POPULATION GENETICS AND EVOLUTIONARY CHANGE 445 



Table 19. 1. Results of One Deiiionstriilion 

 of the Genetic Drift Model 



isolated populations may come to differ from each other in characteristics 

 which are of no practical importance in the lives of the animals concerned. 

 We have mentioned repeatedly that drift is a phenomenon of small 

 populations. That it is such may at first glance seem to restrict its action 

 to inhabitants of oceanic islands and members of other small, isolated 

 communities. Yet, as noted earlier (p. 350), even large, widely ranging 

 assemblages of animals are commonly at least partially divided into 

 smaller breeding groups. A species of field mouse, for example, may range 

 over an entire state and include millions of individuals. If the principles of 

 random mating prevailed, any given individual would be equally likely to 

 mate with any other individual, of opposite sex, among those millions. Yet 

 that is not the condition which actually obtains. An individual living in one 

 river valley is more likely to mate with an individual in the same valley 

 than with an individual in another one. An individual in one patch of 

 woods is more likely to mate with another individual in the same wood lot 

 than with an individual in woods separated from it by open country. 

 Furthermore, field studies reveal that many animals establish rather defi- 

 nite "home ranges" beyond the limits of which they seldom stray and 

 within which they repel intruders of the same species and sex. Thus the 



