Equilibrium speciation dynamics in a model adaptive radiation of island lizards.

The relative importance of equilibrium and nonequilibrium processes in shaping patterns of species richness is one of the most fundamental questions in biodiversity studies. If equilibrium processes predominate, then ecological interactions presumably limit species diversity, potentially through diversity dependence of immigration, speciation, and extinction rates. Alternatively, species richness may be limited by the rate at which diversity arises or by the amount of time available for diversification. These latter explanations constitute nonequilibrium processes and can apply only to biotas that are unsaturated or far from diversity equilibria. Recent studies have challenged whether equilibrium models apply to biotas assembled through in situ speciation, as this process may be too slow to achieve steady-state diversities. Here we demonstrate that speciation rates in replicate Caribbean lizard radiations have undergone parallel declines to equilibrium conditions on three of four major islands. Our results suggest that feedback between total island diversity and per-capita speciation rates scales inversely with island area, with proportionately greater declines occurring on smaller islands. These results are consistent with strong ecological controls on species richness and suggest that the iconic adaptive radiation of Caribbean anoles may have reached an endpoint.