Der Artbegriff in Zoologie,Phylogenetik und Biostratigraphie

The cladistic species concept, as advocated recently by Bonde (1977), Willmann (1985), and others, operates with a “time-bio-species” whose temporal boundaries are defined by successive speciation events. Phyletic lineages without speciations would then correspond to one single species, even if far reaching morphologic changes occur. On the other hand, speciations would be obligatory species boundaries, even if one daughter species is perfectly identical with the parent species. SIMPSON (1951) has already criticized such a procedure as both undesirable and impractical. But objections of this kind have always been ruled out by cladists pretending that purity of theory could not be sacrificed for the sake of mere practical difficulties. A critical reappraisal of the theoretical foundations of the time-bio-species concept shows, however, that the main argument is logically untenable: Speciation means appearance of reproductive isolation within a population. From this “emoment” on there are two biospecies where formerly there was only one. Nothing more than this rather meagre statement results from the biospecies concept, which - as everybody agrees - applies only to synchronous populations and does not allow to define temporal species boundaries. It is therefore impossible to say which one of the two daughter species is new, or if perhaps both of them are new, i. e. whether the parent species survives speciation or not. A decision would only be possible on the base of additional operative criteria allowing to define species boundaries between allochronous populations. Criteria of this kind have never been proposed by cladists. The cladistic species concept is indeed based on an arbitrary decision and the time-bio-species is a purely conventional unit. It should only be used if there are compelling practical and methodological reasons. But, as to the practical aspect, SIMPSON?S (1951) objections are still entirely valid. We may even add that speciation is rather a slow process. Clear-cut boundaries at speciation events exist only in cladograms. One theoretical argument against the use of morphologically defined chronospecies was always that b i ospecies cannot be defined by morphology. This is true, but has nothing to do with the problem of temporal species boundaries. The basic unit of Recent taxonomy is, of course, the biospecies, but this is no longer true for cladistic analysis, even if it starts from Recent faunas. Cladistic analysis depends on synapomorphies, i. e. on morphological characters in a broad sense. Biospecies can only be used as starting point, if they are morphologically distinguishable. But mostly analysis starts from monophyletic groups of several biospecies where monophyly is inferred from morphology. Cladistic analysis does not reconstruct the succession of speciations but of morphogenetic events. This is not a simple technical problem which may be neglected in “pure” theory, but one of methodology. In scientific research the nature of the respective basic unit should agree with the method employed. It makes no sense to work with a species concept that cannot be tested empirically. There has been much debate as to whether or not different species concepts should be used in paleontology and in neontology. But the question is put in the wrong way. As long as only synchronous populations are studied, the biospecies concept should also be used in paleontology. Fossil ostracodes and foraminifera show that dimorphic species can be recognized with the help of Recent homologues. But in phylogenetics we are forced to use an arbitrarily defined conventional basic unit of taxonomy having a temporal dimension. Only the morphologically defined chronospecies allows to describe evolutionary change adequately. On this base biostratigraphy has produced many valuable results, thus proving the efficiency of the concept.