Metrics For The Loss Of Evolutionary History

The traditional accounting method for the loss of evolutionary history is taxa: populations and species for biologists, often genera or families for paleontologists because the vagaries of preservation and correlation make species-level compilations impractical. Conservation biologists have long focused on species, an approach enshrined in the U.S. Endangered Species Act. This reliance on taxa tends to assume, implicitly, that taxonomic entities are a reliable metric to the impact of extinction on ecosystem structure and function, morphological variability, behavior complexity, and developmental processes. This assumption is often far from true. Consequently, conservation biologists have proposed other metrics for identifying critical targets for conservation (Purvis and Hector, 2000), including biogeographic centers of endemic taxa, or hotspots (Meyers et al., 2000), and the characterization of phylogenetic diversity (Faith, 1992a; Faith and Baker, 2006) and evolutionary distinctiveness (Vane-Wright et al., 1991). There is, however, a more important reason for considering the loss of other aspects of evolutionary history, and that is the search for mechanisms underlying patterns of extinction and construction of biodiversity. Ecologists increasingly recognize the importance of a network of interactions in generating biodiversity, including positive feedback relationships among biodiversity, productivity, and stability (Worm and Duffey, 2003; Montoya et al., 2006).

Although paleontologists are aware of the diversity of effects on evolutionary history caused by past extinctions, particularly mass extinctions, we have been slow to develop and apply comparative metrics beyond taxic compilations and estimates of geographic range. Enough work has been done to suggest a range of alternative metrics. Biogeographic structure is an important aspect of evolutionary history that has been considered elsewhere (Jablonski, 2007).

Taxic Diversity

The divisions of the geologic timescale are framed by biotic crises recognized by early geologists as "revolutions" triggering wholesale changes in the biota. Paleontologists have since compiled records of fluctuations in taxonomic diversity for marine taxa (Sepkoski, 1984, 1997), terrestrial plants (McElwain and Punyasena, 2007), vertebrates (Benton, 1989), and various microfossil groups (Rigby and Milsom, 2000). Patterns of extinction and origination have received considerable attention, particularly the decline in "background" extinction rates through the Phanerozoic for marine families and genera (Flessa and Jablonski, 1985) and episodic events of increased extinction. Curiously, as the English geologist John Phillips understood as long ago as the 1840s, extinctions within geologic stages appear pulsed, rather than spread out through the stage (Foote, 2005). Within clades paleontologists have also identified intriguing patterns of replacement where successive subclades replaced earlier clades. For higher-resolution analyses statistical techniques have been developed to account for sampling problems [see Jin et al. (2000) for an application to the end-Permian mass extinction].

Several general lessons emerge from these compilations. First, the persistent decline in extinction rates suggests an increased stability in younger taxa, although this may be a statistical artifact of increased species/genus and species/family ratios (Flessa and Jablonski, 1985). It would be of considerable interest to know whether this apparent increased robustness is real and whether it translates into some of the other metrics described below. Second, patterns of subclade replacement can suggest adaptive

174 / Douglas H. Erwin improvement within the activities of the larger clade, a pattern confirmed by the power of incumbency (Rosenzweig and McCord, 1991). Third, as demonstrated by a recent analysis of Cenozoic mollusks from New Zealand, species and genera exhibit a limited interval of peak abundance, followed by a long decline to extinction. In this system at least, the species at greatest risk of extinction are those already in decline (Foote et al., 2007), although this does not appear to hold true during mass extinctions that may truncate ranges (Foote, 2007). Fourth, mass extinction events periodically upset these patterns, and particularly at the end-Permian mass extinction, trigger pervasive changes in patterns of ecological and evolutionary dominance. Thus over evolutionary time, episodic extinctions has been an important driver for evolution.

Understanding the processes controlling long-term changes in diversity requires identifying and correcting for biases in the fossil record that can be introduced by preservation and sampling. Consequently paleontologists have developed new approaches designed to identify and correct for such biases (Smith, 2001; Crampton et al., 2003; Foote, 2003, 2007; Peters, 2005). These techniques have been applied to correct for biases in our record of the end-Ordovician mass extinction (Krug and Patzkowsky, 2004). As discussed by Alroy (Chapter 11, this volume), the diversity patterns produced by this intensive compilation of taxic diversity largely follow those of Sepkoski's earlier work (Sepkoski, 1982, 1992, 1993). This effort identifies at least three of the five canonical mass extinctions below. However, like other work (Bambach et al., 2002, 2004) it raises questions about the magnitude of other extinction events.

Finally, counting taxa, whether species, genera, or families, assumes that each taxon is equivalent, which is far from true when one considers the differences in diversity or abundance within different groups, much less their evolutionary distinctiveness, morphologic disparity, ecological function, or evolutionary potential.

0 0

Post a comment