Data from: Selection on parental performance opposes selection for larger body size in a wild population of blue tits

There is abundant evidence in many taxa for positive directional selection on body size, and yet little evidence for microevolutionary change. In many species, variation in body size is partly determined by the actions of parents, so a proposed explanation for stasis is the presence of a negative genetic correlation between direct and parental effects. Consequently, selecting genes for increased body size would result in a correlated decline in parental effects, reducing body size in the following generation. We show that these arguments implicitly assume that parental care is cost free, and that including a cost alters the predicted genetic architectures needed to explain stasis. Using a large cross-fostered population of blue tits, we estimate direct selection on parental effects for body mass, and show it is negative. Negative selection is consistent with a cost to parental care, mainly acting through a reduction in current fecundity rather than survival. Under these conditions, evolutionary stasis is possible for moderately negative genetic correlations between direct and parental effects. This is in contrast to the implausibly extreme correlations needed when care is assumed to be cost free. Thus, we highlight the importance of accounting correctly for complete selection acting on traits across generations.

诸多分类群（taxa）中已积累了充足证据，表明生物体型存在正向定向选择，但却鲜有证据能证明其发生了微演化变化。在诸多物种中，生物体型的变异部分由亲本的行为所决定，因此有研究者提出，演化停滞的一种合理解释是，直接效应与亲本效应之间存在负向遗传相关（genetic correlation）。据此，为增大体型而选择的基因会引发亲本效应的协同下降，进而降低下一代的生物体型。我们的研究表明，这类论证隐含假定亲本抚育无需付出代价，而将抚育成本纳入考量后，解释演化停滞所需的遗传架构预测结果将发生改变。我们依托一个大型交叉抚育（cross-fostered）的蓝山雀（blue tits）种群，估算了针对亲本效应的体重直接选择，并证实其为负值。这种负向选择与亲本抚育的成本相符，该成本主要通过当前繁殖力的降低而非存活率下降产生作用。在此情形下，即便直接效应与亲本效应间仅存在中等程度的负向遗传相关，演化停滞也有可能发生。这与假定抚育无成本时所需的极端不合理的相关程度形成鲜明对比。因此，我们强调了正确考量跨世代性状所受完整选择压力的重要性。