Are more diverse parts of the mammalian skull more labile?

Morphological variation is unevenly distributed within the mammalian skull; some of its parts have diversified more than others. It is commonly thought that this pattern of variation is mainly the result of the structural organization of the skull, as defined by the pattern and magnitude of trait covariation. Patterns of trait covariation can facilitate morphological diversification if they are aligned in the direction of selection, or these patterns can constrain diversification if oriented in a different direction. Within this theoretical framework, it is thought that more variable parts possess patterns of trait covariation that made them more capable of evolutionary change, that is, are more labile. However, differences in the degree of morphological variation among skull traits could arise despite variation in trait lability if, for example, some traits have evolved at a different rate and/or undergone stabilizing selection. Here, we test these hypotheses in the mammalian skull usi...哺乳动物颅骨的形态变异分布并不均匀，部分区域的演化分化程度高于其余区域。学界普遍认为，此类变异模式主要由颅骨的结构组织所决定，而颅骨结构组织可通过性状协变异（trait covariation）的模式与幅度加以界定。若性状协变异模式与选择方向一致，则可推动形态演化分化；反之，若其方向与选择方向相悖，则会限制演化分化进程。在此理论框架下，学界普遍认为，变异程度更高的颅骨区域所拥有的性状协变异模式，赋予其更强的演化改变能力，换言之，这些区域具有更高的演化易变性（labile）。然而，即便存在性状演化易变性的差异，颅骨各性状间的形态变异程度仍可能出现分化——例如，若部分性状以不同的速率演化，或经历了稳定选择（stabilizing selection）。本研究以哺乳动物颅骨为研究对象，对上述假说进行检验……