232 I The Process of Evolution 



Biologically sophisticated readers will be familiar with myriad 

 examples in which differences due to different selective pressures 

 ( due to different environments ) have been inferred. For the student 

 a very few more are added. Arctic foxes (following Allen's rule) 

 have short ears and snouts, whereas tropical foxes have long ears 

 and snouts, presumably because the low surface-volume ratio in the 

 former helps conserve body heat, whereas the high ratio in the 

 latter permits the heat to dissipate more rapidly. Indeed, both Allen's 

 and Bergmann's rules seem to be simply functions of the surface-vol- 

 ume ratio problems concerned with heat retention and dissipation. 

 A great many homoiotherms show clinal variation in conformance 

 with these rules, but physiological work to support their validity is 

 mostly lacking. 



Populations of Papilio glaucus have high frequencies of dark fe- 

 males in certain areas, supposedly because these dark-brown females 

 resemble the Aristolochia swallowtail (Battiis philenor), which is 

 also found in these areas. Battus philenor has been shown to be 

 distasteful to birds, and birds have been observed feeding on adult 

 P. glaucus in the field. Selection apparently favored the develop- 

 ment and maintenance of the mimetic form of P. glaucus in these 

 areas. 



In the high Sierra Nevada of California two forms of the butterfly 

 Oeneis chryxus are found, a light form in areas of granite rock and 

 a dark form in areas of basaltic rock. The selection pressure involved 

 has not been discovered, but the correlation suggests the work of an 

 as-yet-undetected visual predator. Similar examples of geographic 

 variation in color and in the habitat of many groups of animals have 

 been reported and could be multiplied indefinitely— geographic vari- 

 ation is ubiquitous, and selection has been shown to play a major 

 role in differentiating most populations that have been studied 

 thoroughly. It is a truism to state that populations of organisms in 

 different places will, under most circumstances, be genetically dif- 

 ferent. 



The variation described in Asclepias seems attributable to a com- 

 bination of differing selection pressures and genetic drift. Asclepias 

 tuberosa is a long-lived perennial growing in colonies of several to 

 many plants. The effective population size is estimated to be less 

 than the median census size of 11 plants. One would expect that, in 

 populations of this small size, genetic drift would become an im- 

 portant factor in evolution, and the data suggest that it is. Neverthe- 

 less, there appears to be a strong selective component in the changes 

 that have occurred along the transect during the 14 years. It is not 

 possible to discuss these in detail, but it is clear that the western 

 genotypes for Z A and Z B are selectively better off than the com- 



