Increase in Number of Species 



stoneflies is Allocapnia pygmaea (Fig. 74), 



173 



Fig. 74. Distribution of the winter stonefly Allocapnia pygmaea showing the 

 disjunct population in the Ozark mountains of Missouri. 



Certain disjunct Ozarkian populations, however, have evolved 

 into species which are distinctive on a morphological basis. Three 

 examples are the winter stonefly Allocapnia sandersoni and the 

 caddisflies Hy dropsy che piatrix and Glyphopsyche rnissoiiri. Each 

 of these species is most closely related to a sister species occurring 

 considerably to the north and east (Ricker, 1952; Leonard and 

 Leonard, 1949). In all three examples the Ozark species differs 

 from its northeastern relative in slight but constant morphological 

 differences comparable in both type and magnitude with differences 

 between closely related pairs of allied sympatric species. 



The time when the Ozark Mountain populations originally be- 

 came separated from the more northeastern populations of the same 

 species is unknown, but the small amount of discontinuity in 

 structure between those which have evolved into sister species, 

 coupled with their geographic location, suggests that these disjunc- 

 tions were associated with events of the Wisconsin glaciation. The 



