Phenotypic selection analysis
To quantify phenotypic selection on traits, multiple regression was used
to provide estimates of selection gradients, whereas individual
univariate models were used to provide estimates of selection
differentials. Selection gradients (β) describe the strength of unique
or direct selection acting on a trait (after controlling for inter-trait
correlations), whereas selection differentials (S) describe ‘total’
selection (Lande & Arnold 1983). For the analysis of β, a multiple
regression model was fitted in which relative fitness (seed number
divided by its mean) was regressed on standardized traits (standardised
by standard deviation) (Lande & Arnold 1983). We also added to the
model the two-level factors of ‘pollination’ (control, hand pollination)
and ‘herbivory’ (addition, removal), and their interaction with all
traits and each other. In this way estimates of β were output for all
traits in all four treatment combinations. Following Sletvold (2019),
the calculated difference in trait β between treatment combinations
thereby provided estimates of pollinator-mediated selection (in the
presence and absence of herbivory), herbivore-mediated selection (with
and without pollinator limitation), and combined pollinator- and
herbivore-mediated selection. We used the ‘emtrends’ function of R
package emmeans (Lenth et al. 2018) to calculate these
differences in β, and to test whether the result differed significantly
from zero (after adjusting p-values for multiple comparisons via
Benjamini–Hochberg correction). Although the multiple regression
included only 27 genotypes for which complete trait data was available,
we still considered these inferences to be robust given that: 1) the
full spectrum of genetic variation in defence was represented (see
above), and 2) no qualitative differences were observed in univariate
regressions regardless of whether some (n = 27) or all (n = 81)
genotypes were used (Table S2). For the analysis of S, univariate
regressions were fitted for each trait individually. Only data from the
‘control pollination/herbivore present’ treatment combination were used
for the estimation of S, as this treatment combination is typically
considered most representative of natural population conditions
(Sletvold 2019).