NATURAL SELECTION 



691 



the female manipulates during copulation 

 (several species of the empidid genus 

 Hilara). The males of the micropezid fly, 

 Nerius fiiscus, combat each other for the 

 possession of the female without the loser 

 being killed by the winner. 



Beebe (1944a) describes a combat be- 

 tween male elephant beetles {Megasoma 

 elephas) in the presence of the female. The 

 function of the conspicuous horns of the 

 male during combat is evidently to tip over 

 the opposing male, thus allowing the victor 

 to copulate with the female, who seems to 

 exert no individual choice. Somewhat simi- 

 lar combats occur between males of the 

 Hercules beetle, Dijnastes hercules (Beebe, 

 1947). 



Secondary sexual characters might also 

 be used for combating or threatening 

 predaceous enemies, but the female might 

 then be expected to have evolved similar 

 defensive adaptations, unless, by division 

 of labor, she was defended by the male. 

 Defense of the females by males is some- 

 times seen, particularly among birds and 

 mammals, but could hardly account for the 

 evolution of the horns of dynastid beetles. 

 Whether a genetic diflference inducing 

 horns in the males is selected differentially 

 on the basis of individual combat is also a 

 question. Such structures are known to be 

 developed through allometric growth and 

 might evolve indirectly by selection of size 

 differences. This is also true of the antlers 

 of the majority of male deer (Huxley, 

 1932; see also p. 638). Secondary sexual 

 characters confined to one sex might have 

 general survival value for the species if 

 they functioned for species recognition, sex 

 recognition, sexual stimulation, coition, 

 fertiUzation, territoriaUty, defense of the in- 

 dividual male, defense of the female, or 

 defense of the family (Huxley, 1938, 

 1938a). 



These possible functions of the horns of 

 the dynastid beetles seem unlikely. It 

 is possible to explain the evolutionary 

 adaptation of some sex characters through 

 intrasexual selection in a sense close to the 

 original Darwinian concept. At the same 

 time we must remember that we have no 

 proof that the winning male is genetically 

 superior to the loser, or that the loser 

 may not find another female. There seems 

 to be a supply of unmated males and fe- 

 males in many populations. It may still 



be possible to explain this peculiar evolu- 

 tion of intrasexual competitive adaptation 

 on the basis of species selection rather than 

 individual selection. 



Birds have probably evolved territorial 

 behavior (p. 412) through more eflBcient 

 spacing of breeding, nesting, and feeding 

 functions. An individual bird or mammal 

 may win a combat in its own territory and 

 lose to the same opponent in the oppo- 

 nent's territory (p. 414). Species exhibiting 

 territorial behavior, however, probably have 

 a selective advantage. 



Both genetically similar and genetically 

 different classes of individuals may occur 

 in certain populations, one set of mecha- 

 nisms integrated with the other set in a 

 functional whole. Sexual division of labor 

 in the social Hymenoptera may be based 

 upon genetic differences— the males being 

 haploid and the females diploid (p. 688). 

 At the same time, the female castes, consist- 

 ing of reproductives, workers, and soldiers, 

 are not genetically different, but are the re- 

 sult of physiological differentiation (Gregg, 

 1942). There is little doubt that such a so- 

 ciety evolved through basic sexual and 

 family systems. 



Likewise, human society evolved social 

 coordination through sexual and family pat- 

 terns. Sex behavior based upon initial 

 genetic differentials is still of fundamental 

 importance in human social life; the human 

 family, like the family in other mammals, 

 is a physiological integration between par- 

 ents and offspring with familial adaptations, 

 such as the mammary glands, genetically 

 and physiologically an evolution of more 

 basic sex patterns. On top of these systems, 

 a social organization has evolved that is 

 based upon conditioned response, intelli- 

 gence, and symbolic learning; each individ- 

 ual has the capacity to fit numerous dif- 

 ferent social niches by means of education. 

 The foundation for the nongenetic psy- 

 chological division of labor may be traced 

 back through various vertebrate aggrega- 

 tions (pp. 411, 664, 686). 



Whether the population is integrated 

 through nongenetic or genetic mechanisms, 

 or both, there are many examples of what 

 would appear to be an evolution of individ- 

 ual or class sacrificial action or behavior. 

 Somatic cells sacrifice the capacity to re- 

 produce new organisms and often die 

 functioning for the benefit of the whole 



