TAXONOMIC CONGRUENCE

Summary Taxonomic congruence is the degree to which classifications of the same organisms contain the same groupings. To aid in understanding such a concept, the types of experimental design for its study, a list of the modes of appraisal of it, a possible explanation for the results and the systematic implications of the problem, are presented. According to theoretical expectations (hypotheses of nonspecificity and Hennig's) if we follow the right methodological steps we should obtain perfect congruence. Nevertheless, the facts indicate that perfect congruence has rarely been obtained. There are two kinds of causes for the lack of perfect congruence: biological and methodological. The biological causes affect the phenetic and evolutionary approaches and are given by the degree of causal interdependence between the two sets of characters used. This interdependence decreases (incongruence increases) by several interacting phenomena (e.g. mosaic evolution). The methodological causes affect all kinds of approaches and are the technical steps that distort the appraisal of congruence. All the characters of the same taxon share their cladistic relationships but not necessarily their patristic and phenetic relationships. Thus, in a purely cladistic approach all kinds of characters have the same value. In an evolutionary or in a phenetic approach, a kind of character will be important if it shares with many others its patristic and phenetic relationships. Congruence is a good criterion for a cladistic approach, but a doubtful one for an evolutionary or phenetic approach, since incongruence could be the result of the biological phenomena that these two classificatory schools try to take into account in the construction of their classifications.