Clegg Sonya M, Degnan Sandie M, Moritz Craig, Estoup Arnaud, Kikkawa Jiro, Owens Ian P F
Department of Zoology and Entomology, University of Queensland, St Lucia, Australia.
Evolution. 2002 Oct;56(10):2090-9. doi: 10.1111/j.0014-3820.2002.tb00134.x.
Theory predicts that in small isolated populations random genetic drift can lead to phenotypic divergence; however this prediction has rarely been tested quantitatively in natural populations. Here we utilize natural repeated island colonization events by members of the avian species complex, Zosterops lateralis, to assess whether or not genetic drift alone is an adequate explanation for the observed patterns of microevolutionary divergence in morphology. Morphological and molecular genetic characteristics of island and mainland populations are compared to test three predictions of drift theory: (1) that the pattern of morphological change is idiosyncratic to each island; (2) that there is concordance between morphological and neutral genetic shifts across island populations; and (3) for populations whose time of colonization is known, that the rate of morphological change is sufficiently slow to be accounted for solely by genetic drift. Our results are not consistent with these predictions. First, the direction of size shifts was consistently towards larger size, suggesting the action of a nonrandom process. Second, patterns of morphological divergence among recently colonized populations showed little concordance with divergence in neutral genetic characters. Third, rate tests of morphological change showed that effective population sizes were not small enough for random processes alone to account for the magnitude of microevolutionary change. Altogether, these three lines of evidence suggest that drift alone is not an adequate explanation of morphological differentiation in recently colonized island Zosterops and therefore we suggest that the observed microevolutionary changes are largely a result of directional natural selection.
理论预测，在小型隔离种群中，随机遗传漂变会导致表型分化；然而，这一预测在自然种群中很少得到定量检验。在此，我们利用鸟类物种复合体——银额绣眼鸟（Zosterops lateralis）成员对岛屿的自然重复定殖事件，来评估仅遗传漂变是否足以解释在形态学上观察到的微进化分化模式。比较岛屿和大陆种群的形态学与分子遗传学特征，以检验漂变理论的三个预测：（1）形态变化模式对每个岛屿而言是特异的；（2）岛屿种群的形态和中性遗传变化之间存在一致性；（3）对于定殖时间已知的种群，形态变化速率足够缓慢，仅由遗传漂变就能解释。我们的结果与这些预测不一致。首先，体型变化的方向始终朝着更大体型，这表明存在非随机过程的作用。其次，最近定殖种群之间的形态分化模式与中性遗传特征的分化几乎没有一致性。第三，形态变化的速率测试表明，有效种群大小不够小，无法仅由随机过程来解释微进化变化的幅度。总之，这三条证据表明，仅遗传漂变不足以解释最近定殖的岛屿银额绣眼鸟的形态分化，因此我们认为观察到的微进化变化很大程度上是定向自然选择的结果。
