64 The Process of Evolution 



Of course, other nuclear constituents may have changed, altering 

 the translation of the code rather than the original information. 



Perhaps the greatest challenge facing embryologists today is the 

 exact elucidation of the mechanisms controlling differentiation. The 

 question is of more than casual interest to the evolutionist. If por- 

 tions of the genotype can somehow be turned on and ofiF ( as is sug- 

 gested by the "puffing" process ) , the operation of selection might be 

 quite different from that in a situation in which the entire genotype 

 always is operant. If a portion of the information that controls, say, 

 the color pattern of a caterpillar is somehow inactivated when the 

 adult tissues are differentiating, it might be possible for selection to 

 alter the larval color but not affect the adult in any way. Equally, 

 if genes affecting hair were inactivated in endoderm tissue, the form 

 and color of the hair could be changed without any effect on the gut. 

 If, on the contrary, such differential activity does not exist, then all 

 changes in genetic information would, to one degree or another, be 

 reflected throughout the life of the organism and at all stages in the 

 life history. In many cases the effect might be so small as to be lost 

 in the normal developmental "noise." In view of the known physical 

 dispersion in the genome of the genes affecting the same character 

 (e.g., the genes controlling wing characters in Drosophila melano- 

 gaster are not concentrated on any one chromosome), the high fre- 

 quency of pleiotropy, and the lack of a known mechanism to act as 

 an off-on switch for major portions of the genotype, it seems most 

 reasonable to assume that the genetic material available in most cells 

 of an organism is essentially identical (except in quantity in endo- 

 polyploid tissues). 



It is known that the cells in different tissues of the same organism 

 (each cell presumably containing identical genetic information) do 

 not have the same complexes of proteins. For protein synthesis, then, 

 there must be specific control mechanisms that regulate the quan- 

 tities of various gene products. 



An interesting model describing such a mechanism has been pro- 

 posed by Jacob and Monod. This model concerns the transcription 

 of the DNA code as discussed in Chap. 1. It is suggested that genes 

 may be classed as either structural genes or regulator genes. The 

 primary product of structural genes is messenger RNA, the synthesis 

 of which is a sequentially oriented process initiated at certain regions 

 of the DNA strands. These regions of initiation are called operators. 

 An operator may control the transcription of more than one struc- 

 tural gene. The adjacent genes controlled by one operator form a 

 unit of transcription, the operon. 



