60 I The Process of Evolution 



ingful biologically only when it is translated through contact with 

 the environment. Indeed, the value of the information is judged only 

 by the translation, not the original. Natural selection operates on the 

 phenotype, not directly on the genotype, which merely determines 

 the responses of the developing organism to the environment. 



Only in recent years have evolutionists given proper attention to 

 the processes of development that result in the production of an adult 

 functioning organism from a fertilized egg or zygote. These processes 

 are interrelated to form a system which Waddington has termed 

 the epigenotype. This may be visualized as a branching system of 

 developmental pathways, each of which leads to one of the compo- 

 nents of the adult form. Because the biochemical reactions deter- 

 mining each path are so interlocked with one another ( as discussed 

 above, there is a strong tendency for the normal end result to 

 be produced even when there is considerable disturbance at 

 early stages. Thus the paths are canalized or buffered as a result 

 of feedback or cybernetic mechanisms interconnecting the 

 paths. 



This epigenetic system must have been the result of natural selec- 

 tion acting upon the genes that afiFect more or less directly the ex- 

 pression of particular characteristics of organisms. However, selec- 

 tion also must have involved the many genes that have as their 

 only obvious phenotypic effect the modification of the expression of 

 other genes. Waddington has pointed out that, in a population of 

 organisms in a given environment, each individual will have its own 

 genotype, and therefore its own epigenotype, which will eventually 

 result in the adult phenotype. Selection to preserve fitness in this 

 particular environment may act to eliminate genotypes that produce 

 deviant phenotypes. It may also act to eliminate individuals that are 

 imperfectly buffered against environmental eflFects. There would 

 thus be selection for a well-canalized epigenetic system. 



Should the environment change, some well-buffered individuals 

 would be likely to respond by producing fit phenotypes without the 

 necessity for immediate genotypic change. After a period of time in 

 the new environment, however, genotypic change is inevitable, and 

 it is to be expected that selection would lead to the stabilization of 

 the new developmental paths. When the organisms are returned to 

 their original environment, it would be found that, as a result of this 

 change in the genotype, they no longer produce their original pheno- 

 type. Thus what was originally a phenotypic (actually epigenetic) 

 response to environmental change becomes incorporated into the 

 genotype, as a result of selection for a well-buffered developmental 



