                         -ad

   Notes:
     * In botany and mycology names at the rank of family and below are
       based on the name of a genus, sometimes called the type genus of
       that taxon, with a standard ending. For example, the rose family
       Rosaceae is named after the genus Rosa, with the standard ending
       "-aceae" for a family. Names above the rank of family are formed
       from a family name, or are descriptive (like Gymnospermae or
       Fungi).

     * For animals, there are standard suffixes for taxa only up to the
       rank of superfamily (ICZN article 27.2).

     * Forming a name based on a generic name may be not straightforward.
       For example, the Latin "homo" has the genitive "hominis", thus the
       genus "Homo" (human) is in the Hominidae, not "Homidae".

     * The ranks of epifamily, infrafamily and infratribe (in animals) are
       used where the complexities of phyletic branching require
       finer-than-usual distinctions. Although they fall below the rank of
       superfamily, they are not regulated under the International Code of
       Zoological Nomenclature and hence do not have formal standard
       endings. The regular, but informal, suffixes listed here are
       supplied by Eugene S. Gaffney & Peter A. Meylan (1988), "A
       phylogeny of turtles", in M.J. Benton (ed.), The Phylogeny and
       Classification of the Tetrapods, Volume 1: Amphibians, Reptiles,
       Birds 157-219 (Oxford: Clarendon Press).

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