A species which has two or more sub-species, are known as poly species. The species that is not sub-divided into subspecies is known monotypic species.
Sub-species
May (1969) defined a subspecies as a geographically locally sub-division of species that differs generally and taxonomically from other sub-divisions of a species. The sub-species are fertile in crosses and intergrade between adjacent subspecies at the meeting place their ranges.
Clines
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A cline is formed by a series of continuous populations in which given character changes slowly. Such geographical character gradients are called clines. The term was introduced by Huxley (1939). They a of two types (a) Intergroup or stepped cline in which changes in charade are constant, (b) Internal cline in which no such variations in the rate ( change in characters is seen. The gradient is more or less constant throughout.
Race
A category which is used in the taxonomy of organisms that consists of a group of individuals within a species that are geographic ecologically and physiologically distinct from other members of the species. The term is also used in the same sense as subspecies.
Supraspecies
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It is the collection of subspecies, which replace one another geographically or physiologically and in which intrinsic isolation has taken place.
Clone
It is a group of individuals all derived from a single individual without sexual combination.
Linage
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It is a radial complex of several lines of descent, which forms a mesh network of evolution within the species.
Many groups of related population live together in the same area and remain distinct from each other without exchanging genes. These are the entities which we usually recognise as sympatric (overlapping) species as opposed to allopathic species which inhabit different areas. However, there are examples of related species which are allopatric in some parts of their distributional range but sympatric in others. A distinct species is one which pass the test of sympatry i.e., retain their identity even thought they live close enough together so that cross fertilization between them is possible. A. good example of sympatry is provided by three species of Rana, R. pipiens, R. syluatica and R. eatesbiana found together in streams and lakes of U.S.A.