40 U.S. NATIONAL MUSEUM BULLETIN 275 



The degree of smoothness of a cline gives some indication of the 

 amount and regularity of gene flow between successively contiguous 

 populations. Because environmental factors vary from region to 

 region, phenotypic responses to the selective pressures of these environ- 

 ments also vary. Genetic exchange between neighboring populations, 

 however, tends to smooth out phenotypic characters that result from 

 this selection, and clinal patterns tend to be gradual and uniform. 



In Libya, clinal patterns of geographic variation generally run from 

 north to south or vice versa. The dipodil, Gerbillus amoenus, is an ex- 

 ception in having a pattern of clinal variation extending generally 

 from east-northeast to west-southwest. This east-northeast west- 

 southwest cline, showing progressively increasing cranial size, is de- 

 monstrable among populations of G. amoenus extending from the Nile 

 Delta to eastern Algeria. Members of this species from northern Egypt 

 and northern Cyrenaica have small, gracile skulls. In the Fezzan, 

 representatives of G. amoenus have markedly larger and more robust 

 skulls, while specimens examined from eastern Algeria are even larger 

 cranially than any of those from Libya. 



In all examples of clinal variation in Libyan rodents, only the broad 

 features of clinal progression are observable, and these clinal gradients 

 may be far more irregular than indicated. Regardless of local differ- 

 ences in clinal pattern, clines in Libyan rodents serve to demonstrate 

 general trends in morphological variation. In many cases, a clinal 

 population represents a stage in speciation, the morphological differ- 

 ences among populations being attributable to differences in their 

 genetic constitution. 



Geographic isolates. Subspecies of Libyan rodents representing geo- 

 graphic isolates have developed rather striking phenotypic characteris- 

 tics as a result of their altered genetic constitutions. These subspecies 

 are: Gerbillus gerbillus aeruginosus; Gerbillus pyramidum hamadensis; 

 Gerbillus campestris patrizii; Meriones caudatus amplus; Psammomys 

 obesus tripolitanus; Psammomys vexillaris vexillaris; Eliomys quercinus 

 cyrenaicus; Eliomys quercinus denticulatus; Jaculus deserti vastus; 

 Jaculus jaculus cujrensis; Ctenodactylus gundi gundi; and Ctenodac- 

 tylus gundi vali. 



Ranges of most of the above subspecies are located on the periphery 

 of the range of their respective species. Many of these peripheral 

 isolates have undergone marked morphological differentiation and are 

 well on the way toward becoming full species. It is probable that in 

 Libya most new species have arisen as a result of the genetic diver- 

 gence of peripheral isolates. 



The range of Gerbillus pyramidum hamadensis is limited to the 

 coastal plain of the Gulf of Sirte and littoral deserts and steppes of north- 

 ern Tripolitania. The nearest populations of gerbils belonging to this 

 species are those of G. p. tarabuli, 200 kilometers farther south in the 



