Recognizable taxonomic unit

The units one has to work with, the taxa from low to high, may be fixed, about the contents of these concepts a number of questions can be asked. Partly these questions are philosophical (does a species, a genus really exist in nature or only in the human mind ), partly they are quite practical (to which criteria must species, genera etc. answer ). Especially about the species concept there has been a lot of discussion and opinions did and do differ. For many a species is a unit that has to be morphologically recognizable with preferentially two constant differences with other species, for others the species is more or leas identical with the hybridization com-munity. Genet.ical "identity" of the components of a species is an important criterium and may lead some to forbid different chromosome numbers within one species. The battle about "biological and "taxonomical or less clear event linnean)" species started in the forties when herbarium taxonomists were confronted with students of... 

Domain A portion of a polypeptide chain that folds on itself to form a compact unit that remains recognizably distinct within the tertiary structure of the whole protein. Large globular proteins often consist of several domains, which are connected to each other by stretches of relatively extended polypeptide. A new taxonomic category above the kingdom level, consisting of the archaea, bacteria, and eukarya. 

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