496 INTRODUCTION TO EVOLUTION 



two species may change their ecological requirements sufficiently 

 so that they no longer compete (pp. 486-488). 



c. Preadaptation (prospective adaptation, Simpson, 1953). At times 

 a subpopulation may attain adaptations of wider applicability 

 than merely to the environmental niche in which it lives at the 

 time. If so the population may take up life in a different environ- 

 ment. Usually this can only happen if the new environmental 

 niche is unoccupied. 



d. The measure of success. Success in evolution is measured by 

 magnitude of genetic contribution to the next generation. Suc- 

 cessful populations or subpopulations are those which ( 1 ) are, or 

 become, so well adapted to life under conditions prevailing that 

 they continue generation after generation to fill completely the 

 environmental niche in which they live, or (2) possess, or become 

 possessors of, adaptations enabling them to invade new environ- 

 mental niches. The first type of successful population will con- 

 tinue to be successful so long as environmental conditions remain 

 substantially unchanged. The second type, capable of change to 

 meet new conditions, may be expected to give rise to evolutionary 

 changes of larger magnitude than those attained by populations 

 that merely become better adapted to prevailing conditions. Ob- 

 viously, however, a given population may be both well adapted to 

 one environmental niche and possessed of attributes which would 

 enable it to enter some other niche if opportunity offered. Prob- 

 ably most populations adapted for life in one niche do not retain 

 sufficient genetic plasticity to permit them to enter a radically 

 different niche or are not presented with opportunity to enter a 

 radically different one, even though they may have the necessary 

 capacity for change. Historically, those populations both possessed 

 of the requisite genetic plasticity and afforded the opportunity to 

 invade radically different niches have been the ones that have 

 made large strides in evolutionary change. 



DIRECTIVE FORCES IN EVOLUTION 



What determines the direction evolution shall take? 

 This is a large question the answers to which are imperfectly known. Space 

 will permit only brief consideration of two important factors concerned: 

 (1) the directive effect of preceding events and (2) natural selection. 



Directive Effect of Preceding Events 



We may think of the first factor as the directive action of past history. 

 The future is always in part determined by the past. To illustrate our mean- 

 ing let us devise a litde game of words. The rules of this game are simple: 



