RODENTS OF LIBYA 41 



Fezzan. This population of G. p. hamadensis represents the northern- 

 most occurrence of the species Gerbillus pyramidum in Libya and as is 

 sometimes typical of populations on the margins of a species range, it 

 is sporadic and localized in occurrence. This subspecies has diverged 

 strikingly morphologically from the nearest related populations in the 

 Fezzan and is approaching full species status. There is no evidence of 

 intergradation between G. p. hamadensis and G. p. tarabuli to the south. 

 Apparently the mountainous areas of the Gebel es Soda and Gebel el 

 Harug el Asued and the vast plateau of the Hamada el Hamra, which 

 are located between the ranges of these two subspecies, act as absolute 

 barriers to dispersal. 



The distinct Meriones caudatus amplus of the Fezzan is another 

 typical peripheral isolate which is near to achieving species distinct- 

 ness. In the case of M. c. amplus, parental populations are to the north 

 and east, rather than to the south as in Gerbillus pyramidum 

 hamadensis. 



The species Psammomys obesus is distributed along the coastal 

 areas of the eastern Mediterranean, northern Egypt, and portions of 

 coastal Libya. The subspecies Psammomys obesus tripolitanus of the 

 coastal plain and littoral deserts along the Gulf of Sirte in Cyrenaica, 

 Libya, represents the westernmost populations of the species Psammomys 

 obesus in North Africa. The Cyrenaican Plateau is interposed between 

 the range of P. o. tripolitanus and Egyptian populations representing 

 the nominate subspecies. The amount of morphological divergence 

 in populations of P. o. tripolitanus is far less than that demonstrated 

 by Gerbillus pyramidum hamadensis or Meriones caudatus amplus. 

 The Cyrenaican Plateau has acted more as a filter in retarding gene 

 flow between populations of Psammomys obesus than as an absolute 

 barrier. 



It is possible that Psammomys vexillaris vexillaris represents a 

 species evolved through the agency of geographic isolation. Although 

 Psammomys vexillaris somewhat superficially resembles Psammomys 

 obesus, it is clearly a separate species and is readily separable from 

 P. obesus by its markedly smaller cranial size and external dimensions. 

 Some time in the past, peripheral populations of the coastal species, 

 P. obesus, may have become separated from the parental population. 

 In time, this isolated population developed different morphological 

 and ecological characteristics as a reflection of its altered genetic 

 constitution. The continuous action of selection has resulted in the 

 evolution of additional isolating mechanisms, both ecological and 

 behavioral, and this inland population of sand rats is now both 

 spatially and reproductively isolated from the parental population. 



The two subspecies of the gundi, Ctenodactylus gundi gundi and 

 Ctenodactylus gundi vali, have probably had an evolutionary history 

 similar to that of Psammomys obesus and Psammomys vexillaris, except 



285-134 O — 68 4 



