xii Contents 



The Source of Variability 59 



The Genetic Mechanism 62 



Change in Genetic Effect 72 



Genetic Variability in Natural Populations 85 



Magnitude of Character Change 90 



The Order in Genetic Change 94 



O Natural Selection 97 



Factors of Natural Selection 100 

 Rate of Change 109 

 Selection in Apomictic Organisms 112 

 Selection in Bisexual Populations 112 

 Summary 113 



Species and Species Change 115 



Recognition of Species 115 



The Species as a Physical Unit 122 



The Species as an Adaptive Unit 135 



/ Increase in the Number of Species 153 



In Apomictic Organisms 154 

 In Bisexual Organisms 157 

 Summary 207 



The Evolution of Communities 209 



The Congregation of Species 211 

 The Evolution of Community Relationships 212 

 Evolution of Complex Communities 218 

 Change in Species Composition 239 

 Summary 242 



