2. Systematics of Living Organisms part 03

Units of Classification :
Species :

Species is the principal natural taxonomic unit, ranking below a genus and denoted by latin binomial (considered as the basic) unit of classification.
It is a group of organisms that can interbreed under natural condition to produce fertile offspring.
It was thought to be an indivisible, stable and static unit.
However in the modern taxonomy, subdivision of species such as sub-species, varities and populations are seen and given more importance.

Genus :

Genus is a taxonomic rank or category larger than species used in the biological classification of living and fossil organisms.
Genus is a group of species bearing close resemblance to one another in their morphological characters but they do not interbreed.
e.g. Tiger, Leopard, Lion all three belong to same genus Panthera.
They have common characters yet are different from each other because their genus is same but species is different.
Another example is genus Solanum.
Brinjal and potato both belong to this genus.

Family :

It is one of the major hierarchial taxonomic rank.
A family represents a group of closely related genera.
e.g. genera like Hibiscus, Gossypium, Sida, Bombax are included in same family Malvaceae.
Cat also belongs to family of leopards, tigers and lions, family Felidae but dog belongs to different family Canidae.

Cohort / Order :

It is taxonomic rank used in the classification of organisms and recognised by nomenclature codes.
An order is a group of closely related families showing definite affinities.
Order thus is a step above family in taxonomic hierarchy.
Members belonging to same order but different families may show very few dis similarities.
e.g. family - Papavaraceae, Brassicaceae, Capparidaceae, etc with parietal plancentation are grouped in order Parietales.
Families of dogs and cats though are different, they belong to same order Carnivora.

Class :

The class is the distinct taxonomic rank of biological classification having its own distinctive name.
A group of higher taxonomic rank than order.
Class is the assemblage of closely allied orders.
Orders Carnivora and order Primates belong to class Mammalia.
Thus monkeys, gorillas, gibbons (Primates) and dogs, cats, tigers (Carnivora) belong to same class.

Division / Phylom :

The division is a category composed of related classes
e.g. division. Angiospermae includes two classes - Dicotyledonae and Monocotyledonae.
In animal classification division is a sub-unit of Category / Phylum.

Sub-kingdom :

Different divisions having some similarities form sub-kingdom.
e.g. The divisions Angiospermae and Gymnospermae will tIt is the highest taxonomic category composed of different subkingdoms.

e.g. sub-kingdom Phanerogams and Cryptogams form the Plant kingdom or Plantae which includes all the plants while all animals are included in kindom Animalia.
The taxonomic categories we have considered so far are broad categories.
Scientists have added sub-categories to these in order to place organisms in more scientific manner.
As we go higher in taxonomical ladder, number of common characters go on decreasing.
If we are comparing two organisms that are related to each other only at division or phylum level, their classification may become difficult.

Yogy's biology