38 evolution: the ages and tomorrow 



environmental forces which are making the selective deci- 

 sions, we get the basic evolutionary procession. Perhaps 

 mutations are nothing more than nature's failure to dupli- 

 cate a given gene perfectly, something the chemist already 

 anticipates and hopes to direct in specific ways. Or, perhaps, 

 there is some underlying control of the process which es- 

 capes our notice. In any case, these changes to a molecule, 

 the irreducible unit of life, are fundamental to all that hap- 

 pens, not only in evolution, but in all the biological sciences. 

 There is a tremendous body of literature dealing with the 

 genetics of all kinds of organisms, including man, to support 

 this view. 



The primary source of variation is, then, the mutation of 

 the gene. In any given species, only a relatively few genes of 

 the total number controlling the individuals need be under- 

 going change, yet the possibility of new structural, physio- 

 logical and behavioral traits is enormous. For instance, in the 

 little fruit fly, whose genetics has been so thoroughly inves- 

 tigated, it is estimated by F. A. Shull that there are about 

 6,000 genes or 3,000 pairs and, if only one per cent of these 

 are undergoing change, the possible number of new com- 

 binations of genes in the whole species is the incredible total 

 of more than 200,000 billion. And this is only in the sim- 

 plest possible case. A great many gene and chromosome 

 factors later add to these possibilities. In the first place, 

 genes often have multiple effects on the organism. A single 

 gene may be effective in controlling several characteristics 

 at one and the same time; or many genes may cooperate in 

 producing a single characteristic. Chromosomes occur in 

 pairs, hence genes are paired; and there is often a multiplic- 

 ity of alternative genes (alleles) in the population at a given 

 location on a pair of chromosomes. A member of a pair of 

 genes may be either dominant or recessive or blend with its 

 allele. It is apparent that all this would add greatly to the 

 already staggering figure given above. But that is not all, for 

 nature has many ways of increasing variation. 



