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NORMS OF REACTION: DO THEY INCLUDE MOLECULAR EVENTS? BRUCE WALLACE* Within any system of logic, there are truths that cannot be demonstrated, andfalsehoods that cannot be revealed.—Paraphrase of Goedel's Proof. [1, p. 1668] In his book Factors ofEvolution, Schmalhausen [2] stresses several important concepts: norms of reaction [3], adaptive norms, and morphoses . Every genotype, according to Schmalhausen, "is characterized by its own specific 'norm of reaction,' which includes adaptive modifications of the organism to different environments." Lewontin [4, 5] has stressed the importance of norms of reaction. These norms place severe constraints on the interpretation of results obtained through analyses of variance, as Lewontin [4] demonstrated in his classic account, "The Analysis of Variance and the Analysis of Causes." Adaptive norms are adaptive modifications that involve striking, discontinuous transformations of the organism. Schmalhausen mentions ecophenotypes as examples. Further examples to be found in elementary texts are provided by (1) the aquatic and terrestrial forms of plants such as Sagittaria and Ranunculus, (2) the diapausing and nondiapausing eggs produced by many species of insects, and (3) the sexual and asexual generations of aphids and rusts. To these examples one can add temperature -dependent sex determination in sea turtles [6] and predatorinduced phenotypic alterations [7]. Morphoses are changes induced in organisms by environmental stresses that rarely, ifever, have been previously encountered during the history of the species. Ionizing radiation was a frequently cited example The author expresses gratitude to Joseph Falkinham, Ernst Mayr, David West, and an anonymous reviewer for their many suggestions. *Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg , Virginia 24061.© 1990 by The University of Chicago. All rights reserved. 0031-5982/90/3303-0692$01 .00 Perspectives in Biology and Medicine, 33, 3 ¦ Spring 1990 \ 323 during the 1940s and 1950s; military, medical, and industrial uses of shortwave, high-energy radiation made it a novel component of the earth's ecology, at least at levels to which organisms became exposed. Among novel environmental stresses, Schmalhausen also included rarely encountered extremes of such standard variables as temperature, humidity, and pH. Morphoses are reactions to stresses that lack a history; unlike adaptive norms or norms of reaction, they may not be adaptive. Adaptive modifications are the present-day expressions of morphoses that accidentally conferred a selective advantage (through continued survival and subsequent reproduction) on individuals and their descendants who, because of their particular genotypes, multiplied within the species through natural selection. The above account places Schmalhausen squarely within the Modern Synthesis of the 1940s. That synthesis embraced genetics [8], systematics [9, 10], and paleontology [H]. Dobzhansky, in his foreword to Schmalhausen [2], states: "The book of I. I. Schmalhausen advances the synthetic treatment of evolution starting from a broad base of comparative embryology, comparative anatomy, and the mechanics of development . It supplies, as it were, an important missing link in the modern view of evolution." The examples of adaptive modifications that were cited by Schmalhausen were appropriate for his time. Continued abrasion of the skin results in calluses, adaptive structures that lessen the risk of injury and infection. Exercise leads to enlarged muscles and an increased cardiac capacity. Such examples, cited as norms of reaction, emphasized that genetic programs rather than specific individuals are the ultimate target of selection. The proof of a pudding is in the eating. Individuals, like servings of puddings, are consumed. The successful recipe, the appropriate information, the properly organized and functioning genetic program persists to the eventual exclusion of other recipes and other programs that prove wanting. Schmalhausen's thesis can be extended into the world of microbes and molecules. The manner in which Escherichia coli utilizes lactose serves as an excellent example. Glucose is the preferred carbon source for this organism. When individual cells are transferred from a glucosecontaining medium to one containing only lactose, several minutes elapse before they gain the ability to utilize the new sugar. This acquisition involves making available for transcription a segment of chromosomal DNA (the he operon) that codes for (among others) an enzyme (ßgalactosidase ) capable of splitting the lactose molecule into the two simple sugars, glucose and galactose. In the absence of lactose, transcription of the lac operon is blocked by a protein repressor...

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Additional Information

ISSN
1529-8795
Print ISSN
0031-5982
Pages
pp. 323-334
Launched on MUSE
2015-01-07
Open Access
No
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