GENETIC CHANGE AND EVOLUTIONARY CHANGE 405 



reached the original red color was but slightly modified. Here we see one 

 mutation with the two effects listed above: it affected the time at which 

 the reaction occurred and it affected the rate of that reaction. 



Of the metabolic processes operative during embryonic development 

 particular interest centers on those affecting growth of the parts and organs 

 of the developing individual. Parts and organs grow at different rates, 

 hence genes controlling these relative rates must have great potency in de- 

 termining the nature of the resulting adult. If so, mutations of genes con- 

 cerned with differential growth could be highly important in producing the 

 variability upon which natural selection operates. Most of the examples 

 of differential growth rates known at present have not been analyzed 

 genetically. We can scarcely doubt, however, that rate-genes, probably 

 complex systems of them, are involved. 



DIFFERENTIAL GROWTH RATES 

 (ALLOMETRY) 



The study of differential growth rates of the kind just 

 mentioned is called allometry (or originally by Huxley, heterogony). Al- 

 though we may seldom think of it, the fact that different parts of the body 

 grow at different rates, and that these rates change from time to time, is a 

 matter of everyday observation. A glance at Fig. 4.14 (p. 66) will remind 

 us that in the human fetus the head grows much more rapidly than does 

 the rest of the body, the legs especially having a slow rate of growth. After 

 birth the situation changes. An adult man is far from being a newborn baby 

 with all parts of the body increased equally in size. Such an adult would be 

 a most ungainly creature: enormous head, large trunk, and short, crooked 

 legs. After birth the head continues to increase in size but at a deceler- 

 ated rate, while the lower portions of the body, particularly the legs, grow 

 at an increased rate. 



Differential growth of one part as compared to another is common in the 

 animal kingdom. Fig. 18.1 shows an example among males of a certain 

 species of beetle. In this species small males have large forelegs as com- 

 pared to males of most beetles. But larger males of the species have fore- 

 legs that are proportionately much larger than are those of the smaller 

 males. In other words, a little increase in body size is accompanied by 

 much increase in foreleg size. Thus the growth rate of the forelegs must be 

 greater than that of the body as a whole. 



In many cases the differential growth rate of one part as compared to 



