The history of the biosphere: Equilibrium and non-equilibrium models of global diversity

Species diversity on the earth has increased through time. The fossil record suggests that the increase may have occurred in several stages, with perturbations caused by mass extinctions and widespread radiations. Various models for global taxonomic diversification have been proposed: equilibrium models where there is a fixed number of species that the earth can support at any time; and non-equilibrium models of two hinds, in which either an equilibrium level exists, but is never reached, or in which there is no equilibrium number at all. At present, all three models can explain the data, although the equilibrium models have been heavily criticized. Detailed reassessments of the fossil record may indicate which models are more probably correct.