34 U.S. NATIONAL MUSEUM BULLETIN 275 



Saharan steppe and Sahara Desert, they are more localized and usually 

 confined to the more mesic areas of the larger oases. Occasionally, 

 house mice are obtained, along with jerboas, gerbils, and jirds, from 

 the sandy periphery of the oases, but these habitats are probably 

 suboptimal. 



Speciation and Geographical Variation 



Most of the species of Libyan rodents satisfy the requirements of 

 the multidimensional, polytypic species being composed of groups of 

 local populations that "actually or potentially" interbreed with each 

 other. In some genetically plastic species, such as Jaculus jaculus and 

 Gerbillus gerbillus, there are varying degrees of gene flow among the 

 various local populations resulting in pronounced phenotypic vari- 

 ability. Other species, which are more restricted ecologically, such as 

 Gerbillus pyramidum, Eliomys quercinus, and Meriones caudatus, are 

 composed of discontinuous allopatric populations, many of which are 

 geographic isolates and thus only potentially capable of interbreeding. 

 Members of each species of Libyan rodents share in a common gene 

 pool and do not outbreed with members of other contiguous or 

 sympatric species. By virtue of these two biological characteristics 

 they conform to modern concepts of "biological species." 



The species Gerbillus gerbillus and Jaculus jaculus serve to illustrate 

 the concept of polytypic species. Each has a wide distribution in 

 Libya and has developed several subspecies in response to selective 

 pressures of the local environment. The pattern of distribution of the 

 subspecies of Jaculus jaculus and Gerbillus gerbillus is essentially 

 continuous, the range of one subspecies overlapping with that of the 

 contiguous subspecies. In some cases, zones of intergradation are 

 demonstrable. 



In Libya, several species of rodents show varying degrees of morph- 

 ological differentiation, and in some cases, certain populations have 

 progressed to the level of subspecific distinctness. Libyan species which 

 best demonstrate this geographic variation and the number of sub- 

 species comprising each are: Ctenodactylus gundi, 2; Eliomys quercinus, 

 3 ; Gerbillus aureus 3 ; Gerbillus campestris 5 ; Gerbillus eatoni 3 ; Gerbillus 

 gerbillus 5; Gerbillus pyramidum 2; Jaculus deserti 4; Jaculus jaculus 5; 

 Meriones caudatus 4; Meriones libycus 2; and Psammomys obesus 2. 



All of the above species are polytypic and are represented by at 

 least two subspecies. Some species of Libyan rodents, however, are 

 remarkably uniform, morphologically, over extensive areas of Libya 

 and, as a consequence, are not divisible into subspecies. Species of 

 rodents which are either monotypic or represented in Libya by a 

 single subspecies are: Gerbillus amoenus; Gerbillus henleyi; Gerbillus 

 kaiseri; Jaculus orientalis; Allactaga tetradactyla; Microtus mnstersi; 



