158 I The Process of Evolution 



such difRcult-to-explain phenomena as the evolution of social be- 

 havior in bees are now yielding to investigation (e.g., Michener, 

 1958). This problem is complicated because the unit of evolution 

 is the colony, not the individual. ( Most members of a hive are non- 

 reproductive. ) Selection in honeybees consists largely of differential 

 reproduction of colonies rather than of individual genotypes. 



Loss of features when they no longer confer selective advantage 

 is one of the most widely observed evolutionary phenomena. Se- 

 lectively neutral characters presumably are rare. The eye, very 

 useful to most animals, may become an easily injured, infection- 

 prone liability to a cave fish. Body hair, which at one time protected 

 human beings against cold and injury, became a happy hunting 

 ground for lice with the invention of clothing. 



SUMMARY 



In this chapter has been given a series of examples of studies 

 of evolutionary changes within populations, chosen for the diversity 

 of approach and material. In addition, inferential evidence bearing 

 upon the efficacy of the selective process is discussed. It becomes 

 apparent that, although it is relatively simple to demonstrate 

 changes, it is much more difficult to partition the responsibility for 

 the changes among the various evolutionary forces. The problem is 

 especially complicated because of complex interactions within 

 genetic mechanisms and developmental systems. Most difficult of 

 all to document is the role of genetic drift. It is nearly impossible to 

 "prove" the efficacy of drift in natural populations, as one can always 

 hypothesize the existence of some as-yet-to-be-discovered selection 

 pressure that could account for the observed phenomena. Indeed, 

 there have been several cases, especially in snails, where differences 

 at one time attributed to the action of drift have been demonstrated 

 to be caused by selection. However, the inevitability of drift, to- 

 gether with results of studies of gene frequencies in small popula- 

 tions of Drosophilo, both in nature and in the laboratory, leads one 

 to believe that drift, interacting with the other pressures, can be an 

 important evolutionary force. Extensive studies still are needed on 

 a wide variety of organisms before broad generalities on the rela- 

 tive contribution of the various forces can be made with real con- 

 fidence. 



REFERENCES 



Allison, A. C. 1959. Metabolic polymorphisms in mammals and their 

 bearing on problems of biochemical genetics. Am. Naturalist 93: 



