Primary Divergence and Species Concepts in Ferns

Abstract

Primary divergence may be defined as differentiation among populations of homoploid organisms caused by natural selection; it often results in the formation of new species or subspecies. This definition excludes the relatively instantaneous processes of polyploidy and hybrid species formation. Monographers of pteridophyte genera name and describe species and have perforce been the individuals forced to deal with problems of primary divergence and the delimitation of species. Since scant literature exists addressing species concepts for pteridophytes, we have contrasted specific examples of usage from modern systematic treatments by various workers. We examined a sample of 50 monographs completed during the last 50 years by 36 authors (Table 1). Only 12 (24%) of these monographs contained any explicit discussion of the criteria for the species concepts and/or infraspecific categories employed, so that our analysis of these topics is based largely on inference. We perceive that three general types of species concepts have been used singly or in concert by pteridologists: the biological, morphological, and (for lack of a better term) concepts. The biological concept is used here in the somewhat restricted sense of genetic intersterility between species and states that if two taxa can cross to produce fertile offspring, then they are the same species. This concept has rarely been used in fern systematics because data on cross-fertilization is difficult to obtain. Hennipman and Roos (1982), however, used the ability of gametophytes to cross as evidence for reducing four taxa accepted by Hoshizaki (1972) as species to subspecies or varieties of Platycerium bifurcatum, even though the subsumed taxa were geographically and/or ecologically distinct. In contrast, the morphological concept is the recognition of discrete taxa on the basis of breaks of form, with no knowledge of the ability to hybridize. Of course, what constitutes such morphological discontinuities can be quite subjective and contentious. This concept is the tacit species concept of most fern monographers who have worked from herbarium specimens alone. The look-alike concept is similar to the morphological concept (and might best be considered a subset of it) in that both delimit taxa by morphological discontinuities. It differs, however, in that it classifies taxa hierarchically as species or varieties based upon relative morphological similarities-an author may classify two morphologically distinct entities as varieties because they look alike, when by strict morphological criteria they might otherwise have been treated as separate species. An example is the Lycopodium obscurum complex studied by Hickey (1977). He clarified the taxonomy of this group by showing that a new taxon, L. isophyllum, was morphologically distinct from L. obscurum and L. dendroideum. Hickey classified isophyllum as a variety of L. obscurum rather than as a separate species, because of closer resemblance of