Determinants of phenotypic difference between populations
The log ratio values for the phenotypic traits included in the analysis
ranged from 0, indicating cases with no difference between populations,
to 3.7 which, when back transformed from log space, corresponds to a
ratio of approximately 39:1 between populations for the given measure.
As expected, we found higher differences in phenotypic traits between
island populations, than between mainland populations, mainland
populations having a log ratio 0.189 lower than island populations (mode
= -0.19, 95%CI = -0.37, -0.01, Table 1a, Fig. 3, Table S6.1a). This
difference in the level of variation corresponds to a ratio of trait
values of approximately 1.44:1 between island populations and 1.15:1 for
mainland populations. We found some support for an interaction between
system type (island or mainland) and geographic distance, i.e.
phenotypic variability between populations tended to increase with
increasing geographic distance on the mainland, but it was constant for
all geographic distances on islands. This effect size corresponds to
variation between mainland populations approximately 500 km apart
matching the variation found between island system populations at any
distance (Table 1a, Table S6.1a, Fig. S6.2a). The effect of other
variables was even weaker (Table 1a, Table S6.1a). Across the random
terms included in the model, most of the variation was associated with
the residual terms, less variation was associated with the study or the
species, and very little effect was attributed towards the phylogenetic
term and response type (Table 1a). The zero adjusted models (Table
S6.1b, Table S6.3a), the reduced model that included phenotypic traits
associated with size (Fig. S6.4a) and models which excluded the
macroclimatic or geographic distance (Table S6.5a, Table S6.6a) produced
qualitatively similar results to the main phenotypic model.