paraphyly

[taxacom]

Dear All, 

Both sides - "paraphyly is real" versus "paraphyly is mistake" may be right but in different aspects. Actually, paraphyly is real only from EXTENSIONAL perspective (contents of taxa), while INTENSIONALLY (character aspect) paraphyly is mistake (meaning of terms: Buck, Hull, 1966). 
    For instance, a genus contains a hundred of species and all of them are descendants of a single ancestor. One can arbitrarily divide the genus into several groups of species, and all of those groups may be meaningless (paraphyly is mistake). However, suppose, one finds such characters that divide a genus into quite natural subgenera. Then, subgenera are paraphyletic extensionally since they do not include all of the original ancestor's descendants, but they are monophyletic intensionally. Thus subgenera are monophyletic and paraphyletic simultaneously, and only in this case paraphyly is real. 
    The subsequent differentiation of subgenera may make them genera or families, so all taxa in the classification may be paraphyletic and monophyletic in their different aspect. So, Mayr is surely one-sided one he says that "paraphyly does not occur in Darwinian classification". Classification itseld has intensional and extensional aspects. Peter Stevens (letter on January 9) designated the extensional aspect as "Darwin's box-in-box classification", while intensional one is a Linnaean hierarchy. It is a single classification but taxa of different levels are included into each other indeed, while characters of different levels are higher or lower to each other. 

Larissa Vasilyeva