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In the Light of Evolution: Volume II—Biodiversity and Extinction
and qualitatively, and the nature of each extinction influences the rate and pattern of evolutionary recovery from the catastrophe.
David Jablonski develops a somewhat similar theme in Chapter 10 by emphasizing the selectivity of mass extinctions with respect to potential risk factors such as body size, species richness, and geographic range. From a consideration of the fossil record for marine organisms (especially bivalve mollusks), the author concludes that every mass extinction event seems to show some degree of selectivity, but also that disproportionately high clade survivorship during mass extinction episodes is consistently associated with the size of the geographic range of genus-level clades. From this and other evidence, the author’s take-home message is that spatial considerations are fundamental to understanding the evolutionary dynamics of biodiversity, including a clade’s susceptibility to extinction and its potential for recovery and expansion following a mass extinction event. These findings have ramifications for the current biodiversity crisis because human activities are altering the geographic distributions of many taxa around the world.
In Chapter 11, John Alroy uses information from a recent web-based “Paleobiology Database” to revisit classical questions about the marine fossil record, such as: Do biotic turnovers occur in pulses that coincide with the boundaries between geological intervals? Did extinction rates decline during the Phanerozoic? Are biotic extinction rates more volatile than origination rates? Do large-scale extinctions exhibit a 26 Myr periodicity as some have claimed? Were the “Big Five” mass extinction events qualitatively distinct from lesser extinction episodes? Alroy’s provisional answers to some of these questions are unorthodox. For example, he suggests that the Big Five are merely the upper end of a continuous spectrum of extinction intensities, such that it is “a matter of taste whether to speak of the Big Five, the Big Three, or just the Big One….” The analyses yield empirical estimates of typical recovery times from mass extinctions. Alroy concludes that the rebound from the ongoing mass extinction will probably take between 15 and 30 million years, if past mass extinction events are any guide.
Moving closer to the present time, late-Quaternary extinctions heavily impacted large mammals especially. The last 50,000 years were witness to the extinction of approximately two-thirds of all genera and one-half of all species of mammal weighing more than 44 kg (about the size of a sheep). Causal factors for this megafaunal extinction have been much debated, with a leading hypothesis being human hunting (overkill) arguably augmented by habitat alteration and climate change. In Chapter 12, Anthony Barnosky examines the situation from the fresh perspective of historical tradeoffs in biomass. An inverse relationship between human biomass and nonhuman megafaunal biomass indicates that before the mass extinction,