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CLASSIFYING A GROUP OF SPECIES OR REVISION GROUP
Revision of group is the delimitation (separation) of species by finding out the common characters amongst species and placing them successively into higher taxonomic categories (order, family, class, etc) on the basis of their similarities and evolutionary relationship (patterns of descent of organism or evolutionary tree of an organism).

In sexually reproducing organisms, reproductive isolation (separation) from other and free interbreeding amongst members under natural conditions is the best criteria for delimitation of species. Evidence from morphology (study of outer structure), anatomy (study of internal structures), embryology (study of embryo), cytology (study of cell), chromosomes, and molecular biology are used to support of the same.

In organism reproducing asexually, morphotaxonomy (classifying the organism on the basis of morphology), cytotaxonomy (classifying the organism on the basis of cell structure and unction), karyotaxonomy (classifying the organism on the basis of nucleus or chromosomes), chemotaxonomy (classifying the organism on the basis of chemical constituents), and biochemical taxonomy (classifying the organism on the basis of biomolecules) are used for their delimitation.
After delimitation of the species, the resemblances amongst the species are measured and correlated characters are selected for further grouping.
Correlated characters are common similar and related features which occur in all the member of the group and used for delimiting.
Each group has some primitive or ancestral character present in whole group.
Also have some conservative characters which do not change much.

The group is distinguished from other group by the presence of derived characters which have evolved (gained) in the group.
Homology indicates how an ancestral character has been modified in the group.

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