The extreme isolation and mid-Pacific origin of the Hawaiian archipela
go has ensured that all indigenous organisms have arrived via long-dis
tance dispersal or have evolved from successfully colonizing species.
Although this isolation has also produced high rates of species endemi
sm in angiosperms (89% or more), that rate in pteridophytes is conside
rably less (76%). The ratio of native species to the estimated number
of original successful colonizing species in angiosperms (3.4) is more
than double that for pteridophytes (1.6). One possible explanation fo
r the lower speciation rate in pteridophytes is that populations of th
ese species are more likely to experience interpopulational gene flow
because of the great vagility of their wind-dispersed spores. We condu
cted isozymic surveys of populations from the island of Hawaii of the
indigenous allotetraploid species Asplenium adiantum-nigrum, putativel
y derived from two strictly European diploid taxa. Our data support mu
ltiple hybrid origins for the populations surveyed, with a minimum of
3, and possibly as many as 17, discrete hybridization events having pr
oduced the genetic diversity observed. Since the parental taxa are not
found in Hawaii, each hybrid lineage must have arrived in the archipe
lago independently of the others. Similar long-distance, repeated disp
ersal events may be occurring between insular and noninsular populatio
ns of other native pteridophytes in Hawaii and in other insular region
s of the world, thus contributing to the relatively low rates of speci
ation and insular endemism in this ancient group of plants.