Diversification rates and the latitudinal gradient of diversity in mammals

Abstract

The latitudinal gradient of species richness has frequently been attributed to higher diversification rates of tropical groups. In order to test this hypothesis for mammals, we used a set of 232 genera taken from a mammalian supertree and, additionally, we reconstructed dated Bayesian phylogenetic trees of 100 genera. For each genus, diversification rate was estimated taking incomplete species sampling into account and latitude was assigned considering the heterogeneity in species distribution ranges. For both datasets, we found that the average diversification rate was similar among all latitudinal bands. Furthermore, when we used phylogenetically independent contrasts, we did not find any significant correlation between latitude and diversification parameters, including different estimates of speciation and extinction rates. Thus, other factors, such as the dynamics of dispersal through time, may be required to explain the latitudinal gradient of diversity in mammals.

Figure 1.

Distribution of diversification rates of genera present in each latitudinal 15° band. Results are shown for (a) the diversification rates for the 232 monophyletic genera extracted from the mammalian supertree [24], estimated using the method of Magallon & Sanderson [42]; (b) the diversification rates for the 100 genera whose trees where specifically reconstructed for this study, estimated using the method of Magallon & Sanderson; and (c) the diversification rates for these 100 genera, estimated by fitting a constant birth–death model. The box plots show median values (bold lines) and quartile values. Notches denote 95% confidence intervals (overlapping notches suggest that medians do not differ significantly). Outliers are defined as points beyond 1.5 times the interquartile range and are indicated by white circles. White diamonds represent means.

Figure 2.

Regression analysis of standardized phylogenetically independent contrasts between latitude and diversification parameters estimated by fitting (a–c) a constant birth–death model and (d–i) a variable birth–death model with exponential variation in speciation and constant extinction. Results are shown for (a) diversification rate, (b) speciation rate, (c) extinction rate, (d) initial diversification rate, (e) initial speciation rate, (f) extinction rate, (g) final diversification rate, (h) final speciation rate and (i) α. Regression coefficients (R²) and corresponding p-values are displayed on the top right of each panel. Regression lines fitted by least squares forcing regression through the origin are also shown.