148 I The Process of Evolution 



There is evidence (summarized by Sheppard, 1961) that certain 

 combinations of characteristics give the best mimicry of different 

 models and are at a selective advantage. Others do not look like any 

 model and are at a disadvantage. In at least some cases, selection 

 seems to have reduced the possibility of the production of poorly 

 protected combinations by increasing the linkage between the loci 

 concerned in producing the pattern. This permits linked groups of 

 loci (supergenes) to be selected as a unit and the superior com- 

 binations to be preserved. (The phenomenon is quite comparable 

 to the holding together of gene constellations in Drosophila popu- 

 lations by inversions. ) In mimetic butterflies disruptive selection 

 may also operate through the accumulation of modifier genes which 

 further perfect the resemblance. This is supported by hybridization 

 experiments in which supergenes are transferred to a new genetic 

 environment. There they do not produce phenotypes that mimic the 

 model as precisely as before. 



Resistance to Antibiotics and Insecticides 



No discussion of evolution would be complete without mention of 

 the response of some organisms to man's attempts to reduce their 

 population size or eradicate them. Striking and important examples 

 of the response of natural populations to human endeavors center 

 around the phenomenon of resistance. Indiscriminate application of 

 insecticides to large areas of the earth's surface has constituted a 

 very potent selective force. In the vast majority of cases, the large 

 population sizes of pest insects have contained sufficient residual 

 variability to allow these insects to develop strains resistant to virtu- 

 ally all the compounds that the ingenuity of the organic chemists 

 can produce. The chemists are, of course, limited severely by the 

 survival requirements of nontarget organisms such as man. Even 

 so, there is considerable evidence that man and his domesticated 

 plants and animals have not escaped damage from powerful syn- 

 thetic pesticides. 



It is interesting to note that insects have met the challenge in 

 diverse ways. There have been examples of behavioral resistance in 

 which insects no longer alight on sprayed surfaces and of many 

 kinds of physiological resistance in which the penetration or action 

 of the insecticide is prevented by various mechanisms. Parallel to 

 insecticide resistance has been the appearance of strains of micro- 

 organisms that are highly resistant to antibiotics. This has been 

 caused by the overuse of these antibiotics by well-meaning doctors 

 when other treatments might suffice or be better. So far, the increase 



