Chapter 19 Summary

The per taxon rate of diversification equals the rate of origination (or speciation) minus the rate of extinction. Analyses of diversity in the fossil record require procedures to correct for biases caused by the incompleteness of the record. Some inferences about rates of diversification and speciation can also be made from time-calibrated phylogenies of living species.
The diversity of skeletonized marine animals has increased during the Phanerozoic, but at varying rates. Diversity appears to have increased in the Cambrian to an approximate equilibrium that lasted for most of the Paleozoic; then, after the mass extinction at the end of the Permian, it has increased, with interruptions and at varying rates, ever since.
The background rate of extinction (in between mass extinctions) has declined during the Phanerozoic, perhaps because higher taxa that were particularly susceptible to extinction became extinct early.
Five mass extinctions (at or near the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous) are recognized. These periods of high extinction rates have been followed by intervals of rapid origination of new taxa. Their diversification was probably released by the extinction of taxa that had occupied similar ecological space. Newly diversifying groups have sometimes replaced other taxa by direct competitive displacement, but more often they have replaced incumbent taxa after those taxa became extinct.
The increase in diversity over time appears to have been caused mostly by adaptation to vacant or underused adaptive zones (“ecological space”), and by the evolution of key adaptations. Diversity has also been affected by biological interactions, whereby new species are often used as resources by other species.
Both paleontological and phylogenetic evidence shows that the increase in diversity in most clades has been diversity-dependent. Such observations imply that diversity tends toward an equilibrium, but diversity seems nevertheless to increase, partly because new and specialized ways of living continue to evolve.