Data from: Measuring selection when parents and offspring interact

Non-social and social selection gradients are key evolutionary parameters in systems where individuals interact. They are most easily obtained by regressing an individual's fitness on the trait values of the individual and its social partner.
In the context of parental care it is more common to regress the trait value of the parents (i.e. the social partner) on a ‘mixed’ fitness measure that is a function of the parent's and offspring's fitness (for example, the number of recruits, which equals parental fecundity multiplied by offspring survival).
For such an approach to yield correct estimates of net-selection, the trait must be sex-limited and not affect the parents’ own survival.
When a trait is not sex-limited, the non-social selection should be weighted by one (because all individuals express the trait) and social selection should be weighted by a half (because the relatedness between parents and the offspring they care for is a half, usually). The ‘mixed’ fitness approach does not give estimates of both components of selection and so they cannot be weighted appropriately.
We show that mixed fitness components are frequently used in place of direct fitness measures in the literature (37% of fecundity selection estimates use a mixed fitness approach), but that the frequency is much higher in some taxa, such as birds and mammals.
We suggest alternative methods that could be used to estimate both social and non-social selection gradients, while at the same time assessing the importance of unmeasured traits.

在个体间存在互作的演化系统中，非社会性选择（non-social selection）梯度与社会性选择（social selection）梯度均为核心演化参数。二者最简便的获取方式，是将个体适合度（fitness）对自身及社会伙伴的性状（trait）值进行回归分析。
在育幼行为（parental care）的研究语境下，更常见的做法是将亲本（即社会伙伴）的性状值，对由亲本自身适合度与后代适合度共同构成的“混合”适合度指标进行回归（例如补充个体数（recruits），其数值等于亲本繁殖力（fecundity）与后代存活率的乘积）。
若要通过此类方法获得准确的净选择（net-selection）效应估计值，该性状必须为性别限制性（sex-limited）性状，且不会影响亲本自身的存活能力。
若性状不具备性别限制性，则非社会性选择的权重应设为1（因所有个体均会表达该性状），而社会性选择的权重通常应设为1/2（因亲本与其抚育后代间的亲缘系数（relatedness）通常为0.5）。但“混合”适合度方法无法同时估算选择的两类组分，因此无法对其进行恰当加权。
本研究表明，文献中常以混合适合度组分替代直接适合度指标开展分析（约37%的繁殖力选择（fecundity selection）估计研究采用了混合适合度方法），且在鸟类、哺乳类等类群（taxa）中该方法的使用频率显著更高。
本研究提出了若干替代方案，可在估算社会性与非社会性选择梯度的同时，评估未测量性状的重要性。