36 U.S. NATIONAL MUSEUM BULLETIN 275 



A similar adaptation to color of the substrate is shown by subspecies 

 of Meriones caudatus. In the Fezzan, the pale and uniform dorsal color 

 of jirds referrable to Meriones caudatus amplus may reflect the pre- 

 vailing pale coloration of the desert substrate. To the north, popula- 

 tions of Meriones caudatus caudatus and M. c. confalonierii which 

 inhabit the darker, more grayish soils of the steppe and coastal plain 

 are correspondingly darker in color with a stronger suffusion of black 

 on the dorsum. 



Cryptically colored individuals in a population of rodents where 

 predation is widespread probably have a selective advantage over 

 those less cryptically colored. In most cases a predator, whether it be 

 an owl, fox, or jackal, would most likely select individuals that were 

 most discordant with the color of their surroundings. 



To determine whether or not these "substrate races" are the result 

 of predator selection would require much more detailed research, 

 but it is the most plausible explanation at present. 



The adaptive significance of this cryptic coloration in Libyan 

 rodents may be related to climatic factors, such as humidity, aridity, 

 and solar radiation or may reflect relative density or sparseness of 

 vegetative cover. 



The subspecies Gerbillus campestris dodsoni shows a high degree of 

 polymorphism in dorsal color throughout its wide range in Libya. In 

 the same local population, individuals range in color from uniformly 

 tan or buff to almost dark brown and demonstrate varying degrees of 

 streaked or variegated patches in the pelage. Members of this sub- 

 species are highly polymorphic in dorsal color in some areas (Tazerbo 

 and Brach Oases) and almost uniform in others (Gebel es Soda). 



This type of color polymorphism in Gerbillus campestris dodsoni 

 may be the result of "balanced polymorphism" resulting from a 

 balance among the genotypes AA, Aa, and aa. This balance is subject 

 to geographic variation, but polymorphism is insured by the selective 

 superiority of heterozygotes over homozygotes. 



Several subspecies of Gerbillus gerbillus all possess orangish dorsal 

 color, which doubtless has a broad adaptive significance in the sandy 

 areas in which these gerbils live. Constant genetic interchange among 

 widely distributed local populations of these gerbils, however, has 

 prevented the formation of distinct discontinuous "substrate races" as 

 in Gerbillus campestris and Meriones caudatus. 



Modes of Speciation 



Gradual speciation, as opposed to instantaneous speciation or 

 macroevolution, has doubtless been the prevailing mode of speciation 

 among Libyan rodents. Of the two types of gradual speciation, 

 geographic speciation has played the primary role in the evolutionary 



