Species and Species Change 



145 



break effectively the homeostatic balance that was previously 

 reached between the species and the old environment. 



Why did this ecological extension happen to only one species in 

 the entire branch? It appears to be unrelated to phylogenetic spe- 

 cialization because H. borealis is intermediate between more primi- 

 tive and more specialized types in its own branch (Fig. 37). It 

 may be of significance that even in the warmer parts of the range 

 where its ancestors occurred, H. borealis far outnumbers its con- 

 geners. The tenets of population genetics hold that such larger 

 populations would have a greater prospective incidence of mutants 

 having adaptive benefits and greater potentiahties for the occur- 

 rence of all possible genie combinations. This is a plausible ex- 

 planation. 



Fig. 60. Distribution of the six most northern species of the caddisfly genus 

 Helicopsijche (because of morphological similarity, H. limnella is considered an 

 outpost population of H. mexicana) . Other members of this complex occur from 

 southern Mexico south. 



