Species and Species Change 



149 



Fig. 62. Distribution of the black grain stem sawfly and the European wheat 

 stem sawfly in the United States in 1937. The dotted hne marks the former 

 northern extension of the black grain stem sawfly. (Adapted from Udine.) 



each species would have been rigidly reduced from a wide to a 

 narrow band. In the hne which eventually became chiefly a heavy- 

 cover or forest group, selection pressures for waiting and springing 

 would not be diluted constantly by pressures for coursing and 

 running. Under this newly restricted ecological setting, slight ge- 

 netic changes favoring waiting and springing, silent tread, and 

 other habits adaptive to the now more restricted environment 

 would have had an opportunity for phenotypic expression. 



GENETIC MECHANISMS AND POPULATION SIZE 



The examples cited for unusual genetic change are associated with 

 large populations, whereas those cited for a reduction of genetic 

 complexity are associated with a decrease in population size. This 

 latter circumstance suggests a parallel with the models of adaptive 

 change based on small populations proposed by Wright (1942, 

 1949) and Simpson (1944). In their models, the population is 

 postulated to be at a precariously low point, poorly adapted to its 



