AGGREGATIONS OF HIGHER ANIMALS 125 



is without effect. Individual genes drift from one 

 state of fixation to another regardless of selection. In 

 large populations, gene frequencies tend to come to 

 equilibrium between mutation and selection, and if 

 selection is severe, there tends to be a fixation of the 

 gene or genes that carry adaptive modifications, and 

 evolution comes to a standstill. 



With a population intermediate in size, when there 

 are enough animals present to prevent fixation of 

 the genes on the one hand, but on the other, not 

 enough animals to prevent a random drifting about 

 the mean values determined by selection and muta- 

 tion, then evolution may occur relatively rapidly. 

 The results obtained will depend upon the balance 

 between mutation rate, selection rate, and the size 

 of the effective breeding population. 



In one more case the effect of differences in sever- 

 ity of selection was worked out by Professor Wright 

 (Figure 22). With a moderate mutation rate, if the 

 selection is relatively weak, mutation pressure may 

 determine the result and the given character will 

 then drift to fixation or, as shown in the diagram, to 

 extinction. As selection pressures increase, selection 

 tends to take charge of the end products, and, if 

 slight, there is a wide variation about a mean; if 

 more intense, the amount of variation becomes less 

 and less. 



