A GENERALIZED TAXON CONCEPT

A generalized taxon concept (GTC) is proposed with a method for reveal ing and ranking difficult taxa at any level in the taxonomic hierarchy . The method is based on cluster quality, defined jointly by the compa ctness of a cluster's contents and its isolation from its informationa l neighbours. The cluster contents are individuals in the case of spec ies and at higher levels, taxa front the rank below. A standard, quali ty threshold value is obtained from clustering accepted taxa in the in formational region. If the quality value of a problem cluster lies at or above the threshold it is accepted as a taxon and ranked with other s at the current level. If it lies below, and is likely to be informat ionally useful, it may be accepted as a sub-taxon such as a subgenus o r subfamily. Provision is made for coarsely scored data. The clusterin g is mainly based on homogeneity, where possible with a rapid, fuzzily cladistic de-weighting of symplesiomorphies by self-graded factors. T he strengths of inter-item reactions such as breeding and DNA-DNA hybr idization may also be used. The method is agglomerative so that it can rapidly reveal polythetic groups which may be riddled with exceptiona l property states caused by long exposure to natural selective forces. All this fits the evolutionary outlook of the GTC, which sees taxa as Fuzzy clusters of populations and lineages sharing much of a genetic memory, moulded by a unique history of evolution and extinction. Pract ical problems of methods based on this and other taxon concepts are br iefly compared. The GTC's approach offers important refinements that c ould be valuable in helping to speed up urgent surveys of biodiversity , especially in the moist tropics. (C) 1997 The Linnean Society of Lon don.