Equilibrium speciation dynamics in a model adaptive radiation of island lizards

Abstract

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.

Fig. 1.

(A–E) Reconstructed speciation-through-time curves under five diversification models fitted to the Anolis phylogeny (Fig. 2, Fig. S1). Green curves denote models without state-dependent diversification; other colors describe speciation dynamics on Cuba, Hispaniola, Jamaica, and Puerto Rico. Models are sorted (left to right, top to bottom) from lowest to highest conditional probability (Table 1). Models with island-specific changes in speciation (D and E) account for P = 0.995 of the total probability of the data taken across all models. The maximum-likelihood estimate of the dispersal rate between islands under the IslandVariable model is shown in F. Rates are shown in relative time units, as the tree was scaled to basal divergence of 1.0.

Fig. 2.

Anolis MCC tree with reconstructed island occupancy probabilities and lineage accumulation curves for Cuba (red), Hispaniola (blue), Jamaica (purple), and Puerto Rico (orange). Occupancy probabilities on internal nodes were estimated under the overall best-fit model (IslandVariable). The MCC tree with all taxon labels is shown in Fig. S1.

Fig. 3.

Island-specific rate-decline parameters as a function of island area for (from left to right) Puerto Rico, Jamaica, Hispaniola, and Cuba. The rate decline parameter is the slope of the relationship between speciation and time (−λ₀/K). Confidence intervals reflect uncertainty in tree reconstruction and represent the 0.025 and 0.975 percentiles of the distribution of parameter estimates taken across the posterior distribution of trees sampled with BEAST.

Fig. 4.

Distribution of species richness for islands of the Greater Antilles predicted under (Top to Bottom) GlobalConstant, IslandConstant, GlobalVariable, and IslandVariable models. Colored lines denote observed species richness on each island; black lines and histograms represent mean values and distributions tabulated from 2,000 datasets simulated under maximum-likelihood parameter estimates for each model. The GlobalVariable model underpredicts species richness on Hispaniola and overpredicts richness on both Jamaica and Puerto Rico relative to the IslandVariable model.