Clegg Sonya M, Frentiu Francesca D, Kikkawa Jiro, Tavecchia Giacomo, Owens Ian P F
Division of Biology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, United Kingdom.
Evolution. 2008 Sep;62(9):2393-410. doi: 10.1111/j.1558-5646.2008.00437.x. Epub 2008 Jun 6.
Pronounced phenotypic shifts in island populations are typically attributed to natural selection, but reconstructing heterogeneity in long-term selective regimes remains a challenge. We examined a scenario of divergence proposed for species colonizing a new environment, involving directional selection with a rapid shift to a new optimum and subsequent stabilization. We provide some of the first empirical evidence for this model of evolution using morphological data from three timescales in an island bird, Zosterops lateralis chlorocephalus. In less than four millennia since separation from its mainland counterpart, a substantial increase in body size has occurred and was probably achieved in fewer than 500 generations after colonization. Over four recent decades, morphological traits have fluctuated in size but showed no significant directional trends, suggesting maintenance of a relatively stable phenotype. Finally, estimates of contemporary selection gradients indicated generally weak directional selection. These results provide a rare description of heterogeneity in long-term natural regimes, and caution that observations of current selection may be of limited value in inferring mechanisms of past adaptation due to a lack of constancy even over short time-frames.
岛屿种群中显著的表型变化通常归因于自然选择，但重建长期选择机制中的异质性仍然是一项挑战。我们研究了一个针对物种殖民新环境提出的分化情景，其中涉及定向选择，迅速转向新的最优状态并随后稳定下来。我们利用一种岛屿鸟类——绿头白脸绣眼鸟（Zosterops lateralis chlorocephalus）三个时间尺度的形态学数据，为这种进化模型提供了一些首批实证证据。自与其大陆同类分离后的不到四千年里，体型大幅增加，并且可能在殖民后的不到500代内就实现了。在最近的四十年里，形态特征在大小上有所波动，但没有显著的定向趋势，这表明维持了相对稳定的表型。最后，当代选择梯度的估计表明总体上定向选择较弱。这些结果罕见地描述了长期自然机制中的异质性，并提醒人们，由于即使在短时间框架内也缺乏稳定性，当前选择的观察结果在推断过去适应机制方面可能价值有限。
