THE ORIGIN OF VARIATION 



above in which selection has not produced more than subspecific differen- 

 tiation even wlien aided by ratlier complete isolation. 



There is no doubt that the majority of geneticists, as well as other stu- 

 dents of evolution, do not agree with Goldschmidt with respect to some 

 or all of his theses. But most will agree that he has amassed a body of 

 highly pertinent evidence which cannot be lightly dismissed. 



Perhaps these views are not so divergent as they seemed only a few 

 years ago. At the center is the theory of the gene, which Goldschmidt 

 conceived in physiological terms at a time when most geneticists were 

 thinking in morphological terms. The implications of physiology for mor- 

 phology and of morphology for physiology were not clear, and much 

 acrimonious debate further clouded the issue. Several recent develop- 

 ments have tended to clarify it. First, the great development of biochemi- 

 cal genetics in recent years has focussed the attention of geneticists upon 

 physiological aspects of the gene. Second, increased interest in pseudo- 

 alleles, whether interpreted as gradually diverging repeats or as different 

 mutational sites in broad genie fields, has tended to blur the limits of the 

 gene, and to leave the corpuscular theory as merely a first approximation. 

 Finally, much recent evidence points to desoxyribonucleic acid (DNA) 

 as the actual ojenic material, and Watson and Crick have devised a verv 

 clever model of its structure which readily relates genie action to a pattern 

 in a biochemical continuum. Nucleic acids are made of nucleotides, each 

 of which consists of a pentose (five-carbon sugar), phosphoric acid, a 

 base, which may be a purine [adenine(A), or guanine(G)] or a pyrimi- 

 dine [cytosine(C) or thymine(T)]. These are polymerized by sugar- 

 phosphate linkages to form long chains, on which the bases appear as side 



Figure 87. The Structure of 

 DNA AS Envisioned hy Wat- 

 son AND Crick. At the left is the 

 pattern of polymerization. At the 

 right, the two intertwined hel- 

 ices represent the sugar-phos- 

 phate part of the niacroniolccule, 

 while the crossbars represent 

 purine-pyrimide side chains 

 which are linked together by hy- 

 drogen bonds. (From Watson 

 and Crick, ^Nature, V. 171, 1953.) 



BASE — SUGAR 



D.NA. 



/ 



I 



\ 



BASE — SUGAR 



/ 



PHOSPHATE 



\ 



PHOSPHATE 



BASE — SUGAR 



/ 



\ 



PHOSPHATE 



BASF — SUGAR 



/ 

 \ 



PHOSPHATE 



BASE — SUGAR 



PHOSPHATE 



/ 



228 



