178 



Increase in Number of Species 



Flywoy populotion 



Eastern proirie 



1 1 1 i I Mississippi valley 



'//, Southeast 



\\V South Allontic 

 J 



Fig. 77. Map showing roughly the main ranges of the four populations of 

 Canada geese nesting in the Hudson-James bay region. The range of the Mis- 

 sissippi Valley geese overlaps the range of the Southeast population chiefly in 

 fall; the range of the South Atlantic population overlaps the range of the 

 Southeast population chiefly in spring. The western limits of the range of the 

 Eastern Prairie population extend farther west than indicated here. The eastern 

 limits of the range of the South Atlantic population probably- extend farther 

 east in some areas than indicated. (After Hanson and Smith.) 



fission, it seems that comparative information about it must await 

 future investigation. 



Becoming genetically incompotihle. The great differences be- 

 tween groups in becoming genetically incompatible are shown 

 in striking fashion by many associated plants and their insect 

 predators. The eastern sycamore Platanus occidcntalis and its 

 Erythroneura leafhopper predators have imdoubtedly had a com- 

 mon history of past isolations, yet the sycamore has persisted as 

 only one species while its original Erythroneura companion has 

 evolved into six. The plant and the insects apparently have different 

 rates for becoming genetically incompatible, and these rates are 

 influenced by selective factors and intrinsic genetic mechanisms. 



Two isolated sister populations would normally be under slightly 

 different selection pressures. Presumably the amount of genetic 



