12 INTRODUCTION TO EVOLUTION 



long-legged individuals to become the parents of a larger proportion of the 

 next generation than do their shorter-legged fellows. As a result, long legs 

 will be possessed by larger numbers of the second generation than pos- 

 sessed them in the first. If this same trend continues for several or many 

 generations, eventually the whole population may come to possess the 

 beneficial change. This, in simplified, nontechnical terms, is the central 

 idea of the positive aspect of natural selection. More complete statements, 

 with discussion of the forces operative, will be found in the closing chap- 

 ters of the book (pp. 450-507). The central idea, however, will be 

 found most useful in interpreting the manifestations of evolution dis- 

 cussed in the chapters immediately following this one. 



We note that positive natural selection resembles positive artificial selec- 

 tion in that in both instances individuals possessing some special attribute 

 are favored to become parents of the next generation. In artificial selec- 

 tion the favored individuals are the ones possessing some quality desired 

 by the breeder. In natural selection the favored individuals are the ones 

 possessing some quality which renders them better adapted than their fel- 

 lows for life under the circumstances in which they find themselves. In 

 both instances the desirable quality or change will be likely to be of more 

 frequent occurrence in the next generation than it was in the former. 

 Progress, in terms of more perfect adaptation to the conditions of life, is 

 the result. 



Postadaptation and Preadapfation 



Our discussion so far has emphasized more and more perfect adaptation 

 to a stable environment in which the species is already living. This type of 

 adaptation is called postadaptation, since the species has already entered 

 the environment, and additional adaptation merely perfects the animal 

 for living under the conditions prevailing. Much evolutionary change is 

 of this nature. 



On the other hand, a bodily change may be of no particular benefit, may 

 indeed even be harmful, in the environment in which the species is living, 

 but would be beneficial in some other environment. If possessors of this 

 change can reach that other environment, they may thrive there, with the 

 result that the change may increase in frequency as generations pass in the 

 new environment. Eventually the change may characterize all inhabitants 

 of the new environment, becoming for this population "standard equip- 

 ment." This phenomenon of a change which, though it may not be benefi- 

 cial in the original environment, fits an animal to invade another en- 



