6 - Patterns over evolutionary time, present mass extinctions

Summary

This chapter is not intended to give a detailed discussion of all we know about eras of stasis and radiation in evolutionary history, but concentrates on a few recent studies that provide evidence for saturation (equilibrium) and nonsaturation (nonequilibrium) in evolutionary history. It also gives a brief account of human impact on diversity.

The fossil record and interpretations

There have been three marine evolutionary floras and faunas, i.e., the Cambrian, Paleozoic and Modern, each with its own degree of diversity, and each subsequent one with higher diversity than the previous one (Jablonski and Sepkoski 1996). Benton (1995, 1998), re-analysing fossil evidence, has shown that there has been an exponential increase in the number of families of continental and marine organisms in geological time to the Recent. Courtillot and Gaudemer (1996) analysed the same data, and arrived at the somewhat different conclusion that equilibria were reached several times but re-established at higher levels after each mass extinction; but they still found an increase over geological time. Jablonski (1999, see also Jackson and Johnston 2001) has shown that fossil data are solid and that the general trend of increasing diversity has not been changed by more recent data collected between 1982 and 1992 (Figure 6.1): there was a sharp rise in diversity in the Cambrian, a Paleozoic plateau interrupted by several mass extinctions, and a sharp rise since the Triassic, also interrupted by several extinction events.