Wolf Jochen B W, Harrod Chris, Brunner Sylvia, Salazar Sandie, Trillmich Fritz, Tautz Diethard
Institute for Genetics, Evolutionary Genetics, University of Köln, 50674 Köln, Germany.
BMC Evol Biol. 2008 May 16;8:150. doi: 10.1186/1471-2148-8-150.
Oceans are high gene flow environments that are traditionally believed to hamper the build-up of genetic divergence. Despite this, divergence appears to occur occasionally at surprisingly small scales. The Galápagos archipelago provides an ideal opportunity to examine the evolutionary processes of local divergence in an isolated marine environment. Galápagos sea lions (Zalophus wollebaeki) are top predators in this unique setting and have an essentially unlimited dispersal capacity across the entire species range. In theory, this should oppose any genetic differentiation.
We find significant ecological, morphological and genetic divergence between the western colonies and colonies from the central region of the archipelago that are exposed to different ecological conditions. Stable isotope analyses indicate that western animals use different food sources than those from the central area. This is likely due to niche partitioning with the second Galápagos eared seal species, the Galápagos fur seal (Arctocephalus galapagoensis) that exclusively dwells in the west. Stable isotope patterns correlate with significant differences in foraging-related skull morphology. Analyses of mitochondrial sequences as well as microsatellites reveal signs of initial genetic differentiation.
Our results suggest a key role of intra- as well as inter-specific niche segregation in the evolution of genetic structure among populations of a highly mobile species under conditions of free movement. Given the monophyletic arrival of the sea lions on the archipelago, our study challenges the view that geographical barriers are strictly needed for the build-up of genetic divergence. The study further raises the interesting prospect that in social, colonially breeding mammals additional forces, such as social structure or feeding traditions, might bear on the genetic partitioning of populations.
海洋是高基因流动环境，传统观点认为这会阻碍遗传分化的积累。尽管如此，分化似乎偶尔会在惊人的小尺度上发生。加拉帕戈斯群岛提供了一个理想的机会，来研究孤立海洋环境中局部分化的进化过程。加拉帕戈斯海狮（Zalophus wollebaeki）是这种独特环境中的顶级捕食者，在整个物种分布范围内具有基本不受限制的扩散能力。从理论上讲，这应该会阻止任何遗传分化。
我们发现，处于不同生态条件下的群岛西部殖民地与中部地区殖民地之间，在生态、形态和遗传方面存在显著差异。稳定同位素分析表明，西部的海狮与中部地区的海狮利用不同的食物来源。这可能是由于与仅栖息在西部的加拉帕戈斯群岛的另一种有耳海豹——加拉帕戈斯海狗（Arctocephalus galapagoensis）的生态位划分。稳定同位素模式与觅食相关头骨形态的显著差异相关。线粒体序列以及微卫星分析揭示了初始遗传分化的迹象。
我们的结果表明，在自由移动条件下，种内和种间生态位隔离在高度移动物种种群遗传结构的进化中起着关键作用。鉴于海狮单系地到达该群岛，我们的研究挑战了遗传分化的积累严格需要地理障碍这一观点。该研究还进一步提出了一个有趣的前景，即在社会性、群居繁殖的哺乳动物中，诸如社会结构或觅食传统等其他因素，可能会影响种群的遗传划分。
