The Species Concept in Pteridophyta with Special Reference to Isoetes

Abstract

The definition, description, and identification of Isoetes species has always been difficult. This is primarily the result of an overall morphological simplicity and extreme phenotypic plasticity that combine to reduce the number of stable and therefore diagnostic characters. Such aspects of the biology of Isoetes have, historically, resulted in rather diverse specific treatments and circumscriptions. Recent studies in the genus have tended to emphasize previously overlooked characters and to apply both modern experimental techniques and the principles of population genetics in order to resolve many of these inconsistencies. Investigations utilizing cytological data, germination and hybridization studies, and protein electrophoresis have shown that over the range of a species its boundaries vary due to clinal variation, non-patterned geographical variation, interspecific hybridization, and the production of fertile allopolyploids. Such approaches have greatly increased our understanding of the dynamics of this genus. At the outset, it should be stated that it is not the objective of this paper to discuss the theoretical implications, or the limitations and benefits of the various species concepts that are currently in existence. Such discussions are numerous and can be found throughout the pages of many of the major systematic journals such as Systematic Botany, Systematic Zoology and Biology and Philosophy. Rather, this paper is designed to illustrate both the current and the historical application of the various species concepts in pteridophytes with particular reference to Isoetes. It is hoped that the reader will come away with an appreciation of the progressive nature involved in the elucidation of species and for the (practical) necessity of applying multiple species concepts during any given systematic inquiry (see also Zander, 1985). To illustrate such processes the following examples will be presented: the I. storkii aggregate of Costa Rica; the I. melanopoda complex of the southeastern United States; and the Isoetes species of northeastern North America. As a prelude to these examples, a brief review of the species concept and a discussion of the role of herbarium material will be presented. Species concepts.-For the purposes of this paper, the species concepts of consequence are: the typological species concept (TSC), the morphological species concept (MSC) and the biological species concept (BSC). The TSC is associated with all poorly known groups. Typically it is associated with rare or at least rarely collected species and is almost always employed when only a single collection is known for the species; i.e., simply, the species is circumscribed upon the only materials available-the type. The MSC, the traditional concept of