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 Carl 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. 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.

Table of contents
1 Early Systems
2 Linnaean taxonomy
3 Modern developments
4 Cladistics
5 Examples
6 Group Suffixes
7 See also:
8 External Links:

Early Systems

The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle.

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), 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:

Several acronym mnemonics have been made for these, for instance King Phillip called out 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

In addition, species are often subdivided into subspecies and other infraspecific categories (see subspecies, variety, subvariety and form).

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 is expected to supplant Linnaean classification. In classifying species, cladists place a priority in achieving coherence 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.

More at: cladistics.

Could add a description of the difficulty in classifying microbes: their features are derived from direct visual observation, but include such procedural characteristics as Gram stain type, motility, ability to form spores, etc. However, given an unknown bacterium with a given set of characteristics, it is in general not possible to predict its phylogeny, toxicity, etc. Other methods, using genes, their DNA, and several types of RNA, are under development.

Examples

The usual classifications of three species follow: The fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans, and the cucumber tree.

Fruit Fly (Drosophila)

KingdomAnimalia
PhylumArthropoda
ClassInsecta
OrderDiptera
FamilyDrosophilidae
GenusDrosophila
Speciesmelanogaster

Human (Homo sapiens)

KingdomAnimalia
PhylumChordata
SubphylumVertebrata
ClassMammalia
SubclassEutheria
OrderPrimates
SuborderCatarrhini
'Family'\Hominidae
GenusHomo
Speciessapiens

Cucumbertree (Magnolia acuminata)

KingdomPlantae
DivisionMagnoliophyta
ClassMagnoliopsida
OrderMagnoliales
FamilyMagnoliaceae
GenusMagnolia
Speciesacuminata

Note in this last example, that most of the taxa are named after the type genus, Magnolia.

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:

TaxonPlantsAlgaeFungiAnimals
Division/Phylum-phyta-phyta-mycota
Subdivision/Subphylum-phytina-phytina-mycotina
Class-opsida-phyceae-mycetes
Subclass-idae-phycidae-mycetidae
Order-ales-ales-ales
Suborder-ineae-ineae-ineae
Superfamily-acea-acea-acea-oidea
Family-aceae-aceae-aceae-idae
Subfamily-oideae-oideae-oideae-inae
Tribe-eae-eae-eae-ini
Subtribe-inae-inae-inae-ina

See also:

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