Data from: Selection on parental performance opposes selection for larger body mass 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）中均存在大量证据表明体型受到正向定向选择（positive directional selection），但鲜有证据显示存在微进化改变（microevolutionary change）。在诸多物种中，体型变异部分由亲本行为所决定，因此有研究者提出进化停滞（evolutionary stasis）的一种解释：直接效应与亲本效应间存在负向遗传相关（genetic correlation）。据此，筛选可提升体型的基因会引发亲本效应的相关衰减，进而降低下一代的个体体型。我们的研究表明，这类论证均暗含了亲本抚育无需付出代价的假设；若将抚育成本纳入考量，则解释进化停滞所需的预测性遗传架构将发生改变。我们依托一个大型蓝山雀（blue tits）交叉抚育（cross-fostered）种群，针对体重这一性状的亲本效应估算了其所受的直接选择强度，结果显示该选择为负值。这种负向选择与亲本抚育的成本相符，该成本主要通过降低当前繁殖力（fecundity）而非存活率产生作用。在此条件下，当直接效应与亲本效应间存在适度负向遗传相关时，即可实现进化停滞。这与假设抚育无成本时所需的、极不合理的极端遗传相关形成鲜明反差。综上，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。