SPECIATION

                  The term species is very confusing and conveys different meanings to different biologists. For a long time, species was considered as an artificial grouping, a “made-made” concept, without any real existence. But, Linnaeus gave it a scientific meaning. Still, the term is highly ambiguous. For example; it can be an aggregate of similar looking individuals to a morphologist or a population of organisms living optimally in area or niche (see glossary) to an ecologist or a group of genetically similar individuals to geneticist and so on. 

                  Therefore, a scientific definition was the need of the day, Dobzhansky and Maryr defined species as populations of similar organisms which inter-breed, i.e., produce fertile offspring (Biological species). Americans, Europeans, Africans and Indians all belong to the same species, i.e., Homo sapiens, because although they might be very different looking, with different behavious, customs, habits external features, etc. and might be separated by long distances too,  but still they can cross-breed, when brought together. Thus, they form one group or one population of inter-breeding individuals and hence belong to the same species.

                On the other hand, tigers, leopards, cheetah, panther, etc. all look very similar to each other and are present in the same jungle, but since they cannot mate or breed with each other, therefore they from different species. Similarly a crow and sparrow might be present on the same branch of a tree, but belong to different species.

               The process of formation of new species from the ancestral or per-existing ones is known as specialization. It occurs either by splitting species (divergence) or by accumulation of splitting of species (divergence) or by accumulation of gradual changes with time in pre-existing species (transformation).

             Speciation can be speculated to occur either gradually or suddenly by the following mechanism. In the sudden type of speciation, the pre-existing species or population undergoes mutation, , to form a new species. Whereas, in the gradual type of specification, the population of a species gets separated by physical or geographical barriers, such as tall mountains rivers, large oceans, forests, etc. These barriers split the population into two or more sub-populations, which is known as isolation. The two sub-populations are exposed to different set of environmental conditions. For example, population A may be exposed to dry conditions, whereas population B may be adapted to wet conditions. 

           Therefore, both the population are subjected to independent selection pressure with respect to its to its own environment, and may also undergo genetic drift. This caused divergence between the populations by accumulation of variations. If the divergence occurs for several generation (1000s of years)., ultimately the two populations become very different from each other, so that they are unable to inter-breed or exchange genes. This leads to reproductive isolation and finally formation of a new species.