HISTORICAL BIOGEOGRAPHY OF ASPLENIUM-ADIANTUM-NIGRUM (ASPLENIACEAE) IN NORTH-AMERICA AND IMPLICATIONS FOR SPECIATION THEORY IN HOMOSPOROUS PTERIDOPHYTES

Theories of plant speciation have generally recognized the importance of allopatry as a potential precursor to the genetic divergence of pop ulations. The relative importance of long-distance dispersal vs. vicar iance events in leading to allopatry, however, has been debated. We ex amined isozymic variability in highly disjunct populations of allotetr aploid Asplenium adiantum-nigrum to test alternative hypotheses on the ir mode of origin. In addition, we assessed the genetic distinctness o f the population from Boulder County, Colorado, which had been propose d as a separate species, A. andrewsii. Our results revealed that sampl es from all continental U.S. populations were isozymically identical a nd, with the exception of two samples from Boulder, displayed no intra populational variability. Continental U.S. populations were most simil ar to those from Hawaii, whereas both of these sets of populations wer e considerably more divergent from samples from Mexico and the Caucasu s. The distribution of alleles and genotypes support the hypothesis th at populations from different geographical regions had unique origins, resulting from at least several independent hybridization and polyplo idization events followed by long-distance dispersal. These results ha ve implications for speciation theory of pteridophytes in documenting the effectiveness of long-distance dispersal in the establishment of d isjunct populations which may set the stage for allopatric speciation. In addition, the data suggest that the Boulder population is not suff iciently distinct to be considered a separate species.