Species Concepts in Pteridophytes: The Treatment and Definition of Agamosporous Species

Abstract

Pteridophyte species have generally been defined on the basis of relatively major morphological differences between sets of populations. At least implicitly, these morphological discontinuities are taken to indicate lack of gene flow and are thought to reflect the genetic discontinuities that make one species distinct from another. When functionally diploid, outcrossing fern species are circumscribed in this way, morphologically recognizable taxonomic species may also be good biological species. Unfortunately, in the case of agamosporous ferns (formerly designated apogamous or apomictic), the criteria used to define a biological species do not apply. Unlike the members of a biological species, agamosporous individuals cannot interbreed. However, they can cross with related sexually reproducing taxa to generate reproductively competent offspring, which biological species are not supposed to do. Thus the reproductive behavior of agamosporous ferns precludes application of a strict biological species concept, and the treatment and definition of agamosporous species is somewhat problematical. This issue merits consideration because agamosporous taxa constitute about 10% of all fern species for which the type of reproduction is known (Walker, 1984 p. 125). In this paper, we review the salient features of the typical life cycle of agamosporous pteridophytes. We then discuss the origins of several agamosporous fern taxa and indicate how we would treat them taxonomically. We conclude with a species concept accommodating both sexual and agamosporous taxa.