Comparative Evolution of Biomes 



317 



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FROM DIPRIONIDAE 

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Fig. 140. Phylogeny and dispersal chart of Neodiprion, a genus of sawflies 

 which feed on conifers. This indicates rigid regional restriction of the eastern 

 and western groups, with a fairly recent dispersal to Asia and, even more re- 

 cently, the dispersal of two species from west to east. 



Exitianiis have dispersed to all continents of the world (Oman, 

 1949; Evans, 1947a,b). 



A situation embodying these differentials in dispersal possibilities 

 is found in the North American mouse Peromyscus maniciiJatus and 

 its ectoparasitic fleas Monopsyllus tJmmbus, M. ivagneri, and Or- 

 chopeos leiicopus. The mouse has an extensive range including 

 many subclimax communities. Two of its ectoparasites, Monopsyl- 

 lus ivagneri and Orchopeas leucopus are also widespread, but 

 Monopsyllus thambus occurs only in the extreme northern portion 

 of the mouse's range (Fig. 141) (Holland, 1958). Should this mouse 

 disperse into South America, its two widespread fleas would prob- 

 ably go with it, but Monopsyllus thambus certainly would not. 



The extensive ranges and relatively frequent dispersal of many 

 subclimax species may explain some situations which have been 

 considered puzzling, for example the Cenozoic dispersal of horses 



