424 INTRODUCTION TO EVOLUTION 



evidence that genetic assimilation of an acquired character is possible. For 

 example, if pupae of the fruit fly, Drosophila, are subjected to a heat shock 

 a certain proportion of them will fail to develop the cross-veins normally 

 present in the wings. By selecting cross-veinless individuals as parents and 

 continuing the treatment and selection in each generation Waddington 

 produced a strain in which the percentage of individuals responding to the 

 heat shock was very high. "It is therefore actually possible to select for 

 capacity to respond to the environment." Eventually strains were produced 

 which lacked the cross-veins even when the pupae were not given heat 

 shocks. The trait originally induced by the environment had become 

 genetically assimilated. In this case it is possible that mutations for cross- 

 veinlessness occurred in the stock and were selected. But Waddington 

 pointed out that such an explanation is not necessary and is rendered un- 

 likely by the fact that the cross-veinless strain difl'ered from the original 

 stock by differences in several genes rather than in one gene pair only. 



In a second series of experiments (Waddington, 1956) an abnormality 

 of the thorax called bithorax was induced by subjecting Drosophila eggs to 

 ether vapor. Again, selection resulted in strains having increased sensitivity 

 to the external stimulus. Three different strains were produced in which 

 some form of the bithorax phenotype occurred even in the absence of the 

 ether treatment. Two of these strains differed from the original stock by 

 single-gene changes; these may have arisen as dominant mutations appear- 

 ing at the right time to satisfy requirements imposed by the artificial selec- 

 tion. The third strain, showing greater phenotypic change, differed from the 

 original stock by more complex genetic differences and was interpreted by 

 Waddington as an instance of genetic assimilation of the acquired character 

 bithorax. 



Genetic assimilation as a possible factor in evolution is an idea of such 

 interest that it will undoubtedly be investigated extensively. An allied con- 

 cept is that which has come to be known as "the Baldwin effect." Various 

 forms of the concept have been advanced, sometimes independently, by a 

 number of investigators; the history of it was summarized by Simpson 

 (1953b). According to this idea, an organism may invade an environ- 

 ment if it is adaptable enough to do so (if its genotype gives a norm of re- 

 action making possible the necessary exogenous adaptation). In other 

 words, it may meet the requirements of the environment by adjustments or 

 accommodations it is able to make in response to that environment. This is 

 valuable; the animal has gained a "toe hold" in the new environment and is 

 able to live there, but each generation is under the necessity of developing 

 its own adaptive characteristics. How much better it would be if the ani- 



