R script 3. R code used to conduct modularity and integration analyses. from Morphological variation under domestication: how variable are chickens?

The process of domestication has long fascinated evolutionary biologists, yielding insights into the rapidity with which selection can alter behaviour and morphology. Previous studies on dogs, cattle and pigeons have demonstrated that domesticated forms show greater magnitudes of morphological variation than their wild ancestors. Here, we quantify variation in skull morphology, modularity and integration in chickens and compare those to the wild fowl using three-dimensional geometric morphometrics and multivariate statistics. Similar to other domesticated species, chickens exhibit a greater magnitude of variation in shape compared with their ancestors. The most variable part of the chicken skull is the cranial vault, being formed by dermal and neural crest-derived bones, its form possibly related to brain shape variation in chickens, especially in crested breeds. Neural crest-derived portions of the skull exhibit a higher amount of variation. Further, we find that the chicken skull is strongly integrated, confirming previous studies in birds, in contrast to the presence of modularity and decreased integration in mammals.

驯化过程长期以来一直是进化生物学家的重点关注对象，为理解选择如何快速改变生物的行为与形态提供了关键洞见。此前针对犬类、牛类及鸽类的研究已证实，驯化类群的形态变异程度显著高于其野生祖先。本研究采用三维几何形态测量学（three-dimensional geometric morphometrics）与多元统计分析方法，量化家鸡的头骨形态、形态模块性（modularity）及形态整合性（integration）变异，并将其与野生原鸡进行对比。结果显示，与其他驯化物种一致，家鸡的形态变异程度高于其野生祖先。家鸡头骨中变异程度最高的区域为颅顶，该区域由膜骨与神经嵴源性骨构成，其形态或与家鸡的脑部形态变异密切相关，尤其在带有冠饰的品种中。头骨中由神经嵴发育而来的区域表现出更高的变异水平。此外，本研究发现家鸡头骨具有显著的形态整合性，这验证了此前针对鸟类的相关研究结论，与哺乳类动物中存在形态模块性且整合性降低的特征形成鲜明对比。