Organisms form natural groups of more or less similar individuals. These groups of individuals can be differentiated on the basis of different phenotype traits and can simultaneously be ordered in a natural hierarchically organized system, i.e. taxonomic system, in which lower-order groups can be gradually associated, on the basis of common traits, to form higher-order groups (taxons). While the differentiation of higher-order taxons is, to a substantial degree, a matter of convention or explicit agreements amongst taxonomists, i.e. professionals concerned with this subject area, most biologists now consider that basic taxonomic units exist in the context of the entire hierarchical system at a certain, very low level of hierarchical ordering, where these units are, in some way, natural, i.e. they exist in nature independently of man and his conventions. Current opinion has it that these units are species. Within species, it is, of course, possible to divide individuals into subgroups of more or less similar individuals – subspecies, geographic races, ecological forms. However, delimitation of these intraspecific taxons tends to be a matter of convention and there basically exist continuous transitions to individual variability.
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