136 



Species and Species Change 



Fig. 56. Variation in the pattern on the undersurface of male wings in the 

 recently extinct California butterfly Glaucopsijche xerces. (After Downey and 

 Lange. ) 



preceding chapter, are associated with artificial changes in the 

 environment. Even so, they give an insight into the possible rate 

 of spread of strongly adaptive traits in a population. Dobzhansky 

 ( 1958 ) has demonstrated marked changes in chromosome type over 

 the range of Drosophila pseudoobscura. During a period of twenty 

 years, chromosome type PP increased greatly and type CP de- 

 creased in almost all tested populations in the western portion of 

 the range (Fig. 57). A trait which gives the larch sawfly Pristiphora 

 erichsoni immunity from its chief parasite has spread extremely 

 rapidly through many thousands of square miles in the Canadian 

 portion of its range (Muldrew, 1953). The trait is a blood-clotting 

 behavior by which internal parasite larvae are encapsulated and 

 killed inside the body cavity of the larval sawfly. This again is a 

 highly adaptive trait conferring high survival which is contrasted 

 with no survival among individuals not possessing it. 



