Natural Selection 111 



tion would rise exponentially by a function determined primarily 

 by the size of the species population, the fecundity of the species, 

 and the rigidity of selection for the new mutation. Thus if a mutation 

 conferred unusual adaptive benefits on an individual bearing it, that 

 individual would leave more offspring than others in the population, 

 and all of its progeny bearing the mutant allele would do likewise. 

 However, if the whole population were large, the individuals bear- 

 ing the mutant would at first be only a small proportion of the total. 

 With the difference in survival rates, however, the new type would 

 tend to increase by a geometric proportion, whereas the old type 

 would tend to increase by an arithmetic proportion or lower, and 

 with time the new type would replace the older type at an increas- 

 ing rate. 



Some measure of the possible speed of these changes in genie 

 proportions is afforded by the results of campaigns to eradicate 

 certain mosquito and house fly populations by chemical control. In 

 some instances, under the powerful selection pressure of the in- 

 secticides, alleles conferring resistance to the control chemicals rose 

 from negligible proportions to virtual homozygosity in two years 

 (Brown, 1960). 



Studies of insecticide resistance afford excellent examples of the 

 phenomenon called preadaptation. Some and probably all of the 

 kinds of alleles conferring insecticidal resistance occurred in wild 

 populations of the insects before these populations were ever ex- 

 posed to insecticides. In mosquitoes and house flies which had not 

 been exposed to insecticides, the resistant alleles occurred in 

 extremely low frequencies but built up to much higher ones as soon 

 as control applications superimposed a selective factor for them. 

 Many evolutionists have suggested that in natural evolution the 

 same type of circumstance has operated frequently. They believe 

 that many mutations conferring little or infrequent advantage may 

 persist at low frequencies and that subsequently changed cir- 

 cumstances (either in the environment or in the genome) exert 

 strong selection pressures for the previously rare mutations. Such 

 considerations indicate a possible indeterminate period of low selec- 

 tion pressure preceding the exponential increase of many alleles 

 after they become highly advantageous to the species. 



Thus in time every trend tends to end at a point where the 

 organism is as well adapted to its environment as is possible within 

 its genetic framework. This means that centrifugal selection 

 gradually leads to a condition of centripetal selection. 



