Whittall Justen B, Hodges Scott A
Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106, USA.
Nature. 2007 Jun 7;447(7145):706-9. doi: 10.1038/nature05857.
Directional evolutionary trends have long garnered interest because they suggest that evolution can be predictable. However, the identification of the trends themselves and the underlying processes that may produce them have often been controversial. In 1862, in explaining the exceptionally long nectar spur of Angraecum sesquipedale, Darwin proposed that a coevolutionary 'race' had driven the directional increase in length of a plant's spur and its pollinator's tongue. Thus he predicted the existence of an exceptionally long-tongued moth. Though the discovery of Xanthopan morgani ssp. praedicta in 1903 with a tongue length of 22 cm validated Darwin's prediction, his 'race' model for the evolution of long-spurred flowers remains contentious. Spurs may also evolve to exceptional lengths by way of pollinator shifts as plants adapt to a series of unrelated pollinators, each with a greater tongue length. Here, using a species-level phylogeny of the columbine genus, Aquilegia, we show a significant evolutionary trend for increasing spur length during directional shifts to pollinators with longer tongues. In addition, we find evidence for 'punctuated' change in spur length during speciation events, suggesting that Aquilegia nectar spurs rapidly evolve to fit adaptive peaks predefined by pollinator morphology. These findings show that evolution may proceed in predictable pathways without reversals and that change may be concentrated during speciation.
定向进化趋势长期以来一直备受关注，因为它们表明进化可能是可预测的。然而，这些趋势本身的识别以及可能产生它们的潜在过程常常存在争议。1862年，在解释马达加斯加彗星兰异常长的花蜜距时，达尔文提出一种协同进化“竞赛”推动了植物距和其传粉者舌头长度的定向增加。因此，他预测存在一种舌头异常长的蛾类。尽管1903年发现的预测长喙天蛾的舌头长度为22厘米证实了达尔文的预测，但他关于长距花进化的“竞赛”模型仍然存在争议。随着植物适应一系列不相关的传粉者，每个传粉者都有更长的舌头，距也可能通过传粉者转变而进化到异常的长度。在这里，我们利用耧斗菜属的物种水平系统发育，展示了在向具有更长舌头的传粉者定向转变过程中距长度增加的显著进化趋势。此外，我们发现了在物种形成事件中距长度“间断”变化的证据，这表明耧斗菜的花蜜距迅速进化以适应由传粉者形态预先定义的适应峰。这些发现表明进化可能以可预测的途径进行而不会逆转，并且变化可能集中在物种形成期间。
