Abstract
The paper outlines the current discussion on the species problem : what actually is a species and how do we recognize such an entity in nature? Six of the major species notions are briefly explained. A distinction can be made between species concepts based on the process of interbreeding and concepts that focus on common descent. Application of these different types of species concept may give incompatible results. It is discussed to what extent we can be certain that the species delimited represent objective realities in nature and are independent of the discovery procedure applied. Further, it is argued that because of the existence of various evolutionary processes, nature can be divided into several natural entities, each representing a species of some sort. The choice of a particular species concept is determined by that aspect or process in nature that forms the focus of one's research agenda. This plea for pluralism should preferably not affect taxonomic procedure ; named species taxa should be based on only one, taxic species concept.
