The high vagility of spores in homosporous pteridophytes has been hypothesized to effect high rates of gene flow among conspecific populations over a considerable geographic area, potentially constraining rates of divergence and concomitantly inception of allopatric speciation. This hypothesis is supported in great measure by the pattern of taxonomic diversity in most genera of pteridophytes within large continental regions. For example, in the pteridophyte flora of temperate eastern North America, it is apparent that taxonomic complexity has rarely resulted from primary geographic divergence within the region. Most pteridophyte species that range widely across eastern North America are not divisible, on the basis of morphological attributes, into sub-regional intraspecific taxa. This morphological homogeneity is generally accompanied by genetic homogeneity as revealed by isozyme surveys that show modest values for range-wide FST. Exceptions occur in Dryopteris cristata and Asplenium platyneuron for which strong geographic patterns of allele frequency divergence are evident. Moreover, notable exceptions to morphological homogeneity occur in some genera, notably Botrychium, Pteridium, and Athyrium, each of which includes a species divisible into two or more closely related sub-regional taxa of contentious taxonomic rank. Allozyme surveys conducted on the latter two genera have revealed that the geographic patterns of morphological divergence are correlated with strong patterns of genetic divergence. Why do some species seem to have maintained genetic integrity while others have become geographically subdivided to greater or lesser degrees? It is hypothesized that differences in biogeographic history and life-history strategies combine to provide at least a partial explanation.

Key words: allozyme, genetic divergence, genetic variation, isozyme, pteridophyte, speciation