Since the time of John Ray, whose own attempt at a definition of species we discussed in Chapter 2, there has been no universally agreed definition of ‘species’; different definitions have been devised by biologists working in different specialist fields. Thus, the word ‘species’ has different meanings for different biologists. Here, we examine five influential definitions that focus on different aspects of pattern and process in evolution. (For thorough reviews of species concepts, see Stuessy, 2009; Wilkins, 2009).

The morphological species concept

Historically, the naming, description and classification of species have been based largely upon morphological details of herbarium specimens, and to a lesser extent living material collected from wild or cultivated sources. This is supplemented by geographical and sometimes ecological information. The aim of the taxonomist is to provide a convenient general-purpose classification of the material, a classification that will serve the needs of biologists in diverse fields.

It is quite obvious that in order to communicate experimental findings to others, by word of mouth, in the literature and through databases, the experimentalist, like any other botanist, must be able to name plants unambiguously. To this end, an International Code of Botanical Nomenclature has been agreed. The development of this Code has a fascinating history (Smith, 1957). By 1900, four rival codes of practice were employed in different herbaria. Discussions of the problem occupied taxonomic sessions at International Botanical Congresses in Vienna (1905), Cambridge (1930) and Amsterdam (1935), and the successive Congresses, now at approximately 5-yearly intervals, are the occasion for continued revision of the Code. The international agreements leading to a unified Code must be recognised as a major achievement.

One meaning of the word ‘species’ is now clarified. We may say that species are convenient classificatory units defined by trained biologists using all the information available. Clearly there is a subjective element in their work, and we must therefore face the fact that there will sometimes be disagreements between taxonomists about the delimitation of particular species, but there is a very large measure of agreement, for all except ‘critical groups’, in regions where the flora has been studied for many years. In the taxonomic process, a type specimen is designated and a diagnosis provided that specifies the important distinguishing characteristics of the ‘new’ species from others of the same group (Stace, 1980; Stuessy, 1990, 2009).