The species is a group of actually or potentially interbreeding natural populations which share in a common gene pool, but are reproductively isolated from other such groups. Interbreeding is very frequency among the individuals of a population and is occasional among the populations of a species, whereas interbreeding is absent among the individuals of different species.
It means there is a free gene flow within the members of a population and a free gene flow could be maintained among the members of different populations of a species, provided they have an opportunity to interbreed. Thus there is great potential of origin of new species.
There are two distinct ways of origin of species from the pre-existing one – splitting of the species into two or more species, known as speciation and transformation of the old species into a new one in due course of time known as transformation in time.
Species population usually has a discontinuous distribution. Distance plays a significant role in the separation of species population in due course of time. The population of a given species grows in size generation after generation. As a result, the organism radiate into progressively larger territory.
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After little generation these populations at the opposite ends of the territory may be too far off so that direct gene flow among their members is not possible because they no longer come into reproductive contact directly.
Although distance isolations is probably most common, there are other kinds of geographic isolation, whereby groups of related organisms become separated by some physical barriers such as sea, mountain, desert, glacier or river. In mountain regions individual mountain ranges provide effective barriers between the valleys. Moreover, homing instinct and territoriality of animals also add to the isolation of individuals into groups of populations.
A species is generally composed of a number of allopathic breeding populations, physically separated to some extent from one another and pursuing independent evolutionary path. Initially these might have a very similar genetic composition, but no two environments, howsoever close these may be, are likely to be identical biologically or physically.
Therefore, each population is exposed to somewhat different selection pressure. Therefore, random mutations together with genetic drifts and selection pressure establish genetic difference and morphological and physiological variations in formerly similar or identical populations.
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These differences gradually accumulate and cause more and more divergence in the genetic constitution of populations and thus lead to the establishment of clines, geographical races and finally the distinct subspecies. So far the geographical races or the subspecies are allopathic being geographically isolated, but when given an opportunity to interbreed or are crossed artificially these produce fertile hybrids.
This means that the genetic modifications appeared so far have not reached to the extent of producing reproductive isolation, though these may still exhibit preference of mating to the members of their own group.
Addition of certain more variations in their gene pool so as to affect their interbreeding, leads to reproductive isolation. Therefore, the groups or subspecies or races become reproductively isolated are ranked as species. The species produced in this manner do not differ in their morphology alone but are segregated by the development of reproductive isolation caused by the differences in their genetic composition and alignment.