84 U.S. NATIONAL MUSEUM BULLETIN 2 75 



the former consisting of G. amoenus, G. campestris, G. henleyi, and 

 G. kaiseri and the latter composed of G. aureus, G. eatoni, G. gerbillus, 

 and G. pyramidum. 



Without doubt, gerbils are the most variable morphologically and 

 the most ecologically tolerant of all the Libyan rodents. They have 

 adapted to practically every available environmental situation. The 

 ability of these gerbils to exploit such a wide array of ecological 

 opportunities indicates a broad genetic constitution. This marked 

 adaptability has enabled them to occupy a vast geographic range in 

 Libya and has contributed to the maintenance of a common mor- 

 phological pattern among the widely scattered populations. 



Owing to the broad genetic constitution of the species of the genus 

 Gerbillus, there is wide variation in color and in cranial and external 

 characters of the component subspecies. Individual subspecies vary 

 markedly in any given character and generally are distinguished 

 from each other by an aggregate of characters rather than by specific, 

 measurable cranial differences. In gerbils, the usual criteria for recog- 

 nizing subspecies, such as differences in color and cranial and ex- 

 ternal dimensions, are employed, but these differences are usually 

 much more subtle than in other genera of Libyan rodents. 



Owing to this genetic plasticity in gerbils, the usual 84 to 93 percent 

 rule (1 to 1.5 standard deviations on either side of the mean) for 

 separating the members of two subspecies by a given character is 

 seldom realized, and the percentage of overlap in a character is 

 usually far greater. 



The ranges of many of the subspecies of gerbils in Libya are con- 

 tiguous, and gene exchange is of common occurrence. These inter- 

 grading populations contain genes from both subspecies and contribute 

 significantly to the maintenance of genetic variability among the 

 populations of gerbils in Libya. 



In most populations of a given species, and particularly in those 

 subspecies which are geographic isolates, genetic fixation occurs 

 rapidly and trends develop toward morphological distinctness. 



Key to the Subgenera of Gerbillus 



Plantar surfaces of hind feet hairy Gerbillus 



Plantar surfaces of hind feet naked Dipodillus 



