348 INTRODUCTION TO EVOLUTION 



Genetic Equilibrium 



Our answer to the question just raised will involve further consideration 

 of the action of natural selection, but before we discuss the latter it will 

 be well to consider forces acting on mutations in the absence of, or in 

 addition to, natural selection. Unlike natural selection, these forces oper- 

 ate without regard to considerations of usefulness or harmfulness of the 

 mutations concerned. They depend upon the operation of the laws of 

 chance or probability in the "shuffling of the genes" discussed in an earlier 

 section of this chapter. 



In that discussion we noted the manner in which pairs of genes are re- 

 assorted generation after generation. The genes of heterozygous brown- 

 eyed parents (Fig. 15.1 ), for example, are separated and then combined in 

 various ways in their children and grandchildren. The genes remain the 

 same (except for the rare occurrence of new mutations) but they are re- 

 assorted generation after generation to produce a collection of homozy- 

 gous brown-eyed individuals, heterozygous brown-eyed individuals, and 

 blue-eyed individuals. Suppose that in a given population there are a mil- 

 lion B (brown-eye) genes and a thousand b (blue-eye) genes. These will 

 be combined in pairs in homozygous brown-eyed individuals (BB), het- 

 erozygous brown-eyed individuals (Bb), and blue-eyed individuals (bb). 

 If these individuals marry at random so far as eye color is concerned 

 (e.g., if there is no tendency for brown-eyed people to prefer brown-eyed 

 mates, or for blue-eyed persons to prefer other blue-eyed persons) the 

 number of B genes and the number of b genes will tend to remain con- 

 stant generation after generation. The genes will be "shuffled" and recom- 

 bined but the numbers of the two kinds of genes present will tend to re- 

 main the same, just as the numbers of aces, kings, queens, jacks, and so 

 on, in a deck of cards remain the same despite the varied assortments of 

 them which may be dealt as hands during a long evening of play. 



The tendency of gene frequencies to remain in equilibrium in succeed- 

 ing generations finds mathematical expression in the Hardy-Weinberg 

 law, discussed at greater length on pages 427-435. Our present purpose 

 will be served by calling attention to the existence of this tendency to es- 

 tablishment and maintenance of genetic equilibrium. This equilibrium will 

 tend to be maintained generation after generation unless disturbed ( 1 ) by 

 fresh mutations from the dominant gene to the corresponding recessive 

 one, or vice versa; (2) by natural selection; (3) by chance (see below). 



If the dominant and recessive genes are both common in the population 

 the contrasting characteristics produced will both occur commonly, as is 



