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.