Data from: Females drive asymmetrical introgression from rare to common species in Darwin's tree finches

The consequences of hybridization for biodiversity depend on the specific ecological and evolutionary context in which it occurs. Understanding patterns of gene flow among hybridizing species is crucial for determining the evolutionary trajectories of species assemblages. The recently discovered hybridization between two species of Darwin's tree finches (Camarhynchus parvulus and C. pauper) on Floreana Island, Galápagos, presents an exciting opportunity to investigate the mechanisms causing hybridization and its potential evolutionary consequences under conditions of recent habitat disturbance and the introduction of invasive pathogens. In this study, we combine morphological and genetic analysis with pairing observations to explore the extent, direction and drivers of hybridization and to test if hybridization patterns are a result of asymmetrical pairing preference driven by females of the rarer species (C. pauper). We found asymmetrical introgression from the critically endangered, larger-bodied C. pauper to the common, smaller-bodied C. parvulus, which was associated with a lack of selection against heterospecific males by C. pauper females. Examination of pairing data showed that C. parvulus females paired assortatively while C. pauper females showed no such pattern. This study shows how sex-specific drivers can determine the direction of gene flow in hybridizing species. Furthermore, our results suggest the existence of a hybrid swarm comprised of C. parvulus and hybrid birds. We discuss the influence of interspecific abundance differences and susceptibility to the invasive parasite Philornis downsi on the observed hybridisation and recommend that the conservation of this iconic species group should be managed jointly rather than species-specific.

杂交对生物多样性的影响取决于其发生的具体生态与进化背景。解析杂交物种种间的基因流模式，对于明确物种类群的进化轨迹至关重要。近日在加拉帕戈斯群岛弗洛里亚纳岛发现的两种达尔文树雀——小树雀（Camarhynchus parvulus）与贫树雀（C. pauper）之间的杂交事件，为在近期生境干扰与入侵病原引入的背景下，探究杂交发生的机制及其潜在进化后果提供了绝佳研究契机。本研究结合形态学、遗传学分析与配对行为观测，旨在探究杂交的发生范围、方向及其驱动因素，并验证杂交模式是否由稀有物种（贫树雀）雌性的不对称配对偏好所导致。研究结果显示，基因渐渗呈现不对称性：从体型更大的极危贫树雀流向更为常见、体型更小的小树雀，这一现象与贫树雀雌性未对异种雄性产生排斥选择相关。配对行为数据分析表明，小树雀雌性存在选择性配对行为，而贫树雀雌性则未表现出该模式。本研究揭示了性别特异性驱动因素如何决定杂交物种种间的基因流方向。此外，本研究结果暗示存在由小树雀与杂交个体构成的杂交群。本文探讨了物种类群丰度差异以及宿主对入侵寄生蝇（Philornis downsi）的易感性对观测到的杂交现象的影响，并建议对这一标志性物种类群的保护应采取联合管理策略，而非单一物种针对性管理。