Due to their limited ranges and inherent isolation, island species have long been recognized as crucial systems for tackling a range of evolutionary questions, including in the early study of speciation.^1,2 Such species have been less studied in the understanding of the evolutionary forces driving DNA sequence evolution. Island species usually have lower census population sizes (N) than continental species and, supposedly, lower effective population sizes (Ne). Given that both the rates of change caused by genetic drift and by selection are dependent upon Ne, island species are theoretically expected to exhibit (1) lower genetic diversity, (2) less effective natural selection against slightly deleterious mutations,^3,4 and (3) a lower rate of adaptive evolution.5, 6, 7, 8 Here, we have used a large set of newly sequenced and published whole-genome sequences of Passerida species (14 insular and 11 continental) to test these predictions. We confirm that island species exhibit lower census size and Ne, supporting the hypothesis that the smaller area available on islands constrains the upper bound of Ne. In the insular species, we find lower nucleotide diversity in coding regions, higher ratios of non-synonymous to synonymous polymorphisms, and lower adaptive substitution rates. Our results provide robust evidence that the lower Ne experienced by island species has affected both the ability of natural selection to efficiently remove weakly deleterious mutations and also the adaptive potential of island species, therefore providing considerable empirical support for the nearly neutral theory. We discuss the implications for both evolutionary and conservation biology.

由于其有限的范围和固有的孤立性，岛屿物种长期以来一直被认为是解决一系列进化问题的关键系统，包括在早期的物种形成研究中。^{1 , 2}在理解驱动 DNA 序列进化的进化力量方面，此类物种的研究较少。岛屿物种的人口普查种群规模（ N ）通常低于大陆物种，据推测，有效种群规模（Ne）也较低。鉴于遗传漂变和选择引起的变化率都取决于Ne，理论上预计岛屿物种会表现出（1）较低的遗传多样性，（2）对轻微有害突变的自然选择效率较低^{，3、4和（ 3}）适应性进化率较低。5、6、7、8在这里，我们使用了大量新测序和发表的 Passerida 物种全基因组序列（14 个岛屿和 11 个大陆）来测试这些预测。我们确认岛屿物种的人口普查规模和Ne较低，这支持了岛屿上较小的可用面积限制了Ne上限的假设。. 在岛屿物种中，我们发现编码区的核苷酸多样性较低，非同义多态性与同义多态性的比率较高，适应性替代率较低。我们的研究结果提供了强有力的证据，表明岛屿物种经历的较低Ne影响了自然选择有效去除弱有害突变的能力以及岛屿物种的适应潜力，因此为近乎中性的理论提供了相当大的经验支持。我们讨论了对进化和保护生物学的影响。