NATURAL SELECTION: II 501 



rates of evolution in specific groups of organisms is almost completely lack- 

 ing. Different evolutionary lines are observed to differ in rate of change, 

 and one evolutionary line may be observed to undergo alteration in rate of 

 change during the course of its history. But there is little clear evidence 

 that differences or changes in mutation rate underlie the differences and 

 changes in rate of evolution. Indeed, there is some evidence to the con- 

 trary (cf. Stebbins, 1949). At the same time we must remember that our 

 present actual knowledge of mutation rates in wild populations of plants 

 and animals is still very fragmentary. 



An interesting discovery in this connection is that of genes which in- 

 crease the rates at which other genes undergo mutation. The possible evo- 

 lutionary significance of such genes is discussed by Ives (1950), who de- 

 scribes a high-mutation-rate gene in Drosophila. This gene has the effect of 

 increasing the mutation rates of other genes about tenfold. Such genes 

 increase the rate of supply of "raw materials" and hence might under some 

 conditions affect the rate of evolution. 



External Factors 



On the whole, evidence available seems to indicate that differing rates of 

 change are more dependent upon external factors than they are upon inter- 

 nal ones, always provided, of course, that the mutation rate is adequate to 

 furnish the raw materials. Analysis of factors involved in determination of 

 rates of change is outside the province of an elementary discussion. Read- 

 ers are particularly referred to the basic treatment accorded the subject by 

 Simpson in his The Major Features of Evolution ( 1953 ) as well as his more 

 general discussion in The Meaning of Evohaion (1949a). Statement of a 

 few general principles and conclusions which seem justified by present 

 knowledge must suffice us here. 



Such quantitative studies as have been possible on rates of evolution 

 among prehistoric animals seem to indicate that there is an average rate of 

 evolution approximated by many evolutionary lines. Simpson has used the 

 term horotelic to designate average rate of evolution. Other evolutionary 

 lines group themselves around a lower mean rate of evolution, termed 

 bradytelic. Clearly, cockroaches, mentioned on a preceding page (p. 164) 

 as having undergone little change since the Pennsylvanian period, are 

 bradytelic. Many forms have changed little over long periods of time dur- 

 ing which other forms underwent great changes. Oysters were practically 

 the same 200 million years ago as they are today. The opossum has changed 

 but little from the closing days of the dinosaurs down to the present. 



