Divergent processes drive parallel evolution in marine and freshwater fishes

Evolutionary comparisons between major environmental divides, such as between marine and freshwater systems, can reveal the fundamental processes governing diversification dynamics. Although processes may differ due to the different scales of their biogeographic barriers, freshwater and marine environments nevertheless offer similar opportunities for diversification in benthic, demersal, and pelagic habitats. Here, we compare the evolutionary patterns and processes shaping teleost diversity in each of these three habitats and between marine and freshwater systems. Using specimens from the National Museum of Natural History, we developed a data set of linear measurements capturing body shape in 2266 freshwater and 3344 marine teleost species. With a novel comparative approach, we contrast the primary axis of morphological diversification in each habitat with the major axis defined by phylogenetic signal. By comparing angles between these axes, we find that fish in corresponding habitats have more similar primary axes of morphological diversity than would be expected by chance, but that different historical processes underlie these parallel patterns in freshwater and marine environments. Marine diversification is more strongly aligned with phylogenetic signal and shows a trend toward lineages occupying separate regions of morphospace. In contrast, ecological signal appears to be a strong driver of diversification in freshwater lineages through repeated morphological evolution in densely packed regions of morphospace. In spite of these divergent histories, our findings reveal that habitat has driven convergent patterns of evolutionary diversification on a global scale.

针对海洋与淡水生态系统这类典型环境分界开展的演化比较研究，可揭示调控物种分化动态的核心过程。尽管生物地理屏障的尺度差异会导致两类环境下的演化过程有所不同，但淡水与海洋生境在底栖（benthic）、近底栖（demersal）与水层（pelagic）三类栖息地中，均为物种分化提供了相似的机遇。本研究针对上述三类栖息地以及海洋与淡水生态系统，对比了塑造硬骨鱼（teleost）多样性的演化模式与过程。本研究依托国家自然历史博物馆的标本，构建了包含2266种淡水硬骨鱼与3344种海洋硬骨鱼体型形态线性测量数据的数据集。本研究采用全新的比较分析方法，将各栖息地内形态分化的主轴线，与由系统发育信号（phylogenetic signal）定义的主轴线进行对比。通过对比两类轴线的夹角，本研究发现：对应栖息地内鱼类的形态多样性主轴线相似度显著高于随机预期，但淡水与海洋环境中这类平行模式的形成机制却受制于不同的历史演化过程。海洋类群的物种分化与系统发育信号契合度更高，且呈现出线系占据各自独立形态空间（morphospace）区域的趋势。与之相反，淡水类群的物种分化则以生态信号（ecological signal）为主要驱动因素：其形态演化在形态空间中密集分布的区域反复发生。尽管两类环境的演化历史存在上述差异，本研究结果仍表明：在全球尺度下，栖息地类型驱动了趋同的演化分化模式。