Taxon:3 



morphological characters. Ultimately, it may be decided that these 

 forms are simply geographical variants of the same species and that, 

 although the majority of each form are recognizable on a morphological 

 basis, they should be considered to be subspecies of a single species. 

 This, then, would fit into the .category of the polytypic species which 

 consists of a number of geographically isolated populations, exhibiting 

 sufficient differences to be called subspecies. 



On the above basis, you can \anderstand my feeling that one has to have 

 a certain amount of courage to describe new species. This is particu- 

 larly so if the worker recognized that, in addition to the possibility 

 of re-describing as new a previously named species, he also may be 

 describing merely subspecies and not valid species. You will recall 

 that Sher and I synonymized some species in Pratylenchus j but if someone 

 presented evidence that Pratylenchus coffeae , P. musicola , and P. mahogani 

 were indeed valid species, I would be the first to agree, providing, of 

 course, the evidence was conclusive. Similarly, if it could be shown 

 that Trichodorus christiei was clearly a synonym of T. minor , this would 

 be entirely acceptable because it would make the taxonoiTQr of the group 

 somewhat easier to handle. Taxonomy should not be solely concerned with 

 descriptions of new species but should be directed toward providing a 

 better understanding of the present tsxonomic categories. 



In addition to the polytypic species concept, there are two additional 

 concepts that are useful to the taxonomist. These are designated by 

 the terms "sympatric" and "allopatric." These words are used to describe 

 two types of species distribution on a geographical basis. They are use- 

 ful to the taxonomist because the terms have application in all work 

 concerning the species. 



Sympatric species - If you extract two species of Tylenchorhynchus from 



a single soil sample, they would be referred to as 

 sympatric species, meaning that they occur together in the same geo- 

 graphical area. In such a case, we can be reasonably certain that we 

 are dealing with two species, since it is very unlikely that two sub- 

 species of the same species could maintain their identity in such a 

 situation. One may assume from the fact that two species occur together 

 that the differences between them are indeed valid and the taxonomist 

 would not hesitate to indicate they were distinct species. 



Allopatric Species - This type of species distribution presents an 



entirely different problem. Here we are dealing 

 with a number of related species that are geographically isolated from 

 each other. They appear to be different on the basis of morphology, 

 but we know nothing about their possible behavior if they were brought 

 together in the same geographical area. In the case of the sympatric 

 species, we can be fairly certain that we are not dealing with sub- 

 species, because they maintain their identity when they are not geo- 

 graphically isolated. In the case of allopatric distribution, we know 

 nothing of the reproductive potentials of the various speciesj their 

 separation is spacial, but we cannot be certain that they would not 

 interbreed if brought together. This type of species distribution 



