CHAPTER VI 

 SELECTION AND ISOLATION 



1. The Effect of the Selection 



As already pointed out, protein molecules appear to have the 

 individuality. Every protein molecule is a replica of a template mole- 

 cule, but it may be impossible to make a replica entirely equal to the 

 template ; presumably every replica may differ slightly from the latter 

 on some point or other. The degree and kind of such a difference may 

 give rise to the individual character in protein molecules. 



It is v^rell recognized that the molecules of even carefully prepared 

 samples of a single protein from a single source may differ perceptibly 

 in such diverse properties as molecular weight, charge density, com- 

 position, molecular configuration, solubility, and biological activity. 

 The conclusion is drawn that all protein preparations prepared so far 

 represent populations of closely related members of a family, not 

 collections of identical molecules (66a). 



Since genes consist of protein molecules, they should likewise 

 possess the individuality, and again since organisms are determined 

 in their character by the genes, they should likewise have the indivi- 

 duality even when they belong to one and the same species. Indivi- 

 dual variations may thus arise. 



Proteins will be changed in their structure by environmental effect 

 but the degree and perhaps also the direction of the change may 

 vary with the difference in the structure. In like manner, each indi- 

 vidual may change the character in different ways even under the 

 influence of the same environmental factor, and as a consequence varia- 

 tions will disperse. 



As stated above, organic evolution is considered to be raised by 

 the gradual change of genes. However, since even the individuals of 

 the same species are thus not endowed with entirely the same series 

 of genes, each may tend to undergo different changes, advancing into 

 different directions. An individual exhibiting an extreme character, 

 therefore, may have a gene tending to undergo an extreme change, 

 and extreme character may be further advanced if the environmental 

 effect, which has given rise to th^ character, continues to exist. 



