Progress and pitfalls in systematics: cladistics, DNA and morphology

The ‘natural system’ of organisms reflects their phylogenetic relationship. It is the result of an historical process and has to be inferred from the available evidence. In the morphological phenotype, historical traces are intermeshed with functional adaptations. Overall similarity, even if quantified, can be a misleading indicator of relatedness. Cladistics uses shared derived character states (synapomorphies) to identify groups of common ancestry. Synapomorphies are mostly inferred from their taxonomic context. If apparently equally valid characters suggest mutually exclusive groups, parsimony is invoked: a phylogenetic reconstruction requiring a minimum of evolutionary steps to describe the present character distribution is accepted as the most likely one. Cladistics sets very stringent requirements for informative characters, and a rigorous analysis of morphology is likely to yield very few reliable characters. The direct analysis of DNA sequences provides theoretically the optimal evidence for phylogenetic reconstruction. In practice, very little of this information is readily accessible. Occasionally major sequence rearrangements can be unequivocal synapomorphies. Many phylogenetic problems can be solved by comparative sequencing of an appropriate segment of DNA. Comparative sequencing of the chloroplast gene rhcL has become the model for such studies. Molecular data have confirmed much traditional taxonomy, elucidated doubtful cases and corrected misinterpretations. Molecular data also have clearly shown the limits of the cladistic approach by revealing both known and previously unsuspected modes of reticulate evolution. Molecular approaches separate phylogenetic reconstruction from biological evaluation and will never replace morphological analysis in Systematics. However, molecular methods also facilitate the direct investigation of morphological evolution by revealing the genetic basis of morphogenesis in model systems or by permitting the genetic analysis of diagnostic character changes by genetic mapping.