Reading9_Orr_Selection

Reading9_Orr_Selection - Testing Natural Selection...

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44 SCIENTIFIC AMERICAN January 2009 GRANT DELIN ome ideas are discovered late in the history of a scientific discipline because they are subtle, complex or otherwise difficult. Natural selec- tion was not one of these. Although compared with other revolutionary scientific ideas it was discovered fairly recently—Charles Darwin and Alfred Russel Wallace wrote on the subject in 1858, and Darwin’s On the Origin of Species appeared in 1859—the idea of natural selection is simplicity itself. Some kinds of organisms sur- vive better in certain conditions than others do; such organisms leave more progeny and so become more common with time. The environ- ment thus “selects” those organisms best adapt- ed to present conditions. If environmental con- ditions change, organisms that happen to pos- sess the most adaptive characteristics for those new conditions will come to predominate. Dar- winism was revolutionary not because it made arcane claims about biology but because it sug- gested that nature’s underlying logic might be surprisingly simple. In spite of this simplicity, the theory of natu- ral selection has suffered a long and tortuous his- tory. Darwin’s claim that species evolve was rap- idly accepted by biologists, but his separate claim that natural selection drives most of the change was not. Indeed, natural selection was not accepted as a key evolutionary force until well into the 20th century. The status of natural selection is now secure, reflecting decades of detailed empirical work. But the study of natural selection is by no means complete. Rather—partly because new experi- mental techniques have been developed and partly because the genetic mechanisms underly- ing natural selection are now the subject of me- ticulous empirical analysis—the study of natural selection is a more active area of biology than it was even two decades ago. Much of the recent experimental work on natural selection has fo- cused on three goals: determining how common it is, identifying the precise genetic changes that give rise to the adaptations produced by natural selection, and assessing just how big a role natu- ral selection plays in a key problem of evolution- ary biology—the origin of new species. Natural Selection: The Idea The best way to appreciate evolution by natural selection is to consider organisms whose life cycle is short enough that many generations can be observed. Some bacteria can reproduce them- selves every half an hour, so imagine a popula- tion of bacteria made up of two genetic types that are initially present in equal numbers. Assume, moreover, that both types breed true: type 1 bac- teria produce only type 1 offspring, and type 2 bacteria produce only type 2s. Now suppose the environment suddenly changes: an antibiotic is introduced to which type 1s are resistant but to which type 2s are not. In the new environment, KEY CONCEPTS Charles Darwin’s theory that evolution is driven by natu- ral selection—by inherited changes that enhance sur- vival—struggled against
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Reading9_Orr_Selection - Testing Natural Selection...

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