Dispersal heterogeneity, rather than dispersal evolution, drives biodiversity patterns

Island biodiversity emerges from a fundamental tension between colonization and divergence. Dispersal enables lineages to reach new habitats and exploit ecological opportunity, but simultaneously increases gene flow that can inhibit the genetic differentiation required for speciation. Although dispersal is widely recognized as a key driver of biodiversity patterns, most theoretical studies treat it as a fixed species attribute, making it difficult to distinguish the effects of dispersal evolution, standing dispersal variation, and the spatial genetic structure generated by dispersal inheritance. Here, we develop a spatially explicit individual-based model in which dispersal is a heritable quantitative trait and compare three dispersal scenarios: fixed dispersal, heritable evolving dispersal, and dispersal reshuffling that maintains trait variation while removing local inheritance. Across structured (archipelago-like) and unstructured landscapes, we evaluate how these different components of dispersal shape colonization, diversification, and long-term community dynamics. We find that dispersal heterogeneity is a stronger determinant of biodiversity than dispersal evolution itself. In structured landscapes, both evolving and reshuffled dispersal substantially increase species richness, spatial turnover, and island colonization relative to fixed dispersal, whereas in unstructured landscapes dispersal variability reduces richness. Speciation and extinction rates remain closely balanced across scenarios, indicating that differences in diversity emerge primarily through changes in colonization and spatial organization rather than through altered diversification rates. When dispersal evolves, newly formed species follow two distinct trajectories, characterized by either increasing or decreasing dispersal capacity after speciation. Contrary to expectations from island-syndrome theory, the relative frequency of these trajectories is only weakly associated with landscape connectivity. Our results show that individual variation in dispersal capacity fundamentally alters diversification dynamics by creating opportunities for both colonization-driven and reduced-gene-flow pathways to speciation. Incorporating dispersal heterogeneity into island biogeography therefore provides a more mechanistic framework for understanding how biodiversity emerges and persists in spatially structured landscapes.