In this section we are to discuss the means by which populations get separated or isolated from each other, first gain the status of'Sub-species and finally evolve mechanisms which prevent them from mating with local populations to be called as a distinct species. In ghort, we are to discuss the mechanisms that are responsible for keeping populations from the access of each other that in turn leads to,origin of new species. You are aware that local populations generally interbreed among themselves ' with only rare cases of outbreeding. Therefore, the genotypes of different populations of a single species may show differences in some or many loci. The resulting phenotypes could be classified into sub-species based on the differences in parts.
These subspecies because of their proximity to other populations tend to meet the other members with the result that a single generally intermediate and variable population is formed. This would result in the loss of the status of subspecies for the population. However, if a sub-species is isolated over a long period of time and the breeding with its relations is prevented, then by continuing to accumulate the genetic differences it may lose its ability to interbreed with the parental species. Under such circumstances, the sub-species gets elevated to the status of a species, satisfying Mayr's definition of a species. When two groups are geographically separated from each other, it is often difficult to determine whether they do not interbreed any more. In other words, could they be referred to as allopatric species? Once they move into the same territory, fail to interbreed and form intermediates, they may said to have passed the test of sympatry or they could be regarded as sympatric species. In many cases it has been noticed that once the species status has been established, selection tends to promote those characters which act as a deterrent for the formation of hybrids, since usually hybrid progeny have a reduced fitness.