Incongruence between cladistic and taxonomic systems
Verne Grant
Section of Integrative Biology, University of Texas, Austin, Texas 78712 USA
Received for publication 21 November 2002. Accepted for publication 11 April 2003.
ABSTRACT
Cladistic and taxonomic treatments of the same plant group usually exhibit a mixture of congruences and incongruences. The question arises in the case of the incongruences as to which version is right and which is wrong. Many cladists believe that cladistics is a superior approach and gives the best results. There are several conceptual and methodological differences between cladistics and taxonomy that cause incongruence. One important conceptual difference is the use of different criteria for grouping: order of branching vs. similarity and difference (clades vs. taxa). Another is the policy regarding paraphyletic groups: to ban them in cladistics but ignore the ban in taxonomy. These two differences automatically lead to some incongruences. One approach is not right and the other wrong; each is operating by its own standards. However, when cladists apply the paraphyly rule to a taxonomic system and conclude that it needs revision to eliminate paraphyly, as cladists often do, they are judging the taxonomic system by a wrong standard. Several differences between the two schools in the use and handling of characters can also cause incongruence. First consider phenetic characters. Taxonomy uses a very wide range of these, whereas phenetic cladistics sets restrictions on the selection of characters, which deprive it of potentially useful evidence. Taxonomic systems generally rest on a broader empirical foundation than phenetic cladistic systems. Next, consider molecular cladistics, which is the leader in the use of DNA evidence. Two sources of incongruence between molecular cladistics and taxonomic systems can come into play here. First, the molecular evidence used in cladistics comes mainly from cytoplasmic organelles, whereas taxonomic systems are based on characters that are determined mainly by the chromosomal genome. More generally, the database in a molecular cladogram is, in itself, too narrow to serve as a foundation for an organismic classification. In cases of incongruence, the molecular evidence can be a reliable indicator of taxonomic relationships sometimes, misleading other times, and may afford no clear basis for a systematic decision. In this situation, it is helpful, indeed necessary, to integrate the molecular evidence with the phenetic evidence and bring more characters to bear on the question.
Key words: cladistics molecular systematics paraphyly phylogenetics systematics taxonomy
FREE PDF GRATIS: American Journal of Botany
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NOTA DESTE BLOGGER:
Apesar de ser um artigo antigo, ele continua veraz na sua abordagem. Razão? A vida é dividida em eucariontes e procariontes, e não há transicionais entre eles. Existem eucariotos unicelulares, e há seis implementações de multicelularidade eucariótica. Os eucariontes podem ser mais subdivididos, como na taxonomia de Lineu.
NOTA DESTE BLOGGER:
Apesar de ser um artigo antigo, ele continua veraz na sua abordagem. Razão? A vida é dividida em eucariontes e procariontes, e não há transicionais entre eles. Existem eucariotos unicelulares, e há seis implementações de multicelularidade eucariótica. Os eucariontes podem ser mais subdivididos, como na taxonomia de Lineu.
(1) "Cladistics is based on inferred phylogenies, which makes for an uncertain foundation. Phylogenies of groups above the species level are, with rare exceptions, unverifiable hypotheses. Taxonomic systems are based on observable characters and do not rest on phylogenetic hypotheses." p. 1268
