436 INTRODUCTION TO EVOLUTION 



human individual. But a species, or a man, who continually gambled 

 everything upon single spins of the wheel of fortune would lead a precari- 

 ous existence. Genetic equilibrium helps to insure that a species will not 

 "put all its eggs in one basket" in undergoing evolutionary change. Radical 

 change may lead to progress; it may also hustle a species down a blind 

 alley to speedy extinction. 



A further conservative function of the equilibrium tendency arises from 

 the manner in which it keeps a store of recessive genes continually in ex- 

 istence even though individuals homozygous for those genes rarely appear. 

 As we noted earlier (p. 349), there is no tendency for recessive genes to 

 "die out." This fact should be clearly evident from the examples showing 

 the workings of the Hardy-Weinberg law. In the last example, while only 

 about 6 percent of the hamsters are gray, over 37 percent of them "carry" 

 the gray gene (i.e., are heterozygous). These heterozygous individuals thus 

 form a reservoir of "gray" genes which can be drawn upon in producing 

 future gray individuals. If there is no advantage to be gained from being 

 gray, this matter remains of little importance, but if at any time or under 

 any conditions grayness, or any associated physiological effect, becomes 

 an asset, the reservoir of "gray" genes may assume great significance for 

 the species. We have mentioned this matter of the importance of hetero- 

 zygotes before (pp. 346-347) and shall return to it again (pp. 457-468). 



Having established a foundation of understanding concerning the tend- 

 ency to genetic equilibrium we shall now turn our attention to the forces 

 which tend to modify or upset that equilibrium and hence to lead to evolu- 

 tionary change. 



MUTATION PRESSURE AND 

 GENETIC EQUILIBRIUM 



In earlier chapters we have emphasized mutations as 

 the raw material of evolution. It will be recalled that mutations are changes 

 in genes and that, having occurred, they are inherited in accordance with 

 the Mendelian principles we have been discussing. 



We noted (p. 337) that the Dutch botanist, Hugo De Vries, first em- 

 phasized the importance of mutations in evolution. Indeed, he proposed a 

 "mutation theory" of evolution intended not only to supplement but in 

 large measure to supplant the Darwinian theory of natural selection. But 

 that was before the principles of genetic equilibrium we have just been 

 discussing were understood. 



Before discussing in more detail the role of the laws of probability in de- 

 termining the fate of mutations, we should note that the occurrence of mu- 



