Genetic variance is not equal for all multivariate combinations of traits. This inequality, in which some combinations of traits have abundant genetic variation while others have very little, biases the rate and direction of multivariate phenotypic evolution. However, we still understand little about what causes genetic variance to differ among trait combinations. Here, we investigate the relative roles of mutation and selection in determining the genetic variance of multivariate phenotypes. We accumulated mutations in an outbred population of Drosophila serrata and analyzed wing shape and size traits for over 35,000 flies to simultaneously estimate the additive genetic and additive mutational (co)variances. This experimental design allowed us to gain insight into the phenotypic effects of mutation as they arise and come under selection in naturally outbred populations. Multivariate phenotypes associated with more (less) genetic variance were also associated with more (less) mutational variance, suggesting that differences in mutational input contribute to differences in genetic variance. However, mutational correlations between traits were stronger than genetic correlations, and most mutational variance was associated with only one multivariate trait combination, while genetic variance was relatively more equal across multivariate traits. Therefore, selection is implicated in breaking down trait covariance and resulting in a different pattern of genetic variance among multivariate combinations of traits than that predicted by mutation and drift. Overall, while low mutational input might slow evolution of some multivariate phenotypes, stabilizing selection appears to reduce the strength of evolutionary bias introduced by pleiotropic mutation.

对于所有多变量性状组合，遗传方差不相等。这种不平等，其中一些性状组合具有丰富的遗传变异，而另一些则很少，这使多变量表型进化的速率和方向产生了偏差。然而，我们仍然对导致性状组合之间遗传变异不同的原因知之甚少。在这里，我们研究了突变和选择在确定多变量表型的遗传变异中的相对作用。我们在锯缘果蝇的远交种群中积累了突变并分析了超过 35,000 只苍蝇的翅膀形状和大小特征，以同时估计加性遗传和加性突变（协）方差。这种实验设计使我们能够深入了解突变的表型效应，因为它们在自然远交种群中出现并受到选择。与更多（更少）遗传变异相关的多变量表型也与更多（更少）突变变异相关，这表明突变输入的差异导致遗传变异的差异。然而，性状之间的突变相关性强于遗传相关性，并且大多数突变方差仅与一种多变量性状组合相关，而多变量性状之间的遗传方差相对更均等。所以，选择与分解性状协方差有关，并导致多变量性状组合之间的遗传变异模式不同于突变和漂移预测的遗传变异模式。总体而言，虽然低突变输入可能会减缓某些多变量表型的进化，但稳定选择似乎降低了多效性突变引入的进化偏差的强度。