Biologists working at the popiihtion level of organization have 

 been oriented in large degree by the characteristics of the organ- 

 isms studied. For instance, cytological features of genetic systems 

 are more readily studied in Drosophila and Oenothera than in 

 Papilio or Sequoia. Unusual combinations of circumstances have 

 presented opportunities for studying the operation of natural 

 selection in certain organisms, organisms about which there may 

 be little or no cytogenetic information. Much of our knowledge 

 is gleaned from work on organisms of economic importance, 

 such as crops, domestic animals, and pests. Thus circumstances 

 have made it impractical to produce a unified description of all 

 aspects of evolution within populations. 



The theory of population genetics has been created largely to 

 treat diploid, outcrossing organisms. It is therefore convenient 

 to present this body of theory and related examples from nature 

 before discussing the cojiiplexities of systems controlling recom- 

 bination in various kinds of organisms. It is hoped that eventually 

 a theory may be constructed which will consider the interactions 

 of the genetic system of an organism and the evolutionary forces 

 acting upon the organism. In the meantime, the warning of Norbert 

 Wiener inust be kept in mind: It is very difficult to study the inter- 

 actions of two systems with very different rates of time course. This 

 is true when we attempt to understand history on the basis of 

 day-to-day human behavior or when we try to understand 

 phylogenetic history on the basis of individual gene changes 

 in contemporary organisms. 



