RODENTS OF LIBYA 37 



history of the Libyan rodent fauna. Sympatric speciation, or speciation 

 without geographic separation, may be operative in Libyan rodents, 

 but evidence to support geographic speciation is far greater. 



Sympatric speciation. The presence in Libya of closely related sym- 

 patric species, differing more in ecological or biological characteristics 

 than in morphological characters, is suggestive of a type of sympatric 

 speciation. The following pairs of closely related species of Libyan 

 rodents have greater ecological than morphological differences and, in 

 this respect, resemble sibling species: 



Gerbillus eatoni and Gerbillus gerbillus 

 Jaculus deserti and Jaculus jaculus 



The above pairs are sympatric over portions of their range in Libya 

 yet do not interbreed. Isolating mechanisms, which segregate each 

 member of the species pair, involve habitat selection and behavioral 

 characteristics which usually have a genetic basis and are not to be 

 construed with geographic isolation, a topic that will be discussed later. 



Habitat isolation contributes significantly to spatial segregation of 

 these species of Libyan rodents; in the present study, examples will be 

 limited to this type of isolating mechanism. 



In many areas of coastal Tripolitania and Cyrenaica, Gerbillus 

 eatoni and Gerbillus gerbillus occur together without loss of genetic 

 identity. In these coastal habitats, G. gerbillus is almost exclusively 

 confined to localized sandy areas of the coastal plain or to the barren 

 margins of the nearby hamada, and G. eatoni seems to prefer the hard 

 clay substrate of the coastal plain. Vegetative cover of the sandy areas 

 and hamadas is generally extremely sparse, whereas that of the 

 majority of the coastal plain is relatively dense. This habitat segrega- 

 tion is largely instrumental in the prevention of introgression of the 

 gene pools of the respective species. In the absence of this habitat 

 exclusion, G. eatoni, owing to its more static gene pool, would doubtless 

 be "swamped out" by the more widely ranging and more adaptable 

 G. gerbillus. 



The dipodils, Gerbillus amoenus and Gerbillus campestris, also illus- 

 trate the principle of habitat isolation very well. These two species 

 are sympatric throughout most of Libya and show no evidences of 

 hybridization. Gerbillus campestris is highly polymorphic, both crani- 

 ally and in dorsal color, whereas G. amoenus is remarkably uniform 

 morphologically. The latter species is normally confined to sandy 

 margins of the oases or to gravel plains of the steppes. The larger 

 G. campestris shows a wide ecological tolerance and occupies a wide 

 variety of habitats, including the sparsely vegetated gravel plains of 

 the steppes, the margins of sand seas, the sandy environs of the 

 oases, the dense chapparal vegetation of the Cyrenaican Plateau, 

 and adjacent densely vegetated coastal plain. The preferred habitat of 



