The classification of the Megascolecoid earthworms: an investigation of Oligochaete systematics by computer techniques

Abstract

SUMMARY ‘Classica’ classifications of Megascolecoid earthworms are reviewed and the assumptions on which they were based are described. Formerly morphological series were regarded as phylogenetic series and member species were believed to be ancestors and descendants living together contemporaneously. ‘Family‐tree’ type classifications were constructed on these sequences and the systems were regarded as ‘natural’, i.e. phylogenetic, classifications. More recently Omodeo (1958), Lee (1959) and Gates (1959) separately rejected the concept of classifying new taxa by definition, that is, placing them in the appropriate places in a family‐tree according to the weighting given to single characters. Instead each proposed a new classification for the Megascolecoid earthworms based on overall similarities. The three classifications are incompatible. Taxonometric methods are described by which a less subjective approach to the problem of Megascolecoid classification is possible by taking into account a relatively large number of attributes. Thirty species, representing twenty‐nine genera of Megascolecoid earthworms were examined. Forty‐three attributes, eighteen quantitative and twenty‐five qualitative, were scored and the assessments computed to produce a matrix of similarity coefficients. The quantitatives were computed to permit comparisons to be made over the total range of the character states assessed (similar to the requirements of Cain & Harrison, 1958), while the qualitatives were treated on a match or mismatch basis. The matrix of similarity coefficients was examined by two methods. Firstly, by clustering by single linkage, ‘Sneath's method’, to produce a dendrogram and secondly, by transforming the original matrix by ‘Gower's formula’ and calculating the latent roots and vectors of which the first and the second were employed as co‐ordinates in a vector diagram. Two assemblages of genera could be detected in the dendrogram but they did not cluster in the vector diagram. Instead they formed a scatter with genera of one assemblage occupying one‐half of the diagram and genera of the other assemblage one quadrant of the remaining half. The fourth quadrant contained a third assemblage comprising genera which, due to their low similarity coefficients, could not be recognized in the dendrogram as having any special mutual similarity. The pattern of arrangement of the genera was found to coincide largely with the classification previously proposed by Gates and to disagree with those proposed by both Omodeo and Lee. It is concluded that: (a) Problems of oligochaete systematics are suited for study by computer techniques, (b) The results of newer taxonometric procedures need not be a variance with the results obtained by more traditional methods, (c) In this particular investigation, the classification proposed for the Megascolecoid earthworms by Gates (1959), has been confirmed. (The ‘key’ or ‘spot’ characters by which the component family groups may be recognized are firstly, the structure of the prostatic glands and ...

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