358 INTRODUCTION TO EVOLUTION 



vidual mutations but also to those combinations of mutations (genes) dis- 

 cussed earlier in the chapter (pp. 334-336). 



Origin of Races and Species 



So far we have not accounted for a great amount of evolutionary 

 change; we have shown how a population might come to possess a new 

 mutation or combination of mutations. But we have noted previously that 

 one mutation is usually a small change. Few, if any, races or subspecies 

 differ from each other by a single mutation. How can we account for the 

 origin of larger differences such as those distinguishing separate races and, 

 especially, separate species, genera, and so on? Addition of one mutation 

 to another probably accounts for many of these larger differences. Our 

 hypothetical population acquires a certain favorable characteristic, as de- 

 scribed above. In later generations a second mutation arises which is an 

 improvement on, or addition to, the first one. Natural selection now works 

 on this second mutation until some generations later the whole population 

 comes to possess it. Thus step by step through the long expanses of geo- 

 logic time greater and greater evolutionary change is produced by natural 

 selection. The change as we have described it will be in the nature of 

 more perfect adaptation to the environment in which the animals are \i\- 

 ing, i.e., postadaptation (pp. 12-13). 



Continuing with our hypothetical population, let us suppose that the ani- 

 mals' environment changes — (1) as a result of geologic change in the re- 

 gion or (2) as a result of the animals' migration into a different region 

 from that formerly inhabited. Now a premium may be placed on differ- 

 ent characteristics from those formerly favored. As a result different muta- 

 tions will prove advantageous in the "struggle for existence," and in con- 

 sequence the population will gradually come to differ from its ancestors 

 living under the conditions formerly prevailing. Thus diversity arises be- 

 tween a population living in one set of environmental conditions and ances- 

 tral or "sister" populations living in other environments. The amount of 

 diversity will at first be slight, but it may increase until the populations be- 

 come separate subspecies, and even eventually separate species, genera, 

 and so on. 



Earlier in this chapter we saw how the evolution of the long legs of the 

 horse might be explained according to Lamarck's theory of the inherit- 

 ance of acquired characters (p. 341). It may help to fix in mind the es- 

 sentials of natural selection if we ask how the same evolutionary change 

 can be explained by the theory of natural selection. 



