The main conflict In the species discussion may be summarized as follows: the population geneticist is mainly Interested in gene flow and, therefore, has to prefer a biospecies concept. On the other hand, the practicing taxonomist and the palaeontologist are mainly interested in similarity and, hence, have to use a morphological species concept. This conflict is fundamental and cannot easily be resolved. It is often stated that higher levels of classification are even more problematic than the species definition. Using a genetical criterion based on Interspecific hybridization, it is suggested that a systematic category above the genus level may be defined rather objectively: two organisms belong to the same basic type If (I) they are able to hybridize or (ii) they have hybridized with the same third organism. In principle It Is possible to check experimentally by artificial insemination or artificial pollination if two biparental individuals belong to the same basic type. The basic type category thus may prove to be open to empirical validation. Advantages and problems of this basic type criterion are discussed.

A general summary follows on a few basic types of the plant and animal kingdom that have been described. Based on rather limited data it appears to emerge that (i) the basic type criterion can be applied successfully in animal as well as plant taxonomy, (Ii) a clear gap of overall similarity Is found between different basic types, (iii) within basic types a variety of mlcroevolutionary processes may help to understand speciation, and (iv) the distribution of characters across different species of the same basic type may be discussed under the hypothetical assumption of a large hidden variation potential harboured by a genetically complex ancestral population.

It must be emphasized that only 14 basic types have been described to date. This number is too low to provide for a reliable basis of generalization. Therefore, the basic type concept is only suggested to serve as a preliminary working hypothesis.


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