The Biogeography of Adaptive Radiations and the Geographic Overlap of Sister Species

The biogeography of speciation and what can be learned about the past mode of speciation from current biogeography of sister species are recurrent problems in evolution. We used a trait- and individual-based, eco-evolutionary model to simulate adaptive radiations and recorded the geographical overlap of species during and after evolutionary branching (speciation). We compared the spatial overlap among sister species in the fully saturated community with the overlap at the speciation event. The mean geographic overlap at speciation varied continuously from complete (sympatry) to none (allopatry), depending on local and regional environmental heterogeneity and the rate of dispersal. The distribution of overlap was, however, in some cases considerably bimodal. This tendency was most expressed at large values of regional heterogeneity, corresponding to sharp environmental contrasts. The mean geographic overlap also varied during the course of a radiation, sometimes with a consistent negative trend over time. The speciations that resulted in currently observable end community sister species were therefore not an unbiased sample of all speciations throughout the radiation. Postspeciation range shifts (causing increased overlap) occurred most frequently when dispersal was high or when local habitat heterogeneity was low. Our results help us understand how the patterns of geographic mode of speciation emerge. We also show the difficulty in inferring the geographical speciation mode from phylogenies and the biogeography of extant species.