150 I The Process of Evolution 



of these organisms has been countered largely by a scramble to 

 find new chemical weapons to use against them, rather than by the 

 application of methods that are biologically more sophisticated and 

 infinitely more beneficial in the long run. 



Such alternative approaches are well known. Some resistant insects 

 prove to be less viable than their nonresistant relatives when they 

 exist in an environment free of insecticide. In these cases, moratona 

 on insecticide applications would give time for the forces of natural 

 selection to return the populations to their previously susceptible 

 condition. By intelligent use of insecticides at critical moments, a 

 reasonable level of control may be attained with a minimum danger 

 of creating resistant strains (a danger maximized by "broadcast" 

 application ) . Where moratoria are not feasible, multiple applications 

 of many different poisons may make it impossible for the popula- 

 tion simultaneously to develop resistance to all. Because it is not 

 possible, however, to affect one member of a community without 

 affecting the entire ecosystem, this approach is fraught with un- 

 foreseen dangers. It is well known that accumulation and concentra- 

 tion of chemical poisons take place in members of food chains. And, 

 if the target organisms are not eliminated, their predators and the 

 predator's predators may be severely affected. 



In addition to this, many pestiferous insects are more readily and 

 economically controlled by interfering with their biology in a non- 

 chemical way than they are by the application of insecticides. For 

 example, the draining of swamps in which mosquitoes breed, the 

 introduction of predators to control imported pests, the releasing of 

 multitudes of sterilized males to compete with wild males for mates, 

 and the dissemination of laboratory-grown pathogens have all 

 proved to be effective. Similarly, classical methods of antisepsis 

 will often deal with highly resistant microorganisms much more 

 effectively than newer and costlier antibiotics. 



LABORATORY POPULATIONS 



The term artificial selection pertains to man's control of the geno- 

 types that contribute to the gene pool of succeeding generations. 

 Artificial selection is carried out by both plant and animal breeders 

 and also by scientists wishing to study the effects of selection in the 

 laboratory. It may have a purposiveness directed at a single trait, 

 another respect in which it differs from natural selection, which 

 operates on all the phenotypic characters affecting fitness and has 

 no purpose. Nevertheless, even under the most carefully controlled 

 laboratory conditions, natural selection still operates in conjunction 



