Origination and extinction components of taxonomic diversity: Paleozoic and post-Paleozoic dynamics

—Changes in genus diversity within higher taxa of marine animals on the temporal scale of a few million years are more strongly correlated with changes in extinction rate than with changes in origination rate during the Paleozoic. After the Paleozoic the relative roles of origination and extinction in diversity dynamics are reversed. Metazoa as well as individual higher taxa shift from one mode of diversity dynamics to the other. The magnitude of taxonomic rates, the relative variance of origination and extinction rates, and the presence or absence of a long-term secular increase in diversity all fail to account for the shift in importance of origination and extinction in diversity changes. Origination and extinction rates both tend to be diversity-dependent, but different modes of diversity-dependence may contribute to the change in diversity dynamics from the Paleozoic to the post-Paleozoic. During the Paleozoic, there is a weak tendency for extinction rates to be more diversity-dependent than origination rates, whereas after the Paleozoic the two rates are about equally diversity-dependent on average. Mike Foote. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637. E-mail: [email protected] Accepted: 21 June 2000 Components of Diversity Change during the Phanerozoic How does variation in diversity during the history of life depend on variation in origination and extinction rates? Any change in taxonomic diversity necessarily implies a difference between origination and extinction rates, but by itself it says nothing about the magnitude of the rates or how they may have changed individually. Although diversity can change if the underlying taxonomic rates are constant, these rates in fact appear to have fluctuated considerably over the scope of the Phanerozoic (Raup and Sepkoski 1982; Van Valen 1984; Raup 1991; Foote 1994; Sepkoski 1998). The central question I will address concerns the relationship between these fluctuations in origination and extinction rate and the corresponding changes in diversity. Does diversity change tend to result more from change in origination rate or change in extinction rate, or is there no systematic pattern? The answer has obvious implications for diversity dynamics (Alroy 1998). For example, one could imagine an extreme case in which extinction rate within a clade is approximately constant, perhaps for intrinsic biological reasons, while origination rate varies as a result of evolutionary innovations and ecological opportunities (Walker and Valentine 1984; Gilinsky and Bambach 1987). In such a case, most of the variation in diversity within a clade would be attributable to variation in origination rate. Conversely, if variation in diversity is correlated primarily with variation in extinction rate, this might imply a characteristic rate of origination that is offset by external perturbations leading to extinction. Whether temporal variation in diversity more clearly reflects variation in origination rate or variation in extinction rate is likely to be related to temporal and taxonomic scale. Some analyses of extant clades that are a few million years old have suggested that reconstructed branching patterns are best explained by a model in which there is no extinction (Hey 1992), although simulation studies have shown that it may be difficult to identify past extinction when only living representatives of the clade are studied (Hey et al. 1998; cf. Raup 1983; Nee et al. 1995; Harvey and Rambaut 1998). At the scale of animals as a whole over the Phanerozoic, there has been a net diversity increase since the Paleozoic, implying an increase in the difference between origination and extinction rates, but this increase has been accompanied by a decline in both rates (Raup 579 DIVERSITY DYNAMICS and Sepkoski 1982; Van Valen 1984). At the level of classes and orders over hundreds of millions of years, these higher taxa tend to experience a secular decline in family origination rate, but extinction rate may be relatively stationary (Van Valen 1985a; Van Valen and Maiorana 1985; Gilinsky and Bambach 1987). Thus, the long-term diversity history of many groups seems to be attributable to changes in origination rate rather than extinction rate. There is considerable variation in rate superimposed on these secular trends, however. It is this finer-scale variation on which I will focus. Analysis of genera and families within higher taxa suggests that variation in extinction rate was more important than variation in origination rate in determining diversity changes during the Paleozoic and that the relative importance of extinction and origination reversed from the Paleozoic to the Mesozoic. Note that the question of temporal variation which I address is distinct from that of variation among biologic groups. A few studies have attempted to factor differential diversification of related taxa into origination and extinction components. For example, Cheetham and Jackson (1996) suggested that two coeval genera of cheilostome bryozoans have had similar histories of origination, and that the difference in their respective species richness is therefore due to a difference in extinction rate. Conversely, Van Valen (1973) compared artiodactyl and perissodactyl mammals and found similar extinction rates, suggesting that the greater diversity of artiodactyls is attributable to a higher origination rate. Another related question concerns the relative importance of stages of allopatric speciation in net rates of origination. Allmon (1992) has cogently argued that differences in speciation rate may be better understood mechanistically if one investigates differences in rates of isolate formation, isolate persistence, and isolate divergence.