Numerical Taxonomy

Numerical Taxonomy Edited by

Joseph Felsenstein

Springer-Verlag Berlin Heidelberg New York Tokyo 1983

TAXONOMIC CONGRUENCE: A

B~IEF DISCUSSIO~

Jorge V. Cr isci División Plantas Vasculares Museo de La Plata, 1900 La Plata, ~rgentina Taxonomic congruence is the degree to which classifications of the same organisms postulate the same groupings. Identical classifications are said to be perfectly congruent. Tllis paper attemps to provide a theoretical background for a discus sion of the concept of taxonomic congruencc. To this end, it ~resents: the types of experimental designs for the study of taxonomic congruence, a possíble explanation for the results and the systemat ic implications of the problem . Experimental-test designs. Any study on taxonomic congruence exceeds the observational approach and it also needs an experimental component By experimental, I mean an analytical approach where sorne variables are control led . It is, therefore, desirable to clearly distinguish the severa! types of experimental-test design for the study of taxonomic congruence. Though the three basi c types I will describe undoubtedly do not exhaust the matter, they are the most frequently exemplified in the literature on the subject. The first type of ex'perimental-test design involves variance of techniques: we analyze the congruence between classifications based on the same set of characters but obtained by different classi ficatory techniques. T~e second type involves variance of source of characters: we analyze t~e congruence bctween classifications based on differen~ sets of c~aracters but obtained by the same classificatory technique. The third type involves variance of technique and source of characters and is a combinati on of the first two types. It consists of an analysis of. the congruence between classifications bas~d on different sets of characters but obtainetl by the same classificatory technique. Then the process is repeated using a different classificatory tech· nique . Finally it is considered whether the congruence obtained with the first technique is higher or lower than the one obtained with the second technique. The second and third type are the most interesting from the systematic viewpoint, therefore, t~e emphasis of this paper will be on them. I refer the reader to the numerous studies on congruence, notably ~neat~ and Sokal, 1973, where cong ruence is appraised by measuring the NATO AS! Series, Vol. G 1 Nwncrk:al Tuonomy. Edi1cd by J. Fclscnstcin C Spnnaer.Vcrlq lkrlin Hcidclbc:r¡ l983

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93 agree~ent betw~en

the similarity values of two similarity matrices (usin.g a correlation coeffi · ent) and ••ickevich, 1978, where congruence is appraised by rueasuring the agreement between groupings of two taxonomic structures (using a consensus index). The explanation. The facts indicate that perfect congruence or complete incongruence has never been obtained. The values vary according to t~e group under study, the characters used, the methodology applied, and the mode of appraisal of congruence. ~ccording to theoretical expectations, if we follow the right methodological steps we should obtain perfect congruence. This expectation could be based on one of two very dissimilar hypot~eses. One has been proposed by pheneticists and the other by ciadists. The first one is the hypothesis of nonspecificity as proposed by Sokal and Sneath (1963), which states that there are ne;> distinct large classes of genes that affect exclusively one class of characters such as Morp~ological, physiological or ethological ones, or that affect special r egions of the organism such as head, skeleton , or leaves. Thus, separate sets of loci controlling, for example, morphological characters independently of chcmical ones .are not expected to exist. Therefore, a broad sampling of the genotype could be achieved through a relatively restricted sampling of the phenotype. If this were true , data taken from different sets of characters should yield like classifications. The second hypothesis comes from the cladists. According to Hennig (1968) two sets of characters mus t produce identical classifications since a ll the characters of an organísrn share the same genealogy, but this perfect congruence will be achi eved only through a cladístic technique, which by definition bases its classification on the cladogram of the organisms . Two sets of characte r s of the same organisr.is shoul d yield · fully congruent c ladograms since there is only one true c l adogram for a given set of organisms regardless of the set of characters on whic~ it is based. The two proposed hypotheses seem to be well elaborated, although in sorne way they contradict each othe1·. But let us conside r the main fac t , which is that perfect congruence was never obtained either with many characters or using a cladistic technique. The reasons for the lack of perfect congruence are twofold sincc there are two kinds of c auses for it: biological and methodological ones. The biological causes affect ruai nly the ~henetic approach (I will

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also.the evolutionary school of systematics among the approaches affected by these causes) and are given by the degree of causal interdependence between the two sets of char acters used. This interdependence decreases . or, in other uords t he i ncongruence increases, by the following interacting phenomena: inc~ude

a- the different activity of the genes in different cells of the same organism. For example, an epidermal cell has the same genotype as a hepatic cell but the manifestation of the genotype i s diffe rent i n each of thom. b- phenotypic p l asticity, that is the ability of a genotype to produce adaptively variant phenotypes by means of development or physiologi.cal modifications. e- somatic mutations, that are r esponsiblc fo r the fact that in sorne organisms there are slightly different genotypes i n the same individual. d- the different pressures of natural selection (d i ffering in nature, intensity, and direction) over different stages or parts of the same individual, especially when thos e stages or parts are not synchronic. e- mosaic evolution, it has been sho~m that the rates of evolution of various kinds of characters are often di ffe rent. Some may r ush far ahead while others stagnate. Ideally, a cladistic tcch.nique should not be aff ected by the biological causes of incongruence, as was stated by Hennig's hypothesis. But as we can s ee there is not a single case of per fe ct congruence for two classifications cons t ruc ted by a cladi s t i c technique. This i s so because a cladogram constructed from a given set of characte rs is only an estimate of the t rue cladogr am . As such it is subject to errors due to what I will call methodological causes. Thes~ methodological causes are the technical steps that could distort, in the case of a cl adistic technique, the true cladistic relationships, and in t he case of a phenet i c technique , the interdependence of characters. These steps include: sampling of characters; detamination of homologies; codification of characters; determinati on of polarity of characters {thi s is a step for cladis tic studies only); algorithm used to const ruc t the es t i mated cladogram or taxonomic structure; the mode of appraisal of congruence. These are sorne of the many technical s t eps that could produce a distortion of the "real congruence". Systematic implications. The core of these implications is expressed

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in thre~ questions that I will a t temp to answer. Ifwe never find perfect congruence, is the~e a kind of character that shows superiority over the othe rs as indicator of taxonomic relationships? Before one addresses the taxonomic value of different kinds of characters, one ought to be very clear about what is meant by taxonomic relationship, since there are at least three kinds of r e lationship between two taxa: a- patristic relationships: these are the similarities due to common ancestry between the taxa, in other words, homologous s imila ri ty. b- cladistic relationships: these reflect the relative recency of common ancestry between the taxa. e- phenetic relationships: these are the similarities based on a set of phenotypic characters of the taxa, these similarities include patristic similarity and non- homologous similarity. The different classificatory schools give different value to these r elationships. For the cladist, taxonomic relationships ~ean cladistic relationships, for the evolut ionary systematist they mean cladistic and patris·tic relationships, and for the pheneticist they mean phenetic relationships. All the characters of the same taxon share their cladistic relationships but not necessarily their patristic and phenetic relationships. Thus, in a pure ly cladistic approach all kinds of characters have the same value. On the contrary, in an evolutionary or in a phenetic approach we are exposed to the possibility of incongruence due to patristic or phenetic relationships. Therefore, in an evolutionary or in a phenetic approach, a kind of character will be important if it is causally connected with many other kinds and by consequence shares with many others its patristic and phenetic relationships. At this time, it is not possible to estab1ish, for all groups, which kind of character fills this requirement. At any rate, it is clear that any evolutionary or phenetic classifi cat ion reflects only the evolution that has ocurred in certain sets of characters, and additional characters from previously little studied stages or parts of the body or classes of characters can be expected to alter the classification . In terms of congruence, what is the general perfomance of the different methodologies? A cladist ic technique would produce congruent classifications as far as the t~chnique is able to achieve the true cladistic history . The current cladistic techniques seem to be sensitive to incongruence

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due to methodological probl ems. ~volut ionary cl assifi ca t ion t echniques and phenet ic techniques would be very sens itive to i ncongruence due to sorne bi ological phenomena (for example, mosaic evolution) . The current evolutionary c lassi fication techniques and phenetic techniques seem to have also sensitivity for reeth.o dolog i cal prob lems . I s congruence u fa i thful cri terion of goodne ss of a methodology? Congruence is a good criterion for a c ladistic approach, but a doub tful one for an evolutionary or phenetic approach because incongruence could be the result of the biological phenomena that these two classi fic atory schools try to take into account in the construction of their classificati ons . Literature cited HENNIG, N. 1968 . Elementos de una si s tem~tica filogen~tica. Buenos Aires : Editorial Un iversi taria de Buenos Aires. 353 pp. 'fICXEVICH, H.F . 1978. Taxonomic Congruence. Syst.:lool., 27:143 - 158 . SNEATH, P.H .A. and R.R. SOKAL. 1973. Numerical Taxonomy; the pri ncip les and practice of numerical cl assification . San Francisco: W. H. Freeman and Co., XV + 573 pp. SOKAL, ll.R. and P.H . SNEATH. 1963. Principles of Numerical Taxonomy . San Franc i sco: W.H. Freeman and Co. 359 pp.