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Unformatted text preview: Copyright 1994 Scientific American, Inc. S CIENTIFIC A MERICAN October 1994 85 S ome creators announce their in- ventions with grand &clat. God proclaimed, Fiat lux, and then ooded his new universe with bright- ness. Others bring forth great discov- eries in a modest guise, as did Charles Darwin in dening his new mechanism of evolutionary causality in 1859: I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection. Natural selection is an immensely powerful yet beautifully simple theory that has held up remarkably well, un- der intense and unrelenting scrutiny and testing, for 135 years. In essence, natural selection locates the mechanism of evolutionary change in a struggle among organisms for reproductive suc- cess, leading to improved t of popula- tions to changing environments. (Strug- gle is often a metaphorical description and need not be viewed as overt com- bat, guns blazing. Tactics for reproduc- tive success include a variety of non- martial activities such as earlier and more frequent mating or better cooper- ation with partners in raising ospring.) Natural selection is therefore a princi- ple of local adaptation, not of general advance or progress. Yet powerful though the principle may be, natural selection is not the only cause of evolutionary change (and may, in many cases, be overshadowed by oth- er forces). This point needs emphasis because the standard misapplication of evolutionary theory assumes that bio- logical explanation may be equated with devising accounts, often speculative and conjectural in practice, about the adap- tive value of any given feature in its original environment (human aggres- sion as good for hunting, music and re- ligion as good for tribal cohesion, for example). Darwin himself strongly em- phasized the multifactorial nature of evolutionary change and warned against too exclusive a reliance on natural se- lection, by placing the following state- ment in a maximally conspicuous place at the very end of his introduction: I am convinced that Natural Selection has been the most important, but not the exclusive, means of modication. N atural selection is not fully suf- cient to explain evolutionary change for two major reasons. First, many other causes are powerful, particularly at levels of biological orga- nization both above and below the tra- ditional Darwinian focus on organisms and their struggles for reproductive suc- cess. At the lowest level of substitution in individual base pairs of DNA, change is often eectively neutral and therefore random. At higher levels, involving en- tire species or faunas, punctuated equi- librium can produce evolutionary trends by selection of species based on their rates of origin and extirpation, whereas mass extinctions wipe out substantial parts of biotas for reasons unrelated to adaptive struggles of constituent species in normal times between such events....
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