HISTORICAL BIOGEOGRAPHY OF ASPLENIUM-ADIANTUM-NIGRUM (ASPLENIACEAE) IN NORTH-AMERICA AND IMPLICATIONS FOR SPECIATION THEORY IN HOMOSPOROUS PTERIDOPHYTES
Ta. Ranker et al., HISTORICAL BIOGEOGRAPHY OF ASPLENIUM-ADIANTUM-NIGRUM (ASPLENIACEAE) IN NORTH-AMERICA AND IMPLICATIONS FOR SPECIATION THEORY IN HOMOSPOROUS PTERIDOPHYTES, American journal of botany, 81(6), 1994, pp. 776-781
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.