Data from: Bill morphology and neutral genetic structure both predict variation in acoustic signals within a bird population

Adaptive evolutionary divergence within a population can be facilitated by associated divergence in mating signals. Acoustic signals are often involved in mate choice, and are also known to diverge spatially in response to a variety of processes. In birds, for instance, variation in bill size and shape can result in correlated changes in vocalizations due to functional constraints on sound production. Acoustic signals can also vary spatially in relation to neutral genetic structure (due to cultural drift) and/or habitat structure (due to acoustic adaptation for optimal sound transmission). Here we test these alternative hypotheses as causes of variation in acoustic signal structure in the Island Scrub-Jay (Aphelocoma insularis), a species that is restricted to one small island (Santa Cruz Island, California, USA) and exhibits spatial genetic structure and microgeographic divergence in bill morphology across short distances and habitat types. We find that bill morphology is related to the structure of the female “rattle” call, a vocalization associated with territorial disputes and male-female interactions. Females with longer, shallower bills produced calls that were more rapid, and those with shallower bills also produced calls that were lower in frequency. In addition, rattle rapidity varied across the island in accordance with neutral genetic structure. Vocal characteristics were not related to habitat structure, suggesting that variation in rattle calls is unlikely to reflect optimization for sound transmission. Our findings indicate that selection on bill morphology and cultural drift can jointly shape variation in acoustic signal structure, even at fine spatial scales within populations.种群内的适应性进化分化，可借助交配信号的协同分化得以推进。声学信号（acoustic signals）常参与配偶选择（mate choice）过程，且已知其会因多种驱动因素发生空间分化。以鸟类为例，喙的大小与形态变异可因发声功能所受的限制，导致鸣叫声（vocalizations）产生相应的关联性变化。声学信号的空间变异还可能与中性遗传结构（neutral genetic structure）相关，其成因包括文化漂变（cultural drift）；或与栖息地结构（habitat structure）相关，其成因是为实现最优声音传播的声学适配（acoustic adaptation）。本研究以仅分布于美国加利福尼亚州圣克鲁兹岛（Santa Cruz Island）这一小型岛屿的岛丛鸦（Aphelocoma insularis，英文名Island Scrub-Jay）为研究对象，该物种存在空间遗传结构（spatial genetic structure），且在短距离范围内及不同生境类型间存在喙形态的微地理分化（microgeographic divergence）。我们针对上述两种替代性假说展开检验，以探究该物种声学信号结构变异的成因。研究结果显示，喙形态与雌性“嘎嘎鸣”鸣啭（rattle call）的结构存在关联——这类鸣啭与领地争端（territorial disputes）及雌雄个体间的互动相关。喙更长且更扁平的雌性个体，其发出的鸣啭节奏更快；而喙更扁平的雌性个体，其鸣啭的频率也更低。此外，“嘎嘎鸣”鸣啭的节奏在全岛范围内的变异符合中性遗传结构特征。声学特征与栖息地结构并无关联，这表明“嘎嘎鸣”鸣啭的变异不太可能是为优化声音传播而产生的适应性调整。本研究结果表明，即便在种群内的精细空间尺度下，针对喙形态的定向选择与文化漂变可共同塑造声学信号结构的变异。