Stankowski Sean, Sobel James M, Streisfeld Matthew A
Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, 97401.
Department of Biological Sciences, Binghamton University, Binghamton, New York, 13902.
Evolution. 2015 Dec;69(12):3054-68. doi: 10.1111/evo.12807. Epub 2015 Nov 17.
Ecological adaptation is the driving force during divergence with gene flow and generates reproductive isolation early in speciation. Although gene flow opposes divergence, local adaptation can be facilitated by factors that prevent the breakup of favorable allelic combinations. We investigated how selection, genetic architecture, and geography have contributed to the maintenance of floral trait divergence and pollinator isolation between parapatric ecotypes of Mimulus aurantiacus. Combining greenhouse, field, and genomic studies, we show that sharp clines in floral traits are maintained by spatially varying selection. Although adaptation breaks down where the ecotypes co-occur, leading to the formation of a hybrid zone, the largely non-overlapping distributions of the ecotypes shield them from immigrant genes, facilitating divergence across most of the range. In contrast to the sharp genetic discontinuities observed across most hybrid zones, we observed a gradual cline in genome-wide divergence and a pattern of isolation by distance across the landscape. Thus, contrary to a long period of allopatry followed by recent re-contact, our data suggest that floral trait divergence in M. aurantiacus may have evolved with locally restricted, but ongoing gene flow. Therefore, our study reveals how the geographic distribution of an organism can contribute to the evolution of premating isolation in the early stages of divergence with gene flow.
生态适应是基因流分化过程中的驱动力，并在物种形成早期产生生殖隔离。尽管基因流会阻碍分化，但局部适应可通过防止有利等位基因组合破裂的因素得以促进。我们研究了选择、遗传结构和地理因素如何促成了橙色沟酸浆（Mimulus aurantiacus）相邻生态型之间花部性状分化和传粉者隔离的维持。通过结合温室、田间和基因组研究，我们发现花部性状的陡峭渐变群是由空间变化的选择所维持的。尽管在生态型共存的地方适应作用会瓦解，导致杂交带的形成，但生态型在很大程度上不重叠的分布使它们免受外来基因的影响，从而促进了大部分分布范围内的分化。与大多数杂交带中观察到的明显遗传间断不同，我们在全基因组分化中观察到了渐变群，并在整个景观中观察到了距离隔离模式。因此，与长期异域分布后近期重新接触的情况相反，我们的数据表明橙色沟酸浆的花部性状分化可能是在局部受限但持续存在的基因流情况下进化而来的。所以，我们的研究揭示了生物体的地理分布如何在基因流分化的早期阶段促进交配前隔离的进化。
