FIGURE 9.1 Similar losses of taxic diversity have very different implications for the loss of evolutionary history depending on the phylogenetic distribution of the extinctions. Three different scenarios are shown, at levels A, B, and C. (A) Seven taxa are lost (33% extinction) but the overall structure of the phylogeny is preserved. (B) An entire clade of seven taxa is pruned, but the remaining structure is preserved. (C) Six taxa are lost but this eliminates the deepest branching clades.

clades, each of which represents unique units with long evolutionary history. This simple example demonstrates how knowledge of the phylogenetic structure is essential to evaluating the amount of evolutionary history lost or at risk, and not surprisingly, conservation biologists have proposed several different metrics for measuring phylogenetic diversity (Vane-Wright et al., 1991; Faith, 1992a; Faith and Baker, 2006). Although some have argued that taxic diversity is a reliable proxy for phylogenetic diversity, empirical studies have convincingly demonstrated the need for phylogenetic analyses. A study of the plants of the fynbos of South Africa, for example, showed that generic richness is strongly decoupled from phylogenetic diversity (Forest et al., 2007). The most direct demonstration of the importance of a phylogenetic framework was a study showing that some 80% of the structure of the underlying phylogeny can survive even a 95% loss of species (Nee and May, 1997), if the extinctions are random. When the phylogenetic structure of an extinction is highly clustered, the effects on evolutionary history can be more severe (Purvis et al., 2000a).

Paleontologists have long recognized the unequal impact of past biotic crises on the disappearance of particular clades, including archaeocyathid sponges in the Early Cambrian; many trilobite clades and numerous problematica during the various Cambrian crises; trilobites, blastoids, and many smaller clades during the end-Permian mass extinction; conodonts at the end-Triassic event; and nonavian dinosaurs, ammonoids, and rudist bivalves during the end-Cretaceous mass extinction. Each such disappearance removed clades of considerable evolutionary distinctiveness. The application of phylogenetic analyses remains sufficiently new