Taxon Concepts Research Articles

A generalized taxon concept

A generalized taxon concept (GTC) is proposed with a method for revealing and ranking difficult taxa at any level in the taxonomic hierarchy. The method is based on cluster quality, defined jointly by the compactness of a cluster's contents and its isolation from its informational neighbours. The cluster contents are individuals in the case of species and at higher levels, taxa from the rank below. A standard, quality threshold value is obtained from clustering accepted taxa in the informational region. If the quality value of a problem cluster lies at or above the threshold it is accepted as a taxon and ranked with others at the current level. If it lies below, and is likely to be informationally useful, it may be accepted as a sub-taxon such as a subgenus or subfamily. Provision is made for coarsely scored data. The clustering is mainly based on homogeneity, where possible with a rapid, fuzzily cladistic de-weighting of symplesiomorphies by self-graded factors. The strengths of inter-item reactions such as breeding and DNA-DNA hybridization may also be used. The method is agglomerative so that it can rapidly reveal polythetic groups which may be riddled with exceptional property states caused by long exposure to natural selective forces. All this fits the evolutionary outlook of the GTC, which sees taxa as fuzzy clusters of populations and lineages sharing much of a genetic memory, moulded by a unique history of evolution and extinction. Practical problems of methods based on this and other taxon concepts are briefly compared. The GTC's approach offers important refinements that could be valuable in helping to speed up urgent surveys of biodiversity, especially in the moist tropics.

Cultivar classification ofPhiladelphusL. (Hydrangeaceae)

The cultivars of the genus Philadelphus are up till now usually classified in five interspecific hybrids, as proposed by A. Rehder some 70 years ago. This system is not satisfactory any more. Instead a new, more stable classification of four cultivar groups is proposed here. This new classification shows influences of Rehder's system, but differs quite a lot in its principles. The new classification is based on practical application (culton concept), while the old classification is based on genetic relationships (taxon concept). The definitions of the cultivar groups are simple and clear, cultivars can be classified better and this system is easy to work with, even for non-specialists. The proposed cultivar groups are: Lemoinei group, Purpureo-maculatus Group, Virginalis Group and Burfordensis Group.

An annotated history of the principles of cultivated plant classification

Important episodes in the history of the classification of cultivated plants are highlighted and commented upon in the light of recent developments in the systematics of cultivated plants. It is shown that throughout history, a division between classifying cultivated plants and plants as found in nature has existed in the minds of most taxonomists, but the slow development of systematic thought in cultivated plant taxonomy has slowed down progress in this field and left ample opportunity for too strong an influence of the taxon concept. As a result, the nomenclature of cultivated plants has always leaned heavily on the International Code of Botanical Nomenclature.

Bases filosóficas de los análisis cladísticos para la investigación taxonómica

Los cambios significativos en la teoría sistemática originados por el uso amplio de métodos cladísticos no han llegado a la mayoría de los taxónomos de habla hispana. Esta contribución revisa tres aspectos de la relación entre la filosofía de la ciencia y la sistemática, basada en el examen de la literatura relevante: I) La ontología de los grupos naturales. Aquí se señala que la filosofía fenética está asociada con la visión ontológica que considera los grupos como “clases”, mientras que la de la cladística es compatible con el concepto de taxa como “individuos”. II) Bases para la clasificación. El debate acerca de estas bases se centra en la cuestión si todas las similitudes en sí mismas (enfoque fenético) o las homologías inferidas a partir de ciertas semejanzas especiales (enfoques evolucionista y cladístico) pueden usarse para construir una clasificación. III) La prueba de hipótesis. Las discusiones conciernen directamente a la índole científica de cada una de las tres principales escuelas de la sistemática. La doctrina fenética que pretende ser “neutral respecto a teorías” y la evolucionista, sin un sistema crítico para evaluar hipótesis, quedan eliminadas como “ciencia” bajo un punto de vista popperiano. La filosofía cladística es un programa científico robusto ya que incorpora un sistema explícito para la refutación de proposiciones a todos los niveles de análisis de caracteres y de cladogramas. En conclusión, se argumenta que la selección de una metodología para la clasificación no debe basarse en ventajas operativas, sino que debe ser una consecuencia de la elección previa de una posición ontológica.

Jurassic Cretaceous boundary palynology in Europe

A brief discussion is given of a sequence of five major problems involved in the making of a palynological contribution to the calibration of a boundary-stratotype section for a period boundary in the Global Stratigraphic Scale. It has been assumed that the Jurassic-Cretaceous boundary description will be in Europe in as Iowa palaeolatitude as possible and that it will not be taken to coincide with any described event of biologic evolution. The most appropriate and useful palynomorphs for close stratigraphic study at this level appear to be those of the Cicatricosisporites and Classopollis groups; Trilobosporites, Aequitriradites, Januasporites, and others are for various reasons considered less useful. A revised succession of Cicatricosisporites taxa is provided through the probable time interval required, and reconciliation of published taxon concepts and names is attempted.

Dental polymorphism and systematics in Saccodon, a neotropical genus of freshwater fishes (Parodontidae, Characoidei)

Population samples of algae‐grazing, mountain stream characins of the genus Saccodon of Panama and northwestern South America are reported with up to four sharply differentiated dental morphs. The morphs differ strikingly in the form and numbers of functional teeth and of preformed replacement teeth. Recognition of the polymorphic nature of these variations necessitates new concepts of taxa at species and generic levels. Two species with similar polymorphism are involved, Saccodon dariensis from Panamá and Colombia, and Saccodon wagneri from the Guayas basin on the Pacific coast of Ecuador. A total of rive morphs have been discovered, three of which are homologous in the two species.