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.