From time to time a mutation occurs in a segment of a polytypic species, such as an ecotype or a subspecies, that limits its ability to exchange genes with other segments of the parent species. The population that develops from the mutant is reproductively isolated and is commonly recognised as a separate species population, distinct from the original species.
Before further mutations completely restricts all gene exchange between the parent population and the new one, there may be for one reason or another some further crossing between them which is known as hybridisation.
Hybrids frequently fail because of the weakness of their phenotypes or through failure to reproduce normally. In plants the doubling of the chromosome content or polyploidy of hybrid individuals sometimes overcomes this sterility.
Species populations that occupy different geographical areas are said to be allopatric, where as those that occupy the same area are said to be sympatric. These terms are now usually applied only to related populations; allopatric populations are defined as segments of an original population that have become separated by spatial isolation (Boughey, 1968).
Lastly, ecospecies is an unit of classification which contains one or more ecotypes, which although interfertile but do not cross or, at least do not produce viable offspring, if crossed with ecotypes of other ecospecies.