Classification and evolution

• The classification and evolution are closely related to each other. Classification helps in categorizing the living organisms according to their characteristic features. While changes in characteristic features help in the process of evolution.
• Evolution is defined as a complex process by which the characteristics of living organisms change due to various factors and are inherited from one generation to the next. Accumulations of such changes lead to the generation of new species or new characters within the species.
• Evolution thus leads to the generation and shaping of the diversity by environmental selection. It never means that complex forms are better than the simple ones as even the simplest prokaryotic forms are able to survive as an independent organism.
• On the basis of accumulation of these changes during the course of evolution organisms are classified as primitive and advance organisms.

• Primitive organisms are those which have ancient body design that has not changed much during the process of evolution. They are simple and belong to lower group of organisms and considered to be older organisms, i.e., have evolved previously during the course of evolution.
• Advanced organisms are those which have evolved their particular designs relatively recently. They are complex and belong to higher group and considered to be young comparatively.

Example:

• Comparatively prokaryotes are primitive and eukaryotes are advanced.

Hierarchy of Categories or Groups:

• The main aim of taxonomic study is to place the organisms in a systematic framework of classification.
• Such arrangement is known as taxonomic hierarchy by which the taxonomic groups are arranged in definite order from higher to lower categories.
• Each category is called taxon or rank. The taxa or categories used in the classification of plants or animals are kingdom, phylum, class, order, family, genus and species. In case of plants, term division is used in place of phylum.

Kingdom:

• It is the top most taxonomic category which contains the maximum number of organisms with minimum similarities among them.
• Example: All animals and plants are placed in separate Kingdom Animalia and Plantae respectively.

Phylum:

• Phylum is the primary subdivision of kingdom in which all groups/ classes of organisms that have the same body plan are placed. The term phylum is used for animals while division is used for plants.
• Example: Kingdom: Animalia

   Phylum: Chordata [animals having notochord]

Class:

• Class is the primary subdivision of phylum which is subdivided into two or more orders depending on their differences in characters.

Example:

Kingdom: Animalia

Phylum: Chordata [Animals having notochord]

Class: Mammalia [Animals that give birth to young ones and have mammary glands]

Order:

•  Order is the subdivision of class which is further subdivided into one or more families.

Example:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora [Animals that depend on other animals for their nourishment]

Family:

• It is defined as collection of similar genera. It can be separated from genera by reproductive and vegetative features.

Example:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: felidae [cats]

Genus:

• It is defined as group of similar species. But it is not mandatory to have many species. Some genera have only one species known as Monotypic. If there are more than one species it is known as polytypic.

Example:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: felidae [cats]

Genus: Panthera

Species:

• Species is a group of similar organisms that are able to interact, perpetuate and interbreed with each other.

Example:

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: felidae [cats]

Genus: Panthera

Species: tigiris [tiger]

• Species is followed by subspecies, varieties and races. These categories are inferior as compared to species.