Kautt Andreas F, Machado-Schiaffino Gonzalo, Meyer Axel
Department of Biology, University of Konstanz, Konstanz, Baden-Württemberg, Germany.
PLoS Genet. 2016 Jun 30;12(6):e1006157. doi: 10.1371/journal.pgen.1006157. eCollection 2016 Jun.
The formation of species in the absence of geographic barriers (i.e. sympatric speciation) remains one of the most controversial topics in evolutionary biology. While theoretical models have shown that this most extreme case of primary divergence-with-gene-flow is possible, only a handful of accepted empirical examples exist. And even for the most convincing examples uncertainties remain; complex histories of isolation and secondary contact can make species falsely appear to have originated by sympatric speciation. This alternative scenario is notoriously difficult to rule out. Midas cichlids inhabiting small and remote crater lakes in Nicaragua are traditionally considered to be one of the best examples of sympatric speciation and lend themselves to test the different evolutionary scenarios that could lead to apparent sympatric speciation since the system is relatively small and the source populations known. Here we reconstruct the evolutionary history of two small-scale radiations of Midas cichlids inhabiting crater lakes Apoyo and Xiloá through a comprehensive genomic data set. We find no signs of differential admixture of any of the sympatric species in the respective radiations. Together with coalescent simulations of different demographic models our results support a scenario of speciation that was initiated in sympatry and does not result from secondary contact of already partly diverged populations. Furthermore, several species seem to have diverged simultaneously, making Midas cichlids an empirical example of multispecies outcomes of sympatric speciation. Importantly, however, the demographic models strongly support an admixture event from the source population into both crater lakes shortly before the onset of the radiations within the lakes. This opens the possibility that the formation of reproductive barriers involved in sympatric speciation was facilitated by genetic variants that evolved in a period of isolation between the initial founding population and the secondary migrants that came from the same source population. Thus, the exact mechanisms by which these species arose might be different from what had been thought before.
在没有地理隔离的情况下物种的形成（即同域物种形成）仍然是进化生物学中最具争议的话题之一。虽然理论模型表明这种最极端的初级分化与基因流的情况是可能的，但只有少数被认可的实证例子。而且即使是最有说服力的例子也存在不确定性；复杂的隔离和二次接触历史可能会使物种错误地看起来是通过同域物种形成而起源的。这种替代情况很难排除。栖息在尼加拉瓜小而偏远的火山口湖中的米达斯丽鱼科鱼传统上被认为是同域物种形成的最佳例子之一，并且由于该系统相对较小且源种群已知，适合用来测试可能导致明显同域物种形成的不同进化情况。在这里，我们通过一个全面的基因组数据集重建了栖息在阿波约湖和希洛亚湖火山口的两个小规模米达斯丽鱼科鱼辐射种群的进化历史。我们在各自的辐射种群中没有发现任何同域物种有差异混合的迹象。结合不同人口模型的溯祖模拟，我们的结果支持一种在同域中开始且不是由已经部分分化的种群的二次接触导致的物种形成情况。此外，几个物种似乎是同时分化的，使米达斯丽鱼科鱼成为同域物种形成多物种结果的一个实证例子。然而重要的是，人口模型强烈支持在湖泊内辐射开始前不久源种群向两个火山口湖的混合事件。这就开启了一种可能性，即同域物种形成中涉及的生殖屏障的形成是由在初始奠基种群和来自同一源种群的二次迁移者之间的隔离期进化而来的遗传变异所促进的。因此，这些物种出现的确切机制可能与之前的想法不同。
