POPULATION GENETICS AND EVOLUTIONARY CHANGE 435 



We answer this question by computing the frequencies of the dominant 

 and the recessive genes, i.e., the values of p and q: 56.25 percent of the in- 

 dividuals have only M genes, and consequently contribute that percentage 

 of M genes to the pool; 37.5 percent of the individuals are heterozygous, 

 half their genes being M, half ni, and thus contribute one-half of 37.5 per- 

 cent, or 18.75 percent, of M genes, as well as 18.75 percent of ni genes. 

 The 6.25 percent consisting of gray hamsters are homozygous mm and 

 hence contribute only m genes, doing so in proportion to their frequency 

 in the population. 



Thus, p = .5625 + .1875 = .75 or 75% 

 q = .1875 + .0625 = .25 or 25% 



We notice immediately that these values of p and q are exactly the 

 same as those for the original population (see above). Substituting them 

 in the Hardy- Weinberg formula will form a mere repetition of the calcu- 

 lation by which we determined the constitution of the first-generation off- 

 spring. Evidently, therefore, the population is now in a state of equilibrium 

 and, as long as unmodified random mating occurs, may be expected to 

 continue 56.25 percent MM, 37.5 percent Mm, and 6.25 percent mm gen- 

 eration after generation. 



Our hypothetical example has demonstrated that when a population is 

 not in genetic equilibrium with regard to a pair of genes it tends to attain 

 such an equilibrium in one generation of random mating. 



Significance of Genetic Equilibrium for Evolution 



So far in this chapter we have devoted attention to the manner in which 

 the laws of chance or probability operate upon gene distribution in ways 

 tending to preserve the status quo — to maintain an unchanging equilib- 

 rium as generations pass. We have noted that not only is there a tendency 

 to maintain such an equilibrium but if the equilibrium is upset there is a 

 tendency to establish quickly a new equilibrium. Evidently this tendency to 

 equilibrium forms a sort of inertia which must be overcome if evolution- 

 ary change is to occur. 



Stating the matter so may give the impression that equilibrium is en- 

 tirely detrimental, and obstructive of progress. We should note, therefore, 

 that the equilibrium tendency is conservative, in the best sense of that 

 much abused word. It tends to conserve sains which have been made in 

 the past and to prevent too rapid change. 'Taking chances" is the price of 

 real achievement and progress in the life of a species, as in the life of a 



