Historical biogeography of Asplenium adiantum‐nigrum (Aspleniaceae) in North America and implications for speciation theory in homosporous pteridophytes

Abstract

Theories of plant speciation have generally recognized the importance of allopatry as a potential precursor to the genetic divergence of populations. The relative importance of long‐distance dispersal vs. vicariance events in leading to allopatry, however, has been debated. We examined isozymic variability in highly disjunct populations of allotetraploid Asplenium adiantum‐nigrum to test alternative hypotheses on their mode of origin. In addition, we assessed the genetic distinctness of the population from Boulder County, Colorado, which had been proposed as a separate species, A. andrewsii. Our results revealed that samples from all continental U.S. populations were isozymically identical and, with the exception of two samples from Boulder, displayed no intrapopulational variability. Continental U.S. populations were most similar to those from Hawaii, whereas both of these sets of populations were considerably more divergent from samples from Mexico and the Caucasus. The distribution of alleles and genotypes support the hypothesis that populations from different geographical regions had unique origins, resulting from at least several independent hybridization and polyploidization events followed by long‐distance dispersal. These results have implications for speciation theory of pteridophytes in documenting the effectiveness of long‐distance dispersal in the establishment of disjunct populations which may set the stage for allopatric speciation. In addition, the data suggest that the Boulder population is not sufficiently distinct to be considered a separate species.