Trends in the sand: directional evolution in the shell shape of recessing scallops (Bivalvia: Pectinidae)

Directional evolution is one of the most compelling evolutionary patterns observed in macroevolution. Yet, despite its importance, detecting such trends in multivariate data remains a challenge. In this study, we evaluate multivariate evolution of shell shape in 93 bivalved scallop species, combining geometric morphometrics and phylogenetic comparative methods. Phylomorphospace visualization described the history of morphological diversification in the group; revealing that taxa with a recessing life habit were the most distinctive in shell shape, and appeared to display a directional trend. To evaluate this hypothesis empirically, we extended existing methods by characterizing the mean directional evolution in phylomorphospace for recessing scallops. We then compared this pattern to what was expected under several alternative evolutionary scenarios using phylogenetic simulations. The observed pattern did not fall within the distribution obtained under multivariate Brownian motion, enab...定向演化（directional evolution）是宏演化（macroevolution）中最为引人注目的演化模式之一。尽管其重要性不言而喻，但在多元数据中检测此类演化趋势仍是一项挑战。本研究针对93种双壳扇贝物种的壳形多元演化展开评估，整合了几何形态测量学（geometric morphometrics）与系统发育比较方法（phylogenetic comparative methods）。通过形态系统空间（phylomorphospace）可视化手段，本研究刻画了该类群的形态多样化历史；结果显示，具有潜居生活习性的类群在壳形上最为独特，且呈现出定向演化趋势。为实证检验这一假说，我们拓展了现有方法，针对潜居扇贝的形态系统空间内的平均定向演化特征进行量化表征。随后，我们通过系统发育模拟（phylogenetic simulations），将观测到的模式与多种备选演化场景下的预期结果进行对比。研究发现，观测到的模式并未落入多元布朗运动（multivariate Brownian motion）模拟所得到的分布范围内，从而为