Ferns and lycophytes present two special problems to those studying species: (1) it is estimated that about 10% of fern and lycophyte species do not reproduce sexually (Walker, 1984), and therefore asexual species deserve special consideration, and (2) when contemporary molecular approaches are applied to widely distributed and (often) morphologically polymorphic species, a series of embedded cryptic species is discovered. Gastony and Windham (1989) reviewed the treatment of agamosporous (asexual) species and since that time, ongoing studies are discovering more examples of groups that are clearly isolated sets of self-perpetuating populations, but are not reproducing sexually (e.g., Windham and Yatskievych, 2003). Gastony and Windham (1989) propose a "genetic" species concept that acknowledges the clearly distinct agamosporous lineages. In many cases, agamosporous groups are also cryptic in that they are triploid autopoly-ploid derivatives of otherwise "normal" diploid species (e.g., Notholaena grayii and
Pellaea andromedifolia discussed in Gastony and Windham, 1989). In cases where the agamosporous lineage cannot be distinguished morphologically from the diploid progenitor, Gastony and Windham (1989) recommend the use of "variety" to identify the agamosporous units. As we learn more about these distinctive evolutionary units, our classification of them can become more sophisticated and responsive to their special characteristics.
One of the more significant advances in fern and lycophyte species biology has been the recognition of cryptic species within formerly circumscribed species. Paris et al. (1989) describe cryptic species as (1) poorly differentiated morphologically, (2) distinct evolutionary lineages that are reproductively isolated, and (3) historically misinterpreted as members of a single species. Prior to the incorporation of molecular data from isozymes and DNA analyses, cryptic species in ferns and lycophytes were difficult to detect because of their (1) simple construction, (2) relatively small number of structural characters available for identification, and (3) considerable plasticity in lineage-defining features (Paris et al., 1989). Through the application of molecular methods cryptic species in many fern and lycophyte lineages have been revealed, e.g., Adiantum pedatum (Paris and Windham 1988), Athyrium oblitescens (Kurihara et al., 1996), the Asplenium nidus complex (Yatabe et al., 2001), Botrychium subg. Botrychium (Hauk and Haufler, 1999), Ceratopteris thalictroides (Masuyama et al., 2002; Masuyama and Watano 2005), Cystopteris (Haufler and Windham, 1991), Gymnocarpium dryopteris (Pryer and Haufler, 1993), Isoetes (Hickey et al, 1989; Taylor and Hickey, 1992), and the Polypodium vulgare complex (Haufler and Windham, 1991; Haufler et al., 1995a). In each of these studies, genetically distinct and geographically restricted species were clearly delineated using molecular approaches. By concentrating on these genetically defined groups, it was possible to discover consistent yet often quite subtle morphological features that correlated with the molecular data. It is likely that these are not isolated examples and that investigation of other groups using molecular methods will reveal additional cryptic species masquerading as polymorphisms in currently recognized species.
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