156 I The Process of Evolution 



and dark and resembles the spring generation in southern locaHties. 

 In the northern populations the individuals are presumed to have 

 genotypes that produce the dwarfing and darkening. Although the 

 critical transfer experiments have not been done, the greater con- 

 stancy of the northern forms in the face of environmental changes 

 supports these presumptions. 



How can one account for the development of the high-altitude 

 races of plants, the high-latitude races of butterflies, and similar 

 phenomena? The environmentally produced changes cannot be 

 directly transmitted to succeeding generations; the Lamarckian 

 idea of the inheritance of acquired characters has long been dis- 

 carded. A method by which such acquired characters could, through 

 selection, become assimilated in the genotype has been proposed by 

 Waddington and is supported by a series of experiments by Wad- 

 dington and others. This genetic assimilation is best explained by a 

 brief example. 



A strain of a wild-type laboratory population of Drosophila 

 melanogaster was subjected to a high-temperature shock during the 

 pupal stage. A few of the adults emerging from the treated pupae 

 showed an abnormal break in the veins of the wings (the "cross- 

 veinless" phenotype). Only those individuals showing the acquired 

 crossveinless phenotype were used as parents for the next genera- 

 tion. After more than a dozen generations, the frequency of cross- 

 veinless flies from treated pupae was over 90 percent, and a few 

 crossveinless flies began to appear from untreated pupae of the 

 selected strain. Crossing these latter flies produced strains that had 

 a high frequency of crossveinless phenotypes in the absence of heat 

 shock. It would appear that an acquired character had become 

 heritable. Actually, in this experiment, selection seems to have fa- 

 vored those genotypes that had a low threshold for producing the 

 favored phenotype. Eventually the threshold was lowered to a point 

 at which no heat shock was necessary to move the developmental 

 sequence to the crossveinless end point. Similar results have been 

 obtained in studies of other characters in Drosophila. 



It is most important to remember that the range of possible viable 

 phenotypes is genetically determined and that selection may alter 

 this range so that phenotypes that previously could be induced by 

 the environment become genetically fixed as the only possible result 

 of gene-environment interaction. Selection may favor phenotypic 

 plasticity where butterflies face different environments in succeed- 

 ing generations. It may operate to produce a highly canalized de- 

 velopment leading to a "winning" phenotype in situations where the 



