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In the Light of Evolution: Volume II—Biodiversity and Extinction
Organismic traits can rise or fall according to the strength of their linkage to broad geographic range or other factors promoting survivorship through those bottlenecks, lending a highly stochastic element to the expansion or demise of individual adaptations or clades. Thinking about the present day, these linkages are unlikely to promote factors beneficial to, or even desirable for, humans or the ecosystems they hope to conserve (see Jackson, Chapter 1, this volume). Given that narrow-ranging genera cannot have wide-ranging species, the net effect must be to deplete specialists in favor of weedy generalists, but this pattern can be ameliorated by survival of clades whose broad ranges arise from the far-flung deployment of individually localized species.
Second, the fossil record is rich in regional extinction events of intermediate intensities, and these can provide insights into present-day biodiversity issues. For example, the Cenozoic history of today’s biodiversity hotspots and coldspots (relative to expectations for their latitudes, for example) may help to predict the potential of these regions to accommodate further diversification, or alternatively to be subject to biotic invasions.
Third, the rules of successful recovery are poorly known, but are important for our understanding of both the larger outlines of the history of life and the future of modern diversity. The inordinate production of evolutionary novelties during recoveries suggests that postextinction dynamics do not simply involve an immediate return to business as usual. At the same time the spatial heterogeneity of recoveries, with significant invasions driving some of the regional patterns, requires a more careful look at the dynamics if we want to avoid biotic homogenization even after the reduction of the pressures on the modern biota. This could be another highly fruitful area at the intersection of paleontology and conservation biology.
Fourth, invasion has always been an evolutionary fact of life (Vermeij, 2005), even across biogeographic barriers and against climate gradients. The out-of-the-tropics model suggests an evolutionary approach to modeling biotic responses to future climate changes and attests to the evolutionary consequences of the stresses on tropical biotas today. If the tropics are the engine of global biodiversity, then driving tropical populations into extinction will have a global effect, by cutting off the primary source of new taxa for all latitudes. Further, if invasibility is more closely tied to extinction than to diversity per se, then there is the possibility of a reversal of the diversity flow, increasing the influx of invaders from higher latitudes. A tropical diversity crisis, now or in the geologic past, has profound long-term evolutionary consequences at a truly global scale.
Simply comparing the magnitude of the extinction occurring today, which is undoubtedly severe, with ancient intensities is not the most fruit-