Petren K, Grant P R, Grant B R, Keller L F
Department of Biological Sciences, University of Cincinnati, Cincinnati, OH 45221-0006, USA.
Mol Ecol. 2005 Sep;14(10):2943-57. doi: 10.1111/j.1365-294X.2005.02632.x.
We use genetic divergence at 16 microsatellite loci to investigate how geographical features of the Galápagos landscape structure island populations of Darwin's finches. We compare the three most genetically divergent groups of Darwin's finches comprising morphologically and ecologically similar allopatric populations: the cactus finches (Geospiza scandens and Geospiza conirostris), the sharp-beaked ground finches (Geospiza difficilis) and the warbler finches (Certhidea olivacea and Certhidea fusca). Evidence of reduced genetic diversity due to drift was limited to warbler finches on small, peripheral islands. Evidence of low levels of recent interisland migration was widespread throughout all three groups. The hypothesis of distance-limited dispersal received the strongest support in cactus and sharp-beaked ground finches as evidenced by patterns of isolation by distance, while warbler finches showed a weaker relationship. Support for the hypothesis that gene flow constrains morphological divergence was only found in one of eight comparisons within these groups. Among warbler finches, genetic divergence was relatively high while phenotypic divergence was low, implicating stabilizing selection rather than constraint due to gene flow. We conclude that the adaptive radiation of Darwin's finches has occurred in the presence of ongoing but low levels of gene flow caused by distance-dependent interisland dispersal. Gene flow does not constrain phenotypic divergence, but may augment genetic variation and facilitate evolution due to natural selection. Both microsatellites and mtDNA agree in that subsets of peripheral populations of two older groups are genetically more similar to other species that underwent dramatic morphological change. The apparent decoupling of morphological and molecular evolution may be accounted for by a modification of Lack's two-stage model of speciation: relative ecological stasis in allopatry followed by secondary contact, ecological interactions and asymmetric phenotypic divergence.
我们利用16个微卫星位点的遗传差异来研究加拉帕戈斯群岛景观的地理特征如何构建达尔文雀的岛屿种群结构。我们比较了达尔文雀中遗传差异最大的三个群体，它们包含形态和生态上相似的异域种群：仙人掌雀（勇地雀和大嘴地雀）、尖嘴地雀以及莺雀（绿莺雀和棕莺雀）。因遗传漂变导致遗传多样性降低的证据仅局限于小的外围岛屿上的莺雀。近期岛屿间迁移水平较低的证据在所有三个群体中普遍存在。距离限制扩散假说在仙人掌雀和尖嘴地雀中得到了最有力的支持，距离隔离模式证明了这一点，而莺雀显示出较弱的关系。在这些群体中的八项比较中，仅有一项支持基因流限制形态差异这一假说。在莺雀中，遗传差异相对较高而表型差异较低，这意味着是稳定选择而非基因流导致的限制。我们得出结论，达尔文雀的适应性辐射是在由距离依赖的岛屿间扩散导致的持续但低水平的基因流存在的情况下发生的。基因流并不限制表型差异，但可能增加遗传变异并促进自然选择导致的进化。微卫星和线粒体DNA都表明，两个较古老群体的外围种群子集在遗传上与经历了显著形态变化的其他物种更为相似。形态进化和分子进化的明显脱钩可能是由于对拉克的两阶段物种形成模型的修改：异域状态下的相对生态停滞，随后是二次接触、生态相互作用和不对称表型分化。
