Species : Species is the 'basic unit' of classification. Species is a *** word meaning 'kind' or 'appearance'. John Ray in his book 'Historia Generalis Plantarum', used the term 'species**' and described it on the basis of common descent (origin from common ancestors) as a group of morphologically similar organisms. Linnaeus considered species, in his book 'Systema Naturae', as the basic unit of classification. Buffon, in his book 'Natural History', proposed the idea of evolution of species which is the foundation for the biological concept of evolution. This biological concept of species (dynamic nature of species) became more popular with the publication of the book "The Origin of Species" by Charles Darwin.
Buffon'* biological concept of species explains that species is an interbreeding group of similar individuals sharing the common 'gene pool' and producing fertile offspring.
Species is considered as a group of individuals which are :
(1) Reproductively isolated from the individuals of other species – a breeding unit.
(2) Sharing the same ecological niche – an ecological unit.
(3) Showing similarity in the karyotype – a genetic unit.
(4) Having similar structure and functional chracteristics – an evolutionary unit.
