104 INTRODUCTION TO EVOLUTION 



tive during embryonic development, and this change in turn is preceded 

 by chemical change in a gene. This fact suggests the question: How do 

 chemical changes in genes cause changes in bodily processes? There is 

 evidence that genes control the nature of enzymes, those catalystlike sub- 

 stances essential to all living processes. When a gene changes, the enzyme 

 it conditions is modified accordingly, and hence the bodily process con- 

 trolled by the enzyme is altered. 



While all mutations are chemical mutations in the sense just stated, the 

 chemical mutations we stress at this point are those having as their princi- 

 pal effect an observable change in the chemistry of the body. Many addi- 

 tional examples are known. A change in a gene which conditions the 

 production of pigment material, melanin, results in the gene's failing in its 

 function. Presumably the changed gene fails to produce an essential en- 

 zyme. Consequently, if an individual inherits such a changed gene from 

 both parents, he will have no pigment, i.e., he will be an albino. Research 

 with the bread mold, Neiirospora, mentioned previously, has brought to 

 light mutations which cause failure in the synthesis of one or another of 

 the amino acids, vitamins, and other organic substances which normal 

 Neurospora can manufacture for itself (see Beadle, 1946). These are all 

 mutations having as their outward manifestations changes in the chemis- 

 try of the organism. 



Retinal Pigments, an Evolutionary Enigma 



The retinas of our eyes contain two kinds of light-sensitive cells, the 

 rods and the cones. The rods function in dim light, the cones in bright 

 light. The rods contain a sensitive pigment commonly called "visual pur- 

 ple" but better termed rhodopsin. It is rose-colored, rather than purple, 

 and it bleaches rapidly when exposed to light. Its properties are impor- 

 tant in the visual process. 



Rhodopsin is found in the eyes of marine fishes, frogs, turtles, birds, and 

 mammals. Evidence, direct and indirect, also indicates its presence in eyes 

 of invertebrates. In fresh-water fishes, however, it is replaced by a differ- 

 ent substance, one actually purple in color, called porphyropsin. Both 

 rhodopsin and porphyropsin are formed in part from vitamin A, different 

 forms of the latter being involved in the two instances. Vitamin A belongs 

 to the class of compounds known as carotenoids, a group of fat-soluble 

 pigments varying from yellow to red and occurring widely in both plant 

 and animal tissues. The principal pigment in carrots is one of them. Wald 

 (1946) has presented evidence that response to light throughout both plant 



