Data from: Lost in the other half: improving accuracy in geometric morphometric analyses of one side of bilaterally symmetric structures

Systematists and evolutionary biologists have widely adopted Procrustes-based geometric morphometrics for measuring size and shape in biology. Many structures, and in fact most animals, are bilaterally symmetric with an internal plane of symmetry (also called object symmetry). Often, when quantifying asymmetric variation is not an aim, only one or the other side is measured and analysed. This approach has been used in hundreds of studies. Its implicit assumption is that the information on the other side is redundant and a single side will therefore produce results mirroring those one would have obtained from the analysis of the entire structure with all its left and right landmarks. However, the extent to which this assumption is met has, to my knowledge, never been explored. Using two example datasets, I will show that congruence may be high in analyses at a macro-evolutionary level but much lower at a micro-evolutionary one, and inaccuracies might especially affect shape. I will discuss some of the other factors that may influence results and will suggest a simple expedient that can improve both the visualization and accuracy of shape analyses in one-side-only studies.

系统分类学家与进化生物学家已广泛采用基于普氏叠加的几何形态测量学（Procrustes-based geometric morphometrics），用于开展生物学领域内体型与形态的量化分析。诸多生物结构——实际上绝大多数动物——均具备以内部对称平面为基准的两侧对称特征，此类对称亦称为物体对称（object symmetry）。在无需量化不对称变异的研究场景中，研究者通常仅对单侧结构进行测量与分析。该分析方法已在数百项研究中得到应用，其隐含假设为：对侧结构的信息存在冗余，因此仅分析单侧所得到的结果，与包含全部左右标志点的完整结构分析结果一致。然而据笔者所知，这一假设的适用范围尚未得到系统探究。借助两个示例数据集，本文将展示：宏观进化层面的分析中，单侧分析与完整结构分析的结果一致性或较高，但在微观进化层面的一致性则显著更低，且误差可能尤其会对形态分析结果造成影响。本文还将探讨其他若干可能影响分析结果的因素，并提出一项简便可行的优化方案，以提升仅单侧分析研究中形态分析的可视化效果与准确性。