MULTIPLE COLONIZATIONS OF ASPLENIUM-ADIANTUM-NIGRUM ONTO THE HAWAIIANARCHIPELAGO

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.