The Evolution of Man | 289 



certain kinds of killing of socially sanctioned types. Thus society can 

 kill its internal enemies (assorted "criminals"), and killing its ex- 

 ternal enemies is encouraged. The social approval of killing out- 

 siders at one time doubtless had considerable selective advantage 

 for the society as a whole, but improved weapons make this no 

 longer true. Certain types of murders within our society, although 

 technically illegal, are approved socially. This is particularly true in 

 situations where members of minority groups are the victims. 



To recapitulate, cultures are extremely diverse and are separated 

 by, among other things, language barriers. In spite of this, there are 

 common threads running through most, if not all, cultures that make 

 it rather obvious that major features of human culture have pro- 

 liferated from a common source. People are generally unaware of 

 their biological and cultural history, and most assume that they 

 were, miraculously, born into the culture which has "The True 

 Word." 



A major interaction between cultural and biological evolution 

 has been in the change of selection pressures. The development of 

 modern medical techniques; the elimination of many large preda- 

 tors; the increase of the food supply through improved agricultural 

 methods; control and evaluation of the environment with furnaces, 

 air conditioners, dams, radar weather-warning systems, and the like 

 have permitted many otherwise nonviable genotypes to persist. The 

 diabetic controls his disease with insulin and reproduces; there is 

 time to lead the congenitally blind man to the storm cellar because 

 of the tornado warning over the radio. Thus some differentials in 

 reproduction have been ironed out by cultural factors in some so- 

 cieties. However, some recently introduced cultural factors have 

 imposed other selective pressures by favoring genotypes that are 

 relatively immune to insecticides in their food, air pollution, nervous 

 tension, heart disease, and cancer. In the last two cases differential 

 reproduction is increased because, with increasing life spans, re- 

 production is carried into the years when these diseases are 

 prevalent. 



The patterns of gene flow in human populations have also been 

 tremendously changed by cultural developments. Systems of trans- 

 portation have steadily improved, moving the entire human popula- 

 tion more and more in a direction of panmixia. However, this trend 

 has been countered to some degree by immigration quotas and other 

 cultural barriers to random mating. 



Other interactions between cultural and biological evolution are 

 obvious. Incest taboos tend to lower the coefficient of inbreeding. 

 Social disapproval of interracial, interreligious, and interclass mar- 



