Density-Dependent Cladogenesis in Birds
(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.