Data from: Lineage fusion in Galápagos giant tortoises

Although many classic radiations on islands are thought to be the result of repeated lineage splitting, the role of past fusion is rarely known because during these events, purebreds are rapidly replaced by a swarm of admixed individuals. Here we capture lineage fusion in action in a Galápagos giant tortoise species, Chelonoidis becki, from Volcano Wolf (Isabela Island). The long generation time of Galápagos tortoises and dense sampling (~1700 individuals) of genetic and demographic data were integral in detecting and characterizing this phenomenon. In C. becki we identified two genetically distinct, morphologically cryptic lineages. Historical reconstructions show that they colonized Volcano Wolf from Santiago Island in two temporally separated events, the first occurring ~199,000 years ago. Following arrival of the second wave of colonists, both lineages co-existed for ~53,000 years. However, they are now fusing back together, as microsatellite data reveal recent and widespread introgressive hybridization. Consistent with evolutionary theory, pre-zygotic isolation is weakest in the most recent colonizers. Forward-in-time projections predict rapid extinction of the older lineage, with negligible potential for genetic rescue. This study provides a rare example of reticulate evolution in action, and underscores the power of population genetics for understanding the past, present and future.尽管学界普遍认为岛屿上的诸多经典物种辐射演化源于反复的谱系分化，但过往谱系融合事件的作用却鲜为人知——因为在这类事件中，纯种个体会迅速被大量混血个体所取代。本研究针对来自伊莎贝拉岛沃尔夫火山的贝氏加拉帕戈斯巨龟（Chelonoidis becki），实时捕捉到了正在发生的谱系融合事件。加拉帕戈斯巨龟漫长的世代周期，加之本研究对约1700个个体开展的高密度遗传与种群统计数据采样，是检测并表征这一现象的关键条件。在C. becki中，我们鉴定出两个遗传分化显著、形态上隐蔽的谱系。历史重建结果显示，二者均从圣地亚哥岛扩散定居至沃尔夫火山，且两次定居事件在时间上相互分隔：首次定居发生于约19.9万年前。第二批定居者抵达后，两个谱系共存了约5.3万年。但如今二者正重新融合：微卫星（microsatellite）数据显示，近期出现了广泛的渐渗杂交（introgressive hybridization）。与进化理论的预测一致，合子前生殖隔离（pre-zygotic isolation）在最晚抵达的定居类群中强度最弱。正向时间预测模型（forward-in-time projections）显示，古老谱系将快速走向灭绝，几乎不存在通过遗传拯救（genetic rescue）实现存续的可能。本研究提供了一个罕见的正在发生的网状演化（reticulate evolution）实例，同时凸显了种群遗传学（population genetics）在解析生物演化的过去、现状与未来层面的强大效力。