Zelditch Miriam Leah, Carmichael Christopher
Department of Zoology and Michigan State University Museum, Michigan State University, East Lansing, MI, 48824.
Evolution. 1989 Jul;43(4):814-824. doi: 10.1111/j.1558-5646.1989.tb05179.x.
Patterns of variation and covariation within populations can influence how characters respond to natural selection and random genetic drift and so constrain the ability of natural selection to modify the phenotype. We examined several potential developmental and functional explanations of character covariation throughout ontogeny using known-age samples of the cotton rat (Sigmodon fulviventer) to identify the causes of covariation and to assess the variability of patterns of covariation throughout postnatal growth. Competing developmental and functional models were fit to samples of orofacial and neurocranial measures by confirmatory factor analysis and evaluated for their ability to reconstruct observed variance-covariance matrices. Samples of successive ages were simultaneously fit to a common model to test the hypothesis that the patterns of developmental and functional integration were invariant between ages. Orofacial characters derived from the same branchial-arch primordium covary early in ontogeny. Subsequently, there is a repatterning of integration that may reflect a transition from developmental to functional sources of integration. Neurocranial characters exhibit even more variation in patterns of covariation: initially, characters appear to comprise a single integrated unit; before puberty, they appear to respond to localized bone growth; after puberty, they form separate calvarial and basicranial components. This ontogenetic variation in patterns of covariation suggests that developmental constraints are transient and flexible and that the consequences of selection may depend upon the age at which it acts.
种群内部的变异和协变模式会影响性状对自然选择和随机遗传漂变的响应方式，从而限制自然选择改变表型的能力。我们使用棉鼠（Sigmodon fulviventer）已知年龄的样本，研究了整个个体发育过程中性状协变的几种潜在发育和功能解释，以确定协变的原因，并评估产后生长过程中协变模式的变异性。通过验证性因子分析，将相互竞争的发育和功能模型拟合到口面部和神经颅骨测量样本上，并评估它们重建观察到的方差协方差矩阵的能力。将连续年龄的样本同时拟合到一个共同模型中，以检验发育和功能整合模式在不同年龄之间不变的假设。源自同一鳃弓原基的口面部性状在个体发育早期协变。随后，整合模式会重新形成，这可能反映了从发育性整合源到功能性整合源的转变。神经颅骨性状在协变模式上表现出更多变化：最初，性状似乎构成一个单一的整合单元；青春期前，它们似乎对局部骨骼生长做出反应；青春期后，它们形成单独的颅盖和颅底部分。协变模式中的这种个体发育变异表明，发育限制是短暂且灵活的，选择的后果可能取决于其作用的年龄。
