114 Natural Selection 



later chapters of this book; many have been expressed as mathe- 

 matical models by Fisher (1930), Haldane (1932), and Wright 

 (1949). By these mechanisms and processes, individuals v^hich are 

 better adapted to their environment tend to survive. The physio- 

 logical and morphological attributes which increase the adapted- 

 ness of an individual are called adaptations. 



The result is that natural selection acts on the individuals of one 

 generation and in so doing determines the genetic composition of 

 the next. 



One aspect of the relationship between genetic change and 

 natural selection is of especial interest from an over-all view of the 

 evolutionary process. Because of its own innate properties, the 

 genetic change mechanism introduces into the population new 

 units which are random as regards selective value. The process 

 of natural selection selects and discards these random units in such 

 a fashion that the persisting units are essentially ordered in a finite 

 number of character changes having definite physiological and eco- 

 logical orientations. 



