Ranker Tom A, Floyd Sandra K, Trapp P Genie
University of Colorado, University Museum, Boulder, Colorado.
University of Colorado, Department of Environmental, Population, and Organismic Biology, Boulder, Colorado, 80309.
Evolution. 1994 Aug;48(4):1364-1370. doi: 10.1111/j.1558-5646.1994.tb05319.x.
The extreme isolation and mid-Pacific origin of the Hawaiian archipelago has ensured that all indigenous organisms have arrived via long-distance dispersal or have evolved from successfully colonizing species. Although this isolation has also produced high rates of species endemism in angiosperms (89% or more), that rate in pteridophytes is considerably 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 for the lower speciation rate in pteridophytes is that populations of these species are more likely to experience interpopulational gene flow because of the great vagility of their wind-dispersed spores. We conducted isozymic surveys of populations from the island of Hawaii of the indigenous allotetraploid species Asplenium adiantum-nigrum, putatively derived from two strictly European diploid taxa. Our data support multiple hybrid origins for the populations surveyed, with a minimum of 3, and possibly as many as 17, discrete hybridization events having produced the genetic diversity observed. Since the parental taxa are not found in Hawaii, each hybrid lineage must have arrived in the archipelago independently of the others. Similar long-distance, repeated dispersal events may be occurring between insular and noninsular populations of other native pteridophytes in Hawaii and in other insular regions of the world, thus contributing to the relatively low rates of speciation and insular endemism in this ancient group of plants.
夏威夷群岛地处偏远且位于太平洋中部，这使得所有本土生物都是通过远距离扩散抵达此地，或者是由成功定居的物种进化而来。尽管这种隔离也导致被子植物的物种特有率很高（89%或更高），但蕨类植物的这一比率要低得多（76%）。被子植物中本土物种与估计的原始成功定居物种数量的比率（3.4）是蕨类植物（1.6）的两倍多。蕨类植物物种形成率较低的一个可能解释是，由于其靠风力传播的孢子具有很强的扩散性，这些物种的种群更有可能经历种群间的基因流动。我们对夏威夷岛本土异源四倍体物种黑木铁角蕨的种群进行了同工酶调查，该物种可能源自两个严格意义上的欧洲二倍体类群。我们的数据支持所调查种群具有多个杂交起源，至少有3次，也可能多达17次离散的杂交事件产生了观察到的遗传多样性。由于在夏威夷未发现其亲本类群，每个杂交谱系必定是独立于其他谱系抵达该群岛的。类似的远距离、反复的扩散事件可能正在夏威夷以及世界其他岛屿地区的其他本土蕨类植物的岛屿种群和非岛屿种群之间发生，从而导致这一古老植物类群的物种形成率和岛屿特有率相对较低。
