Density-dependent cladogenesis in birds

Abstract

A characteristic signature of adaptive radiation is a slowing of the rate of speciation toward the present. On the basis of molecular phylogenies, studies of single clades have frequently found evidence for a slowdown in diversification rate and have interpreted this as evidence for density dependent speciation. However, we demonstrated via simulation that large clades are expected to show stronger slowdowns than small clades, even if the probability of speciation and extinction remains constant through time. This is a consequence of exponential growth: clades, which, by chance, diversify at above the average rate early in their history, will tend to be large. They will also tend to regress back to the average diversification rate later on, and therefore show a slowdown. We conducted a meta-analysis of the distribution of speciation events through time, focusing on sequence-based phylogenies for 45 clades of birds. Thirteen of the 23 clades (57%) that include more than 20 species show significant slowdowns. The high frequency of slowdowns observed in large clades is even more extreme than expected under a purely stochastic constant-rate model, but is consistent with the adaptive radiation model. Taken together, our data strongly support a model of density-dependent speciation in birds, whereby speciation slows as ecological opportunities and geographical space place limits on clade growth.

Figure 1. The Relationship between γ and Clade Size and Age from Pure Birth Simulations

Plot of the γ statistic versus (A) clade size and (B) clade age in 10,000 simulated datasets of the pure birth model. Clade size simulations were run for 18 time units and clade age simulations were run until clades contained 50 species (both using a birth rate of 0.2). The solid line illustrates the relationship between γ and (A) clade size and (B) clade age, fitted using a cubic smoothing spline to capture non-linearity.

Figure 2. The Observed Relationship between γ and Clade Size and Age across Bird Phylogenies

(A) Plot of the γ statistic versus clade size, where clade size is the total number of lineages estimated to be present 2 Mya for 45 bird clades (Table 1). All values below the dashed line are significant at p < 0.05. The least squares regression slope is b = −1.10 ± 0.24, p < 0.001. (B) Plot of the γ statistic versus clade age. Circles and triangles represent passerine and nonpasserine clades, respectively.