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在生物学研究中广泛采用基于普氏叠印法的几何形态测量学（Procrustes-based geometric morphometrics）开展体型与形态的量化分析。许多生物结构，乃至绝大多数动物，均具备双侧对称性，且存在内部对称平面（亦称客体对称，object symmetry）。当研究目标并非量化不对称变异时，研究者通常仅对单侧（左侧或右侧）进行测量与分析，该方法已被数百项研究采用。该方法的隐含假设为：另一侧的相关信息存在冗余，因此仅分析单侧所得结果，与对包含全部左右标志点（landmarks）的完整结构进行分析所得结果完全一致。然而据笔者所知，这一假设的成立程度尚未得到任何探索与验证。本文将通过两个示例数据集展开分析，结果表明：在宏观进化层面的研究中，单侧分析与完整结构分析的一致性较高，但在微观进化层面，二者的一致性会显著降低，且形态量化结果尤其容易出现偏差。本文还将探讨若干可能影响分析结果的其他因素，并提出一项简易可行的优化方案，可同时提升单侧分析研究中形态分析的可视化效果与分析准确性。