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.