The evolution of feather coloration and song in Old World orioles (genus Oriolus)

What is the tempo and mode of evolution – how fast and in what pattern do traits evolve – is a major question of evolutionary biology. Here we studied patterns of evolutionary change in visual and acoustic signals in Old World orioles. Since producing multiple signals may be costly, we also tested whether there was an evolutionary trade-off between the elaboration of those two types of signals. We studied 30 Oriolus taxa using comparative methods and a recent molecular phylogeny. Morphology and plumage hue evolved comparatively slowly, whereas song evolved rapidly. Among individual feather patches, the evolutionary rate of color was slowest in primaries, which are critical for flapping flight, and fastest in patches exposed to observers (mantle and breast). Thus, primaries seem to be under functional constraint while the evolution of visually exposed patches is perhaps shaped by sexual selection. Song evolution was comparatively fast, but also attracted to a single optimum. This may be due to selection for signal efficacy, because all orioles inhabit similar forested habitats. Only color diversity was best fit by a speciational model: the biggest changes in coloration were concentrated at speciation events, thus perhaps linked to the evolution of species recognition. Our analysis did not reveal any evolutionary trade-off between acoustic and visual signals, suggesting that the elaboration of visual and acoustic signals in the Old World orioles evolved independently. Our study shows that patterns of evolutionary change may be surprisingly complex even within a single clade of birds and thus further studies are needed to identify general patterns of signal macroevolution.

演化的速率与模式——即性状演化的快慢与具体形式——是进化生物学的核心议题之一。本研究聚焦旧世界黄鹂（Old World orioles）的视觉与声学信号演化变化模式。由于产生多类信号可能存在成本消耗，本研究同时检验了这两类信号的强化演化之间是否存在演化权衡。本研究采用比较分析法与最新的分子系统发育数据，对30个黄鹂属（Oriolus）类群开展分析。形态性状与羽色色调的演化速率相对缓慢，而鸣唱信号的演化则较为迅速。在单个羽毛区域中，羽色的演化速率在用于振翅飞行的初级飞羽（primaries）中最慢，而在易被观察者察觉的区域（上背与胸部）中最快。由此可见，初级飞羽似乎受到功能约束，而易被视觉识别的羽毛区域的演化则可能受性选择驱动。鸣唱信号的演化速率虽快，但同时趋向单一最优表型。这一现象或源于对信号功效的选择压力——所有黄鹂均栖息于相似的森林生境中。唯有羽色多样性最符合成种模型：羽色的显著变化集中发生于物种形成事件中，因此或与物种识别的演化相关。本研究未发现声学与视觉信号之间存在演化权衡，这表明旧世界黄鹂的视觉与声学信号的强化演化是独立进行的。本研究表明，即便在单一鸟类演化支内，演化变化的模式也可能出人意料地复杂，因此仍需开展进一步研究以揭示信号宏观演化的通用模式。