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slope. We know little about whether the slope and intercept change continuously or shift abruptly at thresholds. For obvious reasons, including some very practical ones relating to the present-day biota, this would be good to know.

We also know relatively little about the determinants of geographic range size at the clade level. Organismic traits such as dispersal ability and ecological strategy must play a role, but interactions with biogeographic context, clade history, and many other factors, including the way that clades extend their ranges by speciation across barriers, serve to decouple geographic ranges at the species and clade levels. For example, the geographic ranges of the 213 marine bivalve genera present today at shelf depths on the eastern Pacific margin from Point Barrow, Alaska to Cape Horn, Chile are not significantly related to the median or maximum ranges of their respective constituent species (Jablonski, 2005). Genera can attain broad ranges via a few widespread species, a mosaic of nonoverlapping but narrow-ranging species, or any combination thereof, each apparently equivalent in a mass extinction event (although this equivalence deserves further study). Genus ranges are not simply species attributes writ large, but involve a dynamic that is set at another hierarchical level, by the complexities of speciation, species extinction, and range expansion.

This strong spatial component to extinction selectivity suggests that survival can be determined by features that are not tightly linked to the organismic and species-level traits that are favored, indeed shaped, during times of lower extinction intensities. Even well-established clades or adaptations could be lost simply because they are not associated with those few features that enhance survivorship during unusual, and geologically brief, high-intensity events. As discussed below, the removal of incumbents and the subsequent diversification of formerly marginal taxa are essential elements of the evolutionary dynamic fueled by major extinctions [see also Erwin (2001), Jablonski (2001, 2005), and Erwin, Chapter 9, this volume].

These results also suggest that hitchhiking effects may be mistaken for direct selectivity more often than generally appreciated. Biological traits tend to covary, even across hierarchical levels, and so selection on one feature can drag others along with it, hampering efforts to pinpoint cause and byproduct. Such hitchhiking was detected for bivalve species richness in Table 10.1. Whenever widespread or restricted taxa tend to occupy nonrandom regions of phenotype space, for example in body sizes, trophic habits, or metabolic rates within a major group, hitchhiking becomes a real possibility, an interesting interaction across hierarchical levels where the extinction probabilities of organism-level characters are conditioned by clade-level properties (Jablonski, 2007). For example, the rudist bivalves of the Cretaceous seas (Order Hippuritoida) represent an extreme repat-



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