ECOLOGY AND ISOLATION 



627 



Drosophila. Some of this graded variation 

 (e.g., in relative heart size) may be non- 

 heritable modification; but the majority of 

 these clines are wholly or mainly genetic. 



EcocHnes often seem to indicate selec- 

 tive eUmination (p. 623) and thus reflect 

 a correlation of function with habitat. Sum- 

 ner (1932) gives an interesting analysis of 

 color variation in the subspecies of Per- 

 omyscus polionotiis (Fig. 230; see also 



is rather uniformly dark-colored, except for 

 a gradation toward a lighter color within 

 20 to 40 miles of the fight sandy strip. 

 Such variation can probably be explained 

 by selection in relation to background color, 

 together with intermixture of coat color 

 genes through interbreeding of contiguous 

 populations (see p. 610). The selection in 

 this case, since it involves animals primarily 

 active at night, is probably the result of 



100 

 90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 



0^ 



% TAIL STRIPE OF TAIL LENGTH 



% COLORED AREA OF TOTAL PELAGE 



10 20 



30 



40 

 39 41 



50 59 



70 80 



90 



104 



S MILES FROM COAST 



Fig. 230. The effect of selection and isolation on pigmentation of deer mice. Left hand 

 measurements are from an island subspecies {Peromyscus polionotus leucocephalus) fiving on 

 white sand off the Florida coast. Low values of colored area (triangles) and tail stripe (cir- 

 cles) indicate fight pigmentation. Measurements at miles are from a coastal subspecies (P. 

 p. albifrons) fiving on white sand on the Florida guff coast. The measurements at 59 and 104 

 miles are from an inland subspecies (P. p. polionotus) living on dark soil in Alabama. The 

 stations at 20, 39, 40, and 41 miles are also on dark soil covered with vegetation and are prob- 

 ably areas of gene flow between areas of selection in relation to soil color. ( Redrawn from 

 Sumner, 1932.) 



Huxley, 1939). Peromyscus p. leucocepha- 

 lus is found on an isolated island of white 

 sand oflF the Florida coast and has a uni- 

 form white coat color throughout the popu- 

 lation. Peromyscus p. albifrons is found on 

 the mainland coastal strip of light sandy 

 soil and exhibits a marked ecocline of in- 

 creased darker pigmentation as the popu- 

 lation approaches the inland dark soil. 

 Peromyscus p. polionotus is found on dark 

 soil behind the coastal strip of light soil and 



predation pressure operating against con- 

 spicuous individuals (see p. 668). Dice 

 (1945, 1947) has experimentally demon- 

 strated the ability of several species of 

 owls to find P. maniculatus in dim light 

 calculated as low as 0.000,000,73 foot- 

 candle or even 0.000,000,08 foot-candle. 

 The possibility of selection in relation to 

 physiological effects of light is not entirely 

 ruled out; nor should we efiminate the 

 possibility of the kind of pigmentation as 



