Spatial isolation is a key driver of population-level variability in
traits and genotypes worldwide. Geographical distance between
populations typically increases isolation, but organisms face additional
environmental barriers when dispersing between suitable habitat patches.
Despite the predicted universal nature of the causes of isolation,
global comparisons of isolation effects across taxa and geographic
systems are few. We assessed the strength of isolation due to geographic
and macroclimatic distance for paired marine island and paired mainland
populations within the same species. Our meta-analysis included
published measurements of phenotypic traits and neutral genetic
diversity from 1832 populations of 112 plant and animal species at a
global scale. As expected, phenotypic differentiation was higher between
marine islands than between populations on the mainland, but spatial
patterns of neutral genetic diversity did not vary between the two
systems. Geographic distance had comparatively weak effects on the
spatial patterns of phenotypes and neutral genetic diversity, but only
phenotypic trait variability showed signal of system-dependence. These
results suggest that spatial patterns of phenotypic variation are
determined by system-dependent eco-evolutionary pressures, while the
spatial variability of neutral genetic diversity might be universal. Our
approach demonstrates that global biodiversity models that include
island biology studies may progress our understanding of the interacting
effects of spatial habitat structure, geographic- and environmental
distances on biological processes underlying spatial population
variability. We formulate future research directions for empirical tests
and global syntheses in the field.