The genus is a collective taxonomic unit consisting of a number of similar or related species. It is distinguished from all other higher categories by being recognised in the scientific names of a group of species. Whilst the scientific trivial name signifies singularity and differentiation, the generic name calls attention to the existence of a group of similar or related species - it relieves the memory.
An objective criterion for generic rank does not exist equivalent, let us say, to that of reproductive isolation for a species criterion. It is therefore impossible to give an objective definition of the genus. A convenient definition is as follows! A genus is a systematic category including one species or a group of species of presumably common phylogenetic origin, which is separated from other similar units by a decided gap.
Genera are tied down by type species and although no one species can be 'typical' of a group of species assigned to a genus, the generic type serves as a fixed point fox’ the generic concept. This situation has been likened to a flat piece of rubber nailed to a table at a single point on its surface. The rubber (generic contents) may be strctched in one direction or another by adding or subtracting species, but it always includes the nail (type species). The species which serves as the type of a genus is, in turn, tied to type specimens, so that the genus is firmly anchored. It is only the extent or limits of the genus that are arbitrary. The type system provides another aid in delimiting genera, i.e. all the species in a genus must resemble the type of that genus more closely than they resemble the types of other genera.
Because species are not evenly distinct from one another, but are arrang¬ed in smaller or larger groups, the genus becomes a logical taxonomic category as the next highest grouping above that of the species. Taxonomic characters that prove generic distinctions do not exist. Literature would have been spared many unnecessary generic names if Linne’s (1757) warning; "The characters do not make the genus, rather it is the genus that give the characters", had more frequently been kept in mind.
The Meaning of the Genus. The genus, as seen by the evolutionist, is a group of species that has descended from a common ancestor. It is a phylogenetic unit. The characters of the genus are thus either the critical characters of the ancestral species or such characters as have been jointly acquired by all the species.
The genus, however, has a deeper significance. Upon closer examination it is usually found, that all the species of a genus occupy a more or less well defined ecological niche. The genus is thus a group of species adapted for a particular mod.e of life and it is probable that all generic characters are either adapted or correlated with adaptive characters.
It should be remembered that the essential property of a species is reproductive isolation, the essential property of 3. genus is morphological distinctness usually correlated with the occupation of distinctly different ecological niches.
Further reading. Mayor, E., Linsloy, E.G., & Usingor, R. K. Methods and Principles of Systematic Zoology, McGraw-Hill, Nov; York (1953).