492 INTRODUCTION TO EVOLUTION 



the male with the brightest colors, the most intricately ornamental plum- 

 age, the most beautiful song, the most skill in courtship wins the female, in 

 competition with his less endowed brethren. In consequence, he fathers a 

 disproportionate share of the next generation, the males of which inherit 

 their father's superior attractions. If females continue generation after 

 generation to select as mates the brightest-colored or otherwise most orna- 

 mental males, male pulchritude may be expected to increase in the species. 



Despite the fact that Darwin marshaled a great array of circumstantial 

 evidence in support of the theory, sexual selection is generally considered 

 to have but limited applicability. Combat between two males, for exam- 

 ple, when it does occur, is seldom to the death, an eventuality which would, 

 of course, permanently eliminate one potential father. Furthermore, ac- 

 cording to the theory, after the fight is over the victor wins the female. Al- 

 though evidence is difficult to accumulate on such matters, it seems that the 

 female not infrequently goes off with the vanquished combatant rather 

 than with the victorious one. Moreover, in species in which the individuals 

 are monogamous, pairing for a season or longer, sexual selection would 

 work only if there were more males than females. If the sexes were equal 

 in number, every male would eventually find a mate regardless of his 

 prowess in combat. In species in which polygamy is the rule sexual selec- 

 tion might be expected to be more effective, since the successful males 

 would amass the largest "harems" of females, or would otherwise mate 

 with the largest number. It is noteworthy that some of the most elaborate 

 displays are possessed by male birds which are polygamous (e.g., the 

 argus pheasant). Under such circumstances, since one male may mate 

 many times, other males not at all, a high premium is placed on elaborate 

 display. Such cases are perhaps our best example of sexual selection in the 

 Darwinian sense. 



What we have said about combat applies also, in the main, to the matter 

 of display on the part of the males for the benefit of the watching females. 

 Evidently some of the display of bright and ornate plumages or other male 

 adornments, accompanied by the courtship ritual appropriate to the species, 

 does have the effect of arousing the female so that she will accept the male. 

 But there is little evidence that females sit in judgment upon the quality of 

 ornamentation or posturing of males of their own species and decide 

 upon that basis which male to accept. Indeed, "in most monogamous birds, 

 display begins only after pairing up for the season has occurred" (Huxley, 

 1938). And in polygamous species, so long as the courtship behavior is ade- 

 quately exciting and orthodox for the species, the females usually are 

 rather passive in the matter of which individual male is accepted. On the 



