Conclusions
We conducted a strong test of the generality of isolation effects, by
comparing geographic and macroclimatic distance effects in paired island
and paired mainland populations within the same species, on a diversity
of response variables measured on a range of taxa. Our results suggest
that while eco-evolutionary pressures that shape phenotypic traits are
likely to differ more between different islands than between mainland
populations, they do not cause consistent between-population differences
in neutral genetic diversity across island and mainland systems. These
findings have deep implications for future models of population
variability at biogeographic scales, which we show can be improved by
considering the spatial structure of species’ habitats in addition to
the commonly employed predictors of environmental conditions or
geographic distances between populations. While small marine islands are
situated at the extreme end of a spatial isolation continuum, they can
serve to understand the interacting causes of spatial population
variability globally. Our findings could may also be useful guides in
conservation decisions. The spatial extent of protected areas could be
tailored to preserve system-dependent biological processes, thus larger
areas may be necessary to preserve similar levels of phenotypic
variability in homogeneous than in spatially more structured habitats.