The taxonomic categories which are always used in
hierarchical classification of organisms are called
obligate or common categories. They are seven in
number. In descending order, these are— kingdom,
phylum or division (for plants), class, order, family,
genus and species.
All the members of taxonomic categories possess some
similar characters, which are different from those of
others. The maximum similarity occurs in species
which is also the lowest category in the hierarchy Of
categories. Similarity of characters decreases with the
rise in hierarchy.

i. Species
Taxonomic studies consider a group of individual
organisms with fundamental similarities as a species.
Species is considered as the lowest or basic taxonomic
category, which consists of one or more individuals of
a population that resemble one another more closely
than individuals of other species.
The members of species interbreed freely and are
reproductively isolated from others. e.g., Mangifera
indica (mango), Solanum tuberosum (potato) and
Panthera leo (lion). All the three names indica,
tuberosum and leo represent the specific epithets while,
the first words Mangifera, Solanum and Panthera are
genera and represent another higher level of taxon or
category.
Each genus may have one or more than-one specific
epithets representing different organisms, but having
morphological similarities. e.g., Panthera has another
specific epithet called tigris and Solanum includes
species like nigrum and melongena.

ii. Genus
Genus comprises a group of related species, which have
more characters in common in comparison to species
of
other genera. In other words, genera are aggregates
closely related species.

iii. Family
Family is a group of related genera with less number of
similarities as compared to genus and species. All the
genera of a family have some common or correlated
features. They are separable from genera of a related
family by important differences in both vegetative and
reproductive features.
Note
A plant family ends in a suffix-aeae and sub-family has
suffix-oideae. While, an animal family has a suffix -idae and
sub-family-inae,

iv. Order
An order is a group of one or more related families that
possess some similar correlated characters, which are
lesser in number as compared to a family or genera.

v. Class
Class is a major category, which includes related orders,
e.g., order—Primata comprises monkey, gorilla and
gibbon and is placed in class—Mammalia along with
order—Carnivora that includes animals like tiger, cat and
dog.
Note
Class-Mammalia has other orders also,

vi. Phylum or Division
Phylum or Division is a taxonomic category higher than class
and lower in rank to kingdom. The term phylum is used for
animals, while division is commonly used for plants.
Both categories consist of morc than one class having some
similar correlated characters.
e.g., phylum—Chordata of animals contains following classes,—
Pisces, Amphibia, Reptilia, Aves and Mammalia.

vii. Kingdom
It is known to be the highest category in taxonomy. This
includes all the organisms which share a set of distinguished
characters, e.g., all the animals belonging to various phyla are
assigned the highest category called kingdom— Animalia.
Similarly, all the plants are kept in kingdom—plantae.
RH Whttaker (1969) assigned five kingdom classification of
organisms.
These are Monera, Protista, Fungi, Plantae and Animalia.

Taxon
Each unit of classification, in fact, represents a rank and is
commonly termed as taxon (Pl. taxa). The term Taxon was first
introduced by ICBN during 1956.
According to Mayr (1964), taxon is a group of any rank that is
sufficiently distinct to be worthy of being assigned a definite
category. In simple words, taxon refers to a group of similar,
genetically related individuals having certain characters distinct
from those of other groups.

TAXONOMY
It is the science of identification, classification and
nomenclature. Based on their special characteristics, all
living organisms can be classified into different
categories. The scientific term for these categories is
taxa. This process of classification is called taxonomy.
Carolus Linnaeus is known as father of taxonomy.
The basis of modern taxonomic studies are external and
internal structures (comparative morphology), along
with the structure of cells (cytology), development
process (embryology) and ecological information of
organisms (ecology). It provides information according
to similarities, dissimilarities and evolutionary
relationships of various organisms.
The concept of classical or old taxonomy exists, since the
time of Aristotle and Theophrastus till the time of
Linnaeus. On the other hand, the concept of modern
taxonomy was given by Julian Huxley (1940).
The basic prcxesses for taxonomic studies are
(i) Organisms are described on the basis of
morphology and other characteristics.
(ii) The description of characteristics helps in the
placement of the organisms in various taxa.
(iii) A new taxon can be framed if the organism is
different from the existing taxa.
(iv) The correct naming of an organism can be done
after placing it in appropriate taxon. A new
organism can be given a new name after following
the standardised rules.

Rules of Binomial Nomenclature
Rules of binomial nomenclature were initially framed by
Linnaeus in his books, Species Plantarum and Systema
Naturae. The rules were revised again by the following
nomenclature codes
(i) International Code for Botanical Nomenclature.
(ii) International Code of Zoological Nomenclature.
(iii) International Code of Bacteriological Nomenclature.
(iv) International Code of Viral Nomenclature.
(v) International Code of Nomenclature for Cultivated
Plants.
The rules framed by Linnaeus and by these codes are as follows
(i) The names are generally in Latin and written in italics.
They are Latinised or derived from Latin irrespective of
their origin.
(ii) The first word in a biological name represents the genus
while, the second component denotes the specific epithet.
(iii) Both the words in a biological name, when
handwritten, are separately underlined or printed in
italics to indicate their Latin origin.
(iv) The first word denoting the genus starts with a capital
letter while, the specific epithet starts with a small
letter, e.g., Mangifera indica.
(v) Name of the author appears after the specific epithet,
i.e., at the end of the biological name and is written in
an abbreviated form, e.g., Mangifera indica Linn Linn
is for Linnaeus.
(vi) When a species is transferred or revised, then the name
of the original worker is retained, but in parenthesis,
e.g., Syzygium cummini (L) skeels.
(vii) No names are recognised prior to those used by
Linnaeus in 1753 for plants in Species Plantarum and in
1758 for animals in the 10th edition of Systema
Naturae.
(viii) Generic and common names may be same, e.g., Gorilla
gorilla.
(ix) The name of categories higher than the rank of genus
are not printed in italics. Bold letters can, however be
used.

Disadvantages of Common Name
(a) All the organisms cannot be named by this method
as there are organisms of different sizes and shapes,
some of which are not directly perceived by man and
thus remain unnamed, e.g., microbes.
(b) An organism may have several names in a given
language, e.g., there are 8 Hindi names of prickly
poppy and water lily has 15 English names.
(c) One common name may have different meanings in
different countries, e.g., maize, means wheat and
other grains in USA and it is called corn in common
wealth countries.
ii. Scientific Names
A scientific name is given by biologists. Each name
represents a particular organism in all parts of the world.
The system of providing scientific names is called binomial
nomenclature.
The scientific names must be
(a) acceptable in every part of the world.
(b) assigned on agreed principles and criteria.
(c) different for each species and not used for
organisms earlier.