THE ORIGIN OF SPECIES 



includes wing movements by the male. Wingless males can mate with 

 normal females, but it takes much longer. Now Sturtevant has shown that, 

 if a normal pair of flies is put into a bottle along with a wingless male, the 

 female will mate after a normal time lapse, but she will mate with the 

 wingless male as readily as with the normal one. It is evident that the 

 courtship of the normal male has hastened receptiveness of the female 

 without influencing her choice of mates. 



Finally, most attempts to determine factors in choice of mates have been 

 inconclusive. It may well be that courtship behavior in general has more 

 to do with sexual excitement than with choice of mates. And it may well 

 be that simple proximity has more influence on choice of mates than does 

 any other factor for most animals. As the problem of sexual selection now 

 stands, then, probably few biologists would care to state categorically that 

 it plays no role in evolution, but equally few would care to ascribe to it a 

 really important role. 



SELECTION AND NONADAPTIVE CHARACTERS 



One of the objections often raised against the theory of natural selection 

 is that it cannot account for the many cases in which differences between 

 related organisms do not have any evident adaptive value. There are 

 several ways in which such cases may be harmonized with the theory. At 

 the outset, it should be pointed out that it would be very difficult to prove 

 in any particular case that there is no adaptive value. The endocrine 

 glands of vertebrates were thought to be without function only a short 

 time ago. The demonstration of their manifold functions awaited the de- 

 velopment of suitable techniques. The same thing is at least potentially 

 possible with respect to any character which appears to have no adaptive 

 significance when studied by present methods. 



A specific possibflity is that an apparently nonadaptive character may 

 have adaptive value during a limited phase of the life cycle. For example, 

 many terrestrial animals return to water for breeding purposes. If studied 

 only during the terrestrial phase, their aquatic adaptations would be 

 rather puzzling. Again, a character may have no selective value under 

 ordinary circumstances, yet be highly valuable in the extremes to which 

 the organism may be occasionally subjected. Thus the plants of the San 

 Francisco Bay region may for many generations never be subjected to any 

 extremes of temperature. Because of this mild climate, a wide variety of 

 introduced plants has been very successfully cultivated. But occasionally, 

 freezing weather does strike, and it wreaks much havoc among the plants, 

 with much greater damage among the introduced plants than among the 

 native plants. As studied during "typical" years, the characteristics which 

 adapt these plants to severe weather are difficult to understand. But the 

 native plants are there not only because they can exploit the usual benign 

 weather, but also because they are capable of withstanding the extremes. 

 The occasional extremes may well constitute the most severe selective 

 force to which these plants must become adapted. 



Lastly, there is the possibility that a character may actually liave no 



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