Revision of Group

Revision of Group

It is the grouping of species into distinct taxa on the basis of their resemblances and differences, development of complexity or simplicity and hence evolutionary relationship. First of all criteria are selected for delimiting a species. In case of sexually reproducing organisms, interbreeding is used as the basic criterion. In case others, morphology, physiology, cytotaxonomy and molecular biology including DNA-matching is resorted ro. The species are then grouped into higher taxonomic categories on the basis of certain common features called correlated characters.

Hierarchy  of Taxonomic Categories

 It is also called Linnaean hierarchy because it was first proposed by Linnaeus. Hierarchy of categories is the arrangement of organisms in a descending order starting from in a descending order starting from kingdom and reaching up to species or an ascending order from species to kingdom. The number of similar characters of categories decreases from lowest rank to highest rank. Hierarchy includes seven obligate categories kingdom, division or phylum, class, order, family, genus and species. The categories are arranged in descending sequence keeping the kingdom at the top. In order to make taxonomic position of species more precise, certain categories have been added to this list. They are called intermediate categories, e,g., subkingdom, super phylum or super division subdivision, super class, subclass, super order, suborder, super family, subfamily, tribe, subspecies, variety, etc.

Taxonomic Categories

A taxonomic category is a rank or level in the hierarchical classification of organisms. There are seven obligate categories and some intermediate categories. The seven obligate categories are as follows.

1.       Species. It is a natural population of individuals or group of populations which resemble one another in all essential morphological and reproductive characters so that they are able to interbreed freely and produce fertile offspring. Mango is species indica of genus Mangifera. Potato is species tuberosum of genus Solanum. Lion is species leo of genus panthera while Tiger is species Tigris of geuns panther. Each species is also called genetically distinct and reproductively isolated natural population. Mayr has defined species as “a groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups” However there are two objections for using interbreeding as a sole criterion for delimitation of a species.

Occasional hydrida occur in nature due to breakdown of mechanical, Physiological spatial and seasonal barriers between species. Interspecific hybrids have been obtained since ages antisocially by man, e. g., mule. Hybrids between closely related species are often obtained in captivity. These hybrids are generally sterile, but some are fertile as well. A few examples are give below:

(a)    Mule (sterile) – Female horse and male donkey

(b)   Hinny (sterile) – Male horse and female donkey

(c)    Tigon (fertile)- Male tiger and female lion

(d)   Liger (fertile) – Male lion and tiger

Sexual reproduction is absent in  prokaryotes and some protiests. In cases and fossils morphological differences, cytotaonomy and chemotaxonomy are resorted.