Fig. 3.

Within-clade directionality parameter distributions within three hierarchical clade levels. The directionality parameter, μ_step, is the maximum-likelihood estimate for the mean rate of directional size change (log₁₀ ml/Myr) within a brachiopod clade. Large clades [classes (n = 4) and orders (n = 11)] all have statistically positive distributions consistent with Cope's rule, whereas small, constituent clades (families, n = 10) are indistinguishable statistically from zero tendency (SI Appendix, Table 5). Distributions were estimated by using Gaussian kernel density estimation with shared bandwidth (0.00361); maximum-likelihood estimates are available in SI Appendix, Table 3. Similarly distinct distributions occur in the joint directionality parameters for each clade level and when the parameters for DRW, URW, and stasis models are combined by using multimodel inference (23, 24).