Scientific classification

Scientific classification is a term used to describe how biologists organize
all life on Earth. The way that forms of life are classified are based
primarily on evolutionary similarity.

The earliest known system of classifying forms of life comes from the Greek
philosopher Aristotle. The next major advance in developing systems for
scientific classification of living beings was by the Swiss professor,
Conrad Gessner (1516 - 1565). He was one of the most voluminous writers on
systematic zoology, and was so highly esteemed that his Historia animalium
was republished a hundred years after his death.

GesnerŐs work, like that of John Johnstone (born 1603), who was of Scottish
descent and studied at St Andrews, and like that of Ulysses Aldrovandi of
Bologna (b. 1522), was essentially a compilation, more or less critical, of
all such records, pictures and relations concerning beasts, birds, reptiles,
fishes and monsters as could be gathered together by one reading in the
great libraries of Europe, travelling from city to city, and frequenting the
company of those who either had themselves passed into distant lands or
possessed the letters written and sometimes the specimens brought home by
adventurous persons.

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), 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
lile Malpighi (1628 - 1694), Swammerdam (1637 - 1680), and Hook (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.

The commencement of anatomical investigations deserves notice here as
influencing the general accuracy and minuteness with which zoological work
was prosecuted, but it was not until a late date that their full influence
was brought to bear upon systematic zoology by Georges Cuvier (1769 - 1832).

Two years after John RayŐs death Carolus Linnaeus (1707 - 1778) was born.
His great work, the Systeina natisrae, 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 RayŐs conception of species, but he
made species a practical reality by insisting that every species shall have
a double Latin name - the first half to be the name of the genus common to
several species, and the second half to be the specific name.

Previously to 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 by his binomial system made it possible to
write and speak with accuracy of any given species of plant or animal. He
proceeded further to introduce into his enumeration of animals and plants a
series of groups, viz, genus, order, class, which he compared to the
subdivisions of an army or the subdivisions of a territory.

Linnaeus's System has been the standard method classifying all forms of life
on Earth until very recently. However, a recent trend in biology since the
1960s, called cladism or cladistic taxonomy, has slowly been becoming more
useful than the Linneaean system, and in recent years has become very
popular. It is now expected that the cladistics system will overtake the
older system. Cladistics requires taxa (named groups in a taxonomy) to be
clades. In other words, cladists argue the classification system should be
reformed to eliminate all non-clades (paraphyletic and polyphyletic groups).
In fact, some cladists have argued for entirely abandoning the Linnaean
system of ranked taxa in favor of clades. A formal code of phylogenetic
nomenclature, the Phylocode, is currently under development for a
cladistic taxonomy that abandons the Linnaean structure.

Linnaeus's System

This system was devised by Carolus Linnaeus (1707-1778).

The 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. Therefore, in theory, one needs know only the lowest
layer (species) of a particular organism in order to uniquely determine the
other six layers. In practice, however, many species actually have the same
species designation, so when specifiying a species, scientists use the
bottom two layers - a system called binomial nomenclature.

The standard 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 called out for good soup. Sometimes tribes, which lie between
families and genera, and races, which lie below species, are also used.
Intermediate ranks may be created by adding prefixes, for instance:

   * Superorder
   * Order
   * Suborder
   * Infraorder

The term varieties is sometimes used in place of subspecies. In
horticulture, it refers to populations modified by selective breeding, for
instance the Peace Rose, a hybrid Tea Rose. At the top of the scale, there
has been a move towards the three domain system. The domains originally were
replacements for the different kingdoms, but often count as a higher rank.

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 Of Biological Classification

The fruit fly so familiar in genetics laboratories is Drosophila
melanogaster. Its usual classification, as well as that of humans, is as follows

Fruit Fly (Drosophila)

Kingdom Animalia
Phylum  Arthropoda
Class   Insecta
Order   Diptera
Family  Drosophilidae
Genus   Drosophila
Species melanogaster

Human (Homo sapiens)

Kingdom   Animalia
Phylum    Chordata
Subphylum Vertebrata
Class     Mammalia
Subclass  Eutheria
Order     Primates
Suborder  Catarrhini
Family    Hominidae
Genus     Homo
Species   sapiens

Cucumbertree (Magnolia acuminata)

Kingdom  Plantae
Division Magnoliophyta
Class    Magnoliopsida
Order    Magnoliales
Family   Magnoliaceae
Genus    Magnolia
Species  acuminata

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:

         Taxon         Plants    Algae     Fungi    Animals
    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

Cladistics

Cladistics (or phylogenetic systematics) describes the evolutionary
relationships between living things based on derived similarity. Cladistics
differs from phenetics, which groups organisms based on overall similarity,
and from more traditional approaches based on "key characters". Based on a
wide variety of information, which includes genetic analysis, biochemical
analysis, and analysis of morphology, relationship trees called "cladograms"
are drawn up to show different possibilities.

Cladistics requires taxa (named groups in a taxonomy) to be clades. In other
words, cladists argue the classification system should be reformed to
eliminate all non-clades (paraphyletic and polyphyletic groups). In fact,
some cladists have argued for entirely abandoning the Linnaean system of
ranked taxa in favor of clades. A formal code of phylogenetic nomenclature,
the Phylocode, is currently under development for a cladistic taxonomy
that abandons the Linnaean structure.