Natural Selection 105 



emergence, the opposite of the previous year. Undoubtedly many 

 conflicting sets of selection pressures are in operation. 



Identical selection pressures may not reoccur for many genera- 

 tions, especially for organisms with a life cycle of a year or less. 

 Competitive selection pressures and weather extremes are often 

 separated by periods of five to 25 years. Thus genetic factors 

 rigorously selected in one year may become diluted due to counter- 

 selection or many other causes before any additional identical 

 selection becomes operative. However, many selection pressures 

 occur continuously for considerable periods of time, especially in 

 the case of structures adapting an organism to better success in a 

 new environment. Thus an animal which had recently adopted a 

 subterranean, digging mode of life might have feet not especially 

 adapted to digging. In this case it would seem that natural selection 

 would favor any genetic change producing feet better suited for 

 digging. Immediate selection, however, might be hindered by 

 pleiotropy and genie competition. 



Many genes are pleiotropic, each affecting several characters, 

 and there is always the possibility that conflicting selection pressures 

 may result from this situation. In chickens, as was stated earlier, 

 rigid selection for long legs results in reduced vitality and fecundity 

 (Lerner, 1958). 



Different genes may themselves be in competition for nuclear 

 substrates, as shown by Glass (1957), so that unforeseen results 

 may follow selection for different combinations of alleles. Here 

 and in pleiotropic conflicts natural selection might result in a 

 biological compromise. 



It is probable that natural selection never produces perfect 

 adaptation. As Simpson (1949) pointed out, many possible varia- 

 tions around a central theme may work equally well to such a 

 degree that any one variant would have little or no selective 

 advantage over the others. He cited as an example the horns of 

 African antelopes (Fig. 43); those of no two species are exactly 

 alike, but all seem to function satisfactorily. 



The correlation of seemingly linked adaptive characters may not 

 be perfect. In several stonefly groups the females have short wings 

 and the extra body segment thus exposed is sclerotized like the 

 other exposed end segments, presumably conferring some kind of 

 protection. In the winter stoneflies of the American genus Allocapnia 

 this correlated change has evolved independently in two lines. In 

 one of these lines, represented by A. vivipara, the wings of the 

 female vary from fairly long wings to short pads ( Fig. 44 ) , resem- 



