Global Variation in the Diversification Rate of Passerine Birds

Net diversification rates were estimated for samples of primarily tribe-to-family-level clades of passerine birds, taking into account extinction as well as speciation. Two samples were used. The first consisted of 37 clades of primarily temperate North American and primarily tropical South American passerines; the second comprised a global set of 90 clades, each distributed within one or more zoogeographic regions. Circumscription and ages of clades were taken from Sibley and Ahlquist's phylogeny based on DNA hybridization, with updates from more recent sequence analysis. Under a homogeneous speciation (rate = lamda) and extinction (rate = mu) process, the expected number of species (N) after t units of time can be described by the expression, N(t)= exp(lamda(1 - kappa) t - kappa/(1 - kappa), where kappa = mu/lamda. A nonlinear least-squares regression for the temperate and tropical American clades with more than one species estimated kappa = 0.938 +/- 0.076 (mean +/- SE), suggesting a high rate of turnover of lineages within clades. Because of the broad confidence limits in kappa, I used values ranging from 0.80 to 0.98 to calculate speciation rates in subsequent analyses, assuming that kappa is uniform among clades and does not vary with latitude. Speciation rate among South American clades exceeded that among North American clades for all kappa, whether monophyletic lineages were included or not. The estimated speciation rate was negatively related to clade age, suggesting that proliferation within clades slows with time. In the global data set, rate of speciation decreased with clade age and increased with the area of the region or regions within which a clade is distributed, and for any given value of kappa the speciation rate was significantly higher in tropical than in temperate regions. Relaxing the assumption of latitude independence in kappa, larger clade size in the tropics could be achieved by various combinations of relative speciation and extinction rates that obscure the underlying causes of global biodiversity patterns. Nonetheless, the results of this analysis clearly indicate that a higher rate of diversification in the tropics contributes to the pervasive latitudinal gradient in diversity observed in passerine birds.