RODENTS OF LIBYA 39 



Geographic speciation. In Libyan rodents, far more evidence is 

 available to support speciation caused by geographic isolation. Be- 

 cause evolution is operative at the level of the local population, it is 

 the degree of isolation of these local populations and the resulting 

 cessation of gene flow between them that is of greatest significance 

 in interpreting evolutionary progress. In some of the examples of 

 geographic speciation discussed below, the subspecies themselves act 

 as incipient species, but it is the geographic isolate which is the main 

 focus of geographic speciation. In many instances geographic variation 

 is clinal. 



Clinal variation. Among Libyan rodents, the species Gerbillus ger- 

 billus, Meriones caudatus, and Gerbillus amoenus demonstrate clinal 

 variation. All of these species show progressive increase or decrease 

 in the size of either external or cranial features. Other morphological 

 and physiological characters of these three species doubtless undergo 

 clinal changes, but detection of these characters requires extremely 

 refined techniques, and, in some cases, living specimens would be 

 required. 



In eastern Cyrenaica, populations comprising the subspecies Ger- 

 billus gerbillus gerbillus, Gerbillus gerbillus psammophilous , and Gerbillus 

 gerbillus aeruginosas demonstrate a regular north to south progression 

 in diminution of cranial and body size. Individuals of the largest body 

 and cranial size are found in representatives of G. g. gerbillus in 

 Giarabub Oasis near the northernmost limits of the range of this 

 species in Libya. The trend of decreasing body and cranial size con- 

 tinues in a progressive fashion from north to south in populations 

 of these gerbils from Gialo, Gasr es Sahabi, and Tazerbo. The extreme 

 in small body and cranial size is shown by members of G. g. aeruginosus 

 from Cufra and Bzema Oases of southeastern Cyrenaica. 



A progressive increase in body and cranial size is clearly notice- 

 able among populations of jirds extending southward from the coastal 

 areas of Tripolitania to the interior of the Fezzan. Because of the 

 relatively few localities from which specimens were examined, this 

 character gradient lacks the uniformity of the cline in populations 

 of Gerbillus gerbillus. The differences in external and cranial dimen- 

 sions between representatives of Meriones caudatus confalonierii from 

 coastal Tripolitania and members of M. c. ampins from the Fezzan 

 are of the degree of specific distinctness. Populations of the two 

 extremes of this character gradient have thus diverged much more 

 than those of the cline in Gerbillus gerbillus. Apparently the more 

 diverse physiography and more varied edaphic conditions of western 

 Libya have proven to be a more severe deterrent to gene flow than 

 has the more uniform terrain of eastern Cyrenaica. 



