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Keeping Pace

Thompson Webb III

Ecology and Evolution: The Pace of Life. K. D. Bennett. 241 pp. Cambridge University Press, 1997. $65 cloth, $24.95 paper.

Evelyn Hutchinson spoke of the ecological theater and its evolutionary play—a connection that Keith Bennett in Ecology and Evolution: The Pace of Life says began with Darwin. Because this theater exists at several time and space scales, we can ask about which scales of ecological settings and what processes contribute most when speciation is the main act. Were Darwin and the originators of the Modern Synthesis correct in their claim that microevolutionary processes could be extrapolated out in time to explain speciation? Not so simple, said Stephen Jay Gould when he described how a search of the fossil record had failed to show "... any clear vector of fitfully accumulating progress [within species on evolutionary time scales of 105 to 106 years], despite expectations that processes regulating [microevolution on the ecological time scale of less than 103 years] should yield such advance." For him this failure represented a paradox and "... our greatest dilemma for the study of pattern in life's history." Bennett, a Quaternary paleoecologist, first offered a solution to Gould's paradox in 1990 and has now elaborated the arguments into a book. He draws on Quaternary research covering the last 20,000 to 2,000,000 years and argues that the paradox is explained by the population shifts and many local extinctions resulting from orbitally induced climate changes with periods of 10,000 to 100,000 years.

Bennett starts by reviewing the development of evolutionary theory from the ideas of Lyell and Darwin through to the Modern Synthesis writing of Dobzhansky, Mayr and Simpson and up to more recent ideas. He next describes the theory and evidence for orbitally driven climate change and its biological consequences. Following Mayr, Bennett lists how Darwin inferred the struggle for existence from three facts that generally apply for standard ecological time scales: the potential for exponential increase in populations, the observed steady-state stability of populations, and the limitation of resources. Bennett notes how the biological consequences of orbitally paced climate changes undercut facts two and three and necessitate a different set of key processes for the struggle for existence over time scales of 10,000 to 1,000,000 years than the processes critical for microevolution.

His full argument uses the following facts and conclusions from the fossil record: Variations in the earth's orbit have induced significant variations in climate at periods of 20,000 to 100,000 years throughout geological time; plant and animal species change individualistically (i.e., independently from one another) in distribution and abundance in response to these orbitally driven climate changes; these independent changes mean that the communities—within which ecologists imagine microevolution as occurring—last for several millennia at most; species have lifetimes of 106 to 107 years and therefore experience 100 or more of the orbitally driven climate changes and the associated isolation and/or mixing of populations along with community changes; so, each species has faced a variety of competitive environments during its lifetime; but the microevolutionary adaptations to the short-term competitive settings in different communities are wiped out by the longer-term climate variations. Microevolutionary changes thus may result in no "clear vector of fitfully accumulating progress" over time scales of 10,000 to 1,000,000 years. The expectations from an understanding of how orbitally driven changes affect the biosphere fit with Gould's observation from the fossil record that no progress is evident on time scales intermediate between the ecological and the evolutionary, and these expectations remove his observation from the realm of paradox.

So far geological evidence has shown that life varies, the earth varies and climate varies. Bennett's book fuses the first and last of these to show how climate change affects macroevolutionary rates. It also sets the stage for choosing the appropriate temporal and spatial scales for the ecological theater in which speciation versus microevolution is the main act. That species survive the orbitally driven climate changes but become extinct over time scales involving what must be significant consequences from tectonic changes gives some hint for the plot of the real play here, and, at even longer time scales, mass extinctions catastrophically alter the cast and the script. A narrow focus on microevolution has led many to miss the main show.—Thompson Webb III, Geological Sciences, Brown University

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