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In the Light of Evolution: Volume II—Biodiversity and Extinction
tion of standing diversity alone, but of regional extinction rates, suggesting a significant role for incumbency [see also Vermeij (2005)]. The polar mollusks may ultimately have come out of the tropics as well, but if so this must have occurred before the 11-Myr window presently available (which would be consistent with the much older genus ages seen at the poles). The interplay of extinction, origination, and immigration is complex, and of course it need not be in a steady state.
Terrestrial animals may well follow a different dynamic (Hawkins et al., 2007b; Weir and Schluter, 2007; Wiens, 2007a). Marine organisms can move down the continental shelf when ice forms at the surface, but terrestrial animals, plants, and fungi do not have that luxury when confronted with a kilometer-thick ice sheet. High-latitude extinction and recolonization are thus almost certainly more important factors on land. Whatever the spatial dynamic near the poles, however, the tropics appear to be a crucial reservoir for biodiversity, with a subset of low-latitude clades expanding out of the tropics over geological timescales. Although this pattern is most readily detected in the shallowest part of the geologic record, thus falling entirely within times away from the major mass extinctions, some evidence suggests that postextinction recoveries are also fueled by the tropics, on land (Kerp et al., 2006) and in the oceans (Jablonski, 1998; Brayard et al., 2006; Krug and Patzkowsky, 2007). The tropics thus appear to be key to the generation and maintenance of global biodiversity across a wide range of boundary conditions.
I have touched on four spatial aspects of ancient extinctions that should be integrated with theoretical and applied approaches to the present-day biota. The fossil record amply demonstrates that the spatial fabric of extinction has profoundly shaped the biosphere. First, broad geographic range probably always buffers clades from extinction, but it becomes most important and clear-cut as the suite of other factors that enhance species and genus survival during normal times become ineffective. It is not yet clear whether the selectivity regime changes steadily with increasing extinction intensity or as a step function (Jablonski, 2005). More intense extinctions may tend to be less selective, which might explain the failure of intrinsic factors to predict extinction risk in the some of the most heavily stressed elements of the modern biota, such as freshwater fishes, amphibians, and Australian marsupials (Duncan and Lockwood, 2001; Fisher et al., 2003).
This shift to a strong spatial component in survivorship during major extinction events greatly increases the likelihood of hitchhiking effects.