In l942 E. Mayr published the classic book, Systematics and the Origin of Species . It is the problem of those who describe new species, the taxonomists, to decide if the differences in populations due to geographic or ecological isolation are sufficient to have created a new species. The act of describing the new species, therefore, indicates that the taxonomist has reached the conclusion that they are sufficient.
Many species have specific differences from all others.
However, with closely related species, extremely similar in appearance, the taxonomist ideally will have to inspect the original material from which each related species was described before reaching a final decision. Furthermore, the taxonomist should have access to series of specimens of the species that have already been described to make sure that this new species does not fit into any of the variations of named species. Along this line, it is also preferable to have more than one specimen of the new species to be sure that one is not dealing only with a unique aberration. A good description of a new species will establish constant and clear differences between it and all other closely related species. When two or more populations of a species are geographically isolated and show consistent differences between them, they are given subspecific names.