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What does the dreaded "E" word mean, anyway
Natural History, Feb, 2000 by Stephen Jay Gould
Interdisciplinary unification represents a grand and worthy goal of intellectual life, but greater understanding can often be won by principled separation and mutual respect, based on clear definitions and distinctions among truly disparate processes, rather than by false unions forged with superficial similarities and papered over by a common terminology In our understandable desire to unify the sciences of temporal change, we have too often followed the Procrustean strategy of enforcing a common set of causes and explanations upon the history or a species and the life or a star--partly, at least, for the very bad reason that both professions use the term "evolution" to denote change through time. In this case, the fundamental differences trump the superficial similarities-and true unity will be achieved only when we acknowledge the disparate substrates that, taken together, probe the range of possibilities for theories of historical order.
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The Darwinian principle of natural selection yields temporal change--evolution in the biological definition--by the twofold process of producing copious and undirected variation within a population and then passing along only a biased (selected) portion of this variation to the next generation. In this manner, the variation within a population at any moment can be converted into differences in mean values (average size, average braininess) among successive populations through time. For this fundamental reason, we call such theories of change variational as opposed to the more conventional, and more direct, models of transformational change imposed by natural laws that mandate a particular trajectory based on inherent (and therefore predictable) properties of substances and environments. (A ball rolling down an inclined plane does not reach the bottom because selection has favored the differential propagation of moving versus stable elements of its totality but because gravity dictates this result when round balls roll down smooth planes.)
To illustrate the peculiar properties of variational theories like Darwin's in an obviously caricatured, but not inaccurate, description: Suppose that a population of elephants inhabits Siberia during a warm interval before the advance of an ice sheet. The elephants vary, at random and in all directions, in their amount of body hair. As the ice advances and local conditions become colder, elephants with more hair will tend to cope better, by the sheer good fortune of their superior adaptation to changing climates--and they will leave more surviving offspring on average. (This differential reproductive success must be conceived as broadly statistical and not guaranteed in every case: in any generation, the hairiest elephant of all may fall into a crevasse and die.) Because offspring inherit their parents' degree of hairiness, the next generation will contain a higher proportion of more densely clad elephants (who will continue to be favored by natural selection as the climate becomes still colder). This process of increasing average hairiness may continue for many generations, leading to the evolution of woolly mammoths.
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